SQL Info

What are variables in PHP?

2026-04-12T20:44:21.662-07:00

Variables store data and always start with $.

$name = "John";

$age = 25;

Difference between echo and print?

2026-04-12T20:43:00.000-07:00

echo	print
Can output multiple strings	Outputs one string only
Faster	Slightly slower
No return value	Returns 1

How to Integrate Google Tag Manager with Drupal 9 - An Easy Step-by-Step Tutorial

2025-07-26T07:23:00.000-07:00

Marketers are always seeking new ways to elevate website engagement and maximize interactivity. Yet, managing the countless third-party integrations and tracking tools implemented to enhance their business can quickly become overwhelming and chaotic. That's when Google Tag Manager comes to the rescue. With its intuitive and user-friendly platform, managing, updating, and tracking the snippets, tags, and integrations on your website has never been easier. From improving website performance to boosting conversion rates, Google Tag Manager offers the solution to streamline your digital marketing efforts and revolutionize your online presence.

Drupal 9 makes integrating with Google Tag Manager easy and effortless. The Google tag manager module in Drupal lets you quickly add, modify and remove tags on your website without touching any code! Read this easy step-by-step guide to learn how to integrate your Drupal 9 site with your Google tag manager.

What is Google Tag Manager and how can it help?

Google Tag Manager is like a toolkit. It has all the tools you need meticulously organized in one place. Tools like Google Analytics, Adwords, personalization tools, A/B testing tools, Remarketing, Native advertising pixels, and much more. All the integration tags can be stored in Google Tag Manager for better access and management. How can it help, you ask?

Marketers find it greatly beneficial as they don’t have to depend on developers to add and modify integration tags on the website. They can easily do it themselves.
Better organization of tags can help marketers’ access and manage their integrations easily.
Updating tags don’t require you to change them on multiple web-pages.
Test if your tags get triggered on any event/action in the preview mode.
You can even check for formatting or security issues before deploying it to live.
Provides an extra layer of data security.

Integrating Google Tag Manager in Drupal 9

Drupal 9 integrates seamlessly with Google Tag Manager and installing it is also a breeze. Now that we know how useful Google Tag Manager is, let’s move on to integrating it with your Drupal 9 website.

Step 1 - Install the module

You can download the Google tag Manager module here.

Or run this command through composer with this command :

       
composer require 'drupal/google_tag:^1.4'

Step 2 - Configure the Module

In the admin toolbar, go to Configuration-> System -> Google Tag Manager

Step 3 - Open Container Page

Once you click on Google Tag Manager, you will see a container page like this –

Step 4 - Add a Container

Click on the Add container button. This is where you will add the Container ID that you had created previously when you signed up with GTM. If not done already, go here to signup and create a container ID for yourself (shown in the below steps).

Step 5 - Create a Container

Under Account setup, give the Account name and your Country name.

Under Container setup, give the Container name and select the target platform as per your requirements.

Click on the Create button.

Step 6 - Get the Container ID

Once done, you will be able to see a popup screen that will display the code snippet that needs to be pasted in the head section. Look out for the Container Id which will look like an alphanumeric word and begins with “GTM”. Here it is “GTM-MXQN9XL”. Copy this code to your clipboard.

Step 7 - Insert the Container ID

Head back to your Drupal setup where you had to add the container ID. Give a name for your container in the Label field and paste the previously copied container ID in the Container ID field. Save the configuration.

Step 8 - And we’re all set!

To verify if the installation of Google tag manager has been successful in your Drupal site, go to the home page of your website and do an “Inspect Element”. If it has been installed properly, you should be able to see it within the <head> tag.

Google Tag Manager is an extremely useful tool for site owners and marketers. Different third-party integrations can be maintained, added and modified easily. This greatly helps marketers in speeding up the process of experimenting and testing out various useful integrations that can lead to better user engagement. Drupal 9 comes loaded with plenty of powerful features and functionalities and tons of handy contributed modules that you can easily install and connect to. As a leading Drupal development company,

What is the purpose of the php.ini file?

2025-05-01T07:08:00.000-07:00

The PHP configuration file, php.ini, is the final and most immediate way to affect PHP's functionality. The php.ini file is read each time PHP is initialized. in other words, whenever httpd is restarted for the module version or with each script execution for the CGI version.

How do you enable error reporting in PHP?

2023-11-13T05:10:00.000-08:00

PHP allows us to choose whether or not to display errors to users. There are three ways to enable or disable error reporting in PHP:

Method 1: You can turn the display error parameter in the php.ini file on or off. When it’s on, errors are shown; when it’s off, there are no errors shown or reported. To do this, locate the display error parameter in the php.ini file, then set its value to “on.” Restart the web server's various services. The PHP script's errors will now be visible to end users. Set the display error parameter to “off” to stop reporting errors. The off argument indicates that users won't see any errors.

Method 2: Using the mini set() function is the second way to enable or stop error reporting. The ability to modify the configuration options in the php.ini file is a built-in feature of the PHP programming language. It requires two inputs: the name of the setting you want to change and the value you want to assign.

Method 3: You can also utilize the error reporting() function, a native PHP function, to enable error reporting. This function controls which PHP faults are reported and sets the error reporting directive during runtime. Given that PHP has multiple error levels, this method also aids in setting the various levels for the runtime (duration) of your script. This function accepts parameters for error names like E ERROR, E WARNING, E PARSE, etc., allowing these errors to be reported.

Sachin vs South Africa Highlights HD🔥 First ODI Double Century hitter | India | Full Match Video

2023-04-16T09:16:00.002-07:00

How to Create Custom Post Types in WordPress

2023-04-16T07:54:00.003-07:00

We only get three built-in content types at the backend when we install WordPress, i.e., posts, pages, and media. However, today WordPress has become quite flexible and advanced.

The approach to adding more post types also has expanded. The diversified usage demands more content types because posts, pages, and media are not enough, and here is where WordPress custom post type comes in handy.

What Is a WordPress Custom Post Type?

Custom post types are used to convert a regular WordPress website into a content management system. As the name suggests, you can use custom post types to create various content types for your website.

Besides that, one can find several post types available by default in WordPress installation, including:

Post – blog post
Page – static page
Attachment – attached media
Revision – post revision
Navigation Menu – nav menu
WordPress supports an unlimited number of Custom Post Types. You can create your custom posts and call them up wherever you want.
For example, if you run a News website and want to add a custom post type titled “News.” Once created, the news post-type would have its own menu in the WordPress dashboard admin area. Additionally, you can create multiple post types, such as Movies, Portfolios, etc.

What Is a WordPress Custom Post Type?

Besides that, one can find several post types available by default in WordPress installation, including:

Post – blog post
Page – static page
Attachment – attached media
Revision – post revision
Navigation Menu – nav menu
WordPress supports an unlimited number of Custom Post Types. You can create your custom posts and call them up wherever you want.
For example, if you run a News website and want to add a custom post type titled “News.” Once created, the news post-type would have its own menu in the WordPress dashboard admin area. Additionally, you can create multiple post types, such as Movies, Portfolios, etc.
Follow the steps below to create a custom post type on a WordPress website:
- Navigate to the function.php file from your WordPress theme directory
- Add the following code to the function.php file
1. /* Custom Post Type Start */
3. function create_posttype() {
4. register_post_type( 'news',
5. // CPT Options
7. array(
8. 'labels' => array(
9. 'name' => __( 'news' ),
10. 'singular_name' => __( 'News' )
11. ),
12. 'public' => true,
13. 'has_archive' => false,
14. 'rewrite' => array('slug' => 'news'),
15. )
16. );
17. }
18. // Hooking up our function to theme setup
19. add_action( 'init', 'create_posttype' );
21. /* Custom Post Type End */
- Once you’ve added the code, the News post-type will automatically appear in the admin area
- When creating custom post types, it is necessary to use init for the hook in add_action(), and the register_post_type() function will take the arguments
1. /*Custom Post type start*/
3. function cw_post_type_news() {
5. $supports = array(
6. 'title', // post title
7. 'editor', // post content
8. 'author', // post author
9. 'thumbnail', // featured images
10. 'excerpt', // post excerpt
11. 'custom-fields', // custom fields
12. 'comments', // post comments
13. 'revisions', // post revisions
14. 'post-formats', // post formats
15. );
17. $labels = array(
18. 'name' => _x('news', 'plural'),
19. 'singular_name' => _x('news', 'singular'),
20. 'menu_name' => _x('news', 'admin menu'),
21. 'name_admin_bar' => _x('news', 'admin bar'),
22. 'add_new' => _x('Add New', 'add new'),
23. 'add_new_item' => __('Add New news'),
24. 'new_item' => __('New news'),
25. 'edit_item' => __('Edit news'),
26. 'view_item' => __('View news'),
27. 'all_items' => __('All news'),
28. 'search_items' => __('Search news'),
29. 'not_found' => __('No news found.'),
30. );
32. $args = array(
33. 'supports' => $supports,
34. 'labels' => $labels,
35. 'public' => true,
36. 'query_var' => true,
37. 'rewrite' => array('slug' => 'news'),
38. 'has_archive' => true,
39. 'hierarchical' => false,
40. );
41. register_post_type('news', $args);
42. }
43. add_action('init', 'cw_post_type_news');
45. /*Custom Post type end*/
$supports: Specifies that the post type is compatible and supports all essential features.
$labels: Specifies that the post type is referred correctly to the admin area.
$args: Specifies a permalink slug of the news and a menu position located just beneath the Posts menu.
Now let’s take a look at before and after adding custom post features to our WordPress website.
– Before adding features

– After adding features
The steps above tell how to register WordPress custom post types to the backend of any theme. Now let’s learn how to add a custom post on a WordPress website.
How to Create a New Custom Post on a WordPress Website
You can create a new custom post on your WordPress website by following the steps below:
- Click the registered custom post type, which in our case is “News.”
- Click Add New.
- Type the title and body of your post.
- Type the excerpt, and set a featured image.
- Click the Publish button to take the new custom post live.
How to Create a Template and Fetching List
Creating a template and fetching a list requires creating a new file named template-news.php.
- Place the newly created file in your theme folder.
- Add the following code to the file:
1. <?php
2. /*Template Name: News*/
3. get_header();
4. query_posts(array(
5. 'post_type' => 'news'
6. )); ?>
7. <?php
8. while (have_posts()) : the_post(); ?>
9. <h2><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></h2>
10. <p><?php the_excerpt(); ?></p>
11. <?php endwhile;
12. get_footer();
13. ?>
Select a Template
- Go to your WordPress dashboard
- Click Pages > Add New
- Create a new page named “News“
- Click Page Attributes on the right side and access the drop-down menu under Template
- Select the new template “News“
- Click the Update button to set your template
Refer to the image below for the visual representation of the above steps:
The image below represents the final display of your listing page:
How to Add Menu for Custom Post Type
Add your custom post type as a part of the Menu options on your WordPress website by following the steps below:
- Go to your WordPress dashboard
- Navigate to Appearance > Menus
- Add the News page to your main menu to display a navigational link to our newly created WordPress custom post type, News
For further reference, check out the image below.
And this is how your website will look on the front end. Check out the image below:
How to Create a Detail Page for Custom Post Type
Create a “detail” page for custom post type by following the steps below:
- Add a new file called single-news.php (located in your WordPress theme)
- Add the following code to the file:
1. <?php
2. get_header();
3. /* Start the Loop */
4. while (have_posts()) : the_post();
5. get_template_part('template-parts/post/content', get_post_format());
6. endwhile; // End of the loop.
7. get_footer();
8. ?>
Now it’s time to see what your detail page looks like:
Wrapping Up!
You have now learned how to create a Custom Post Type in WordPress, which is quite complex but can be easily done by following the steps mentioned in this blog. Check out our other blog posts for more helpful guides on managing your WordPress site, such as reducing WordPress load times. Shall you have any queries or suggestions regarding the blog, feel free to comment below.

Nidahas Trophy 2018 Final Match, Final Over - India vs Bangladesh

2023-04-09T01:06:00.000-07:00

Dinesh Karthik hits 22 runs off Rubel Hossain - 19th over of Nidahas Trophy Final

2023-04-09T01:04:00.003-07:00

Virat kohli 100* off 52 balls aus vs Ind 2nd odi 2013

2023-04-09T01:02:00.002-07:00

Behind the scenes of India's win over Pakistan ICC T20 World Cup 2022

2022-10-23T20:58:00.005-07:00

Difference between Abstract class and Interfaces.

2021-12-20T07:53:00.003-08:00

Abstract class:

Abstract class comes under partial abstraction.
Abstract classes can maintain abstract methods and non abstract methods.
In abstract classes, we can create the variables.
In abstract classes, we can use any access specifier.
By using 'extends' keyword we can access the abstract class features from derived class.
Multiple inheritance is not possible.

Interface:

Interface comes under fully abstraction.
Interfaces can maintain only abstract methods.
In interfaces, we can't create the variables.
In interface, we can use only public access specifier.
By using 'implement' keyword we can get interface from derived class.
By using interfaces multiple inheritance is possible.

What is the difference between MySQL, MySQLi and PDO?

2021-12-20T07:49:00.002-08:00

To understand the difference between MySQL, MySQLi, and PDO, we must know about each one of them individually.

These are nothing but the APIs of PHP that is used to access the MySQL databases and tables. The developers can choose either one of them for their project, however, it must be known that MySQLi cannot be used with PHP 7 and its newer versions. However, developers can use MySQLi with PHP 5, which is now deprecated.

Let’s have some more information about each of them:

MySQL: This was the main extension that was designed to help PHP applications send and receive data from MySQL database. However, use of MySQL has been deprecated and removed as of PHP 7 and its newer versions. This is why it is not recommended for new projects, and that’s the reason why MySQLi and PDO extensions are used more nowadays.
MySQLi: The ‘i’ in MySQLi stands for Improved. Therefore, this is also known as the improved version of MySQL. It has many new features that will be covered later in the article.
PDO – PHP Data Objects: The main advantage of using PDO is that it supports, and provides a uniform method of access to 11 different databases.

PDO supported databases are:

CUBRID
MS SQL Server
Firebird/Interbase
IBM
Informix
MySQL

Oracle
ODBC and DB2
PostgreSQL
SQLite
4D

However, PDO does not allow the usage of all the features available in the present version of the MySQL server. For example, PDO doesn’t allow the support of MySQL’s multiple statements.

Comparing MySQL, MySQLi, and PDO:

Connection to the Database
Error Handling
Data Fetching
API Support
Security

Connection to the database:

MySQL: The MySQL code to connect to the database is:

<?php
  
// Add the hostname, username and password of the database
$connection_link = mysql_connect("host", "username", "password");
  
// Select query for the database
mysql_select_db("database_name", $connection_link);
  
// Set the charset, UTF-8 to be used for projects
mysql_set_charset('UTF-8', $connection_link);
  
?>

MySQLi: In case of MySQLi, there is just a one-line code. The user instantiates a MySQLi instance using the username, password, and name of the database.
<?php

// Database credentials
$mysqli_db = new mysqli('host', 'username', 'password', 'database_name');

?>
PDO: In case of PDO, a new PDO object must be created.
<?php

// Credentials required for connection
$pdo = new PDO('mysql:host=host; dbname=database_name; charset=utf8',
            'username', 'password');

?>
A big advantage of using PDO is that it makes switching the project to another database simpler. Therefore, the only thing to do is to change the connection string and those queries that will not be supported by the new database.

Error Handling: Error handling is the detection, and resolution of application, programming or communication errors. Error handling helps in maintaining the normal flow of program execution, as the errors in the program are deal gracefully, thus making the program run well.

MySQL:
<?php

$my_result = mysql_query("SELECT * FROM table", $connection_link)
or die(mysql_error($connection_link));
?>
The ‘die’ method is used for error handling in MySQL but it is not considered to be a good approach of error handling. This is because die abruptly ends the script and then display the error to the screen. This can make the database prone to hackers.
MySQLi: The error handling in MySQLi if a bit easier. The mysqli::$error (mysqli_error) returns a string description of the last error.
<?php

if (!$mysqli->query("SET a=1")) {
    printf("Errormessage: %s\n", $mysqli->error);
}

?>

PDO: PDO has the best error handling method out of these three. This is because of the availability of the try-catch block. Also, there are some error modes that can be used for error handling.

PDO::ERRMODE_SILENT: It is used to check each result and then check $db->errorInfo() to get error details.
PDO::ERRMODE_WARNING: Warning does not halt the script. This provides run-time warnings and not fatal errors.
PDO::ERRMODE_EXCEPTION: It throws exceptions that show an error raised by PDO. It should not throw a PDOException with your code. It acts much like or die(mysql_error()) when it isn’t caught. But it can catch these PDOException and handle as we want.

We can set these error modes as follow:

<?php
$db->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_SILENT );
$db->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_WARNING );
$db->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
?>

Now, try/catch method can be added as follows:

<?php
  
try {
      
    // Incorrect query
    $db->query('hello');
}
  
catch (PDOException $ex) {
      
    // $ex->getMessage();
      
     // Message to be displayed in
     // case of such an error
    echo "An Error has occurred";
}
  
?>

An advantage of using the try/catch method is that we can set a normal error message to be displayed to the user, instead of an exception message, as that might be difficult for the general users to comprehend.

Data Fetching:

MySQL: General programming loops such as for, or while loop can be used for such a purpose. Suppose there is a table named ‘data’ in the database and we want to output the username from each row of the table. While loop can be used in the following way to do the work.

<?php
  
$my_result = mysql_query('SELECT * from data')
        or die(mysql_error());
  
$num_rows = mysql_num_rows($my_result);
  
while($row = mysql_fetch_assoc($my_result)) {
    echo $row['field1'];
}
  
?>

MySQLi: MySQLi uses a loop for this purpose as well. The code, however, will be a bit different.
<?php

while($row = $my_result->fetch_assoc()) {
    echo $row['username'] . '\n';
}

?>
PDO: PDO has many in-built statements that help in such cases.
- PDOStatement::fetchAll(): It returns the result in the form of an array, containing all of the result rows.
- PDOStatement::fetchColumn(): It fetches a single column from the next row of a result set.
- PDOStatement::fetchObject(): This first fetches the next rows and then returns it as an object.
- PDOStatement::setFetchMode(): It sets the default fetch mode for the statement.
The query is also used for data fetching, as it returns a PDOStatement object that can be used to fetch the data directly by using foreach and for loop.
<?php

// Select query
$stmt = $db->query('SELECT * FROM `data_table`');

// fetchAll is used
$my_results = $stmt->fetchAll(PDO::FETCH_ASSOC);
?>

API Support: When it comes to the API support, PDO provides an object-oriented approach. MySQLi provides a procedural way, much similar to the MySQL. This is the reason why developers coming from a MySQL background prefers using MySQLi. However, object-oriented programmers prefer PDO because of its compatibility with a large number of databases.
Thus, object-oriented programmers prefer PDO, while procedural programmers prefer MySQL and MySQLi.

Security: Database security is used to protect databases and the information they contain from the hackers and their attacks. Hackers generally use SQL injections to disrupt the database. Thus, security from the injections must be ensured.

Both PDO and MySQLi provide SQL injection security.

Suppose a hacker is trying to inject an SQL injection through the ‘firstname’ HTTP query parameter using the POST method:

$_POST['firstname'] = "'; DELETE FROM users; /*"

If the injection escapes, it will be added in the query “as it is”. Thus, it will delete all rows from the users table.

In PDO, manual escaping is there to add security.

$name = PDO::quote($_POST['name']);
$pdo->query("SELECT * FROM users WHERE name = $name");

The difference between PDO::quote() and mysqli_real_escape_string() is that the former escapes the string and the quote, while the latter will only escape the string and the quotes will have to be added manually.

The Difference Between Abstract Class and Interface in PHP

2021-12-20T07:41:00.002-08:00

Introduction

Before diving deep into the difference between abstract class and interface, you must understand one basic thing: these are two completely different classes that cannot be used as an alternative to one another.

Interface classes completely empty the shells while expecting child classes to implement everything for them. Abstract classes not only contain the common piece of information between the shells inside but also expect the child classes within to fill in the gaps.

Let us dive in a bit deeper to actually understand the difference minutely.

If you want to read about PHP Array questions then you can visit here. This will help you crack your PHP interviews.

Interface Class

As we already know, an interface is actually defined by an interface keyword where all the methods are abstract. In addition to this, all the methods declared in this type of class must be declared in public which reflects the true nature of an interface.

Let's help to demonstrate that with an example:

interface Logger {  
    public  
    function execute();  
}  

As you can see above, in the interface, the method body is not defined. Only the name and the parameters are being defined. Now, let's move on to the abstract class.

Abstract Class

In PHP, an abstract class is one being partially implemented by any developer. It might contain at least one abstract method which is basically a method without any written code. It just contains the name and the parameters and has been marked as “abstract”.

By definition, an abstract class is simply a function definition whose purpose is to serve the programmer by telling them the method in question must be implemented in a child class.

Here is an example to demonstrate the abstract class:

abstract class AbstractClass {  
    abstract protected  
    function getValue();  
    public  
    function printOut() {  
        print $this - > getValue().  
        "\n";  
    }  
}  

Now that you have been acquainted with what is an abstract an interface class, its time to delve into their differences, step by step.

Here’s a table depicting the difference between abstract and interface class in PHP.

Interface Class	Abstract Class
Interface class supports multiple inheritance feature	Abstract class does not support multiple inheritances.
This does not contain a data member.	Abstract class does contain a data member.
The interface does not allow containers.	The abstract class supports containers.
An interface class only contains incomplete members which refer to the signature of the member.	Abstract class contains both incomplete(i.e. abstract) and complete members.
Since everything is assumed to be public, an interface class does not have access modifiers by default.	An abstract class can contain access modifiers within subs, functions, and properties.
Any member of an interface cannot be static.	Only a complete member of the abstract class can be static.

Conclusion

Now that you have learned abstract class and interface differences, it will be much easier to implement them on your project.

What is a foreign key? Write a query to implement the same in MySQL.

2021-12-18T08:15:00.001-08:00

A foreign key is used to connect two tables. A FOREIGN KEY is a field (or assortment of it) in one table that alludes to the PRIMARY KEY in another table. The FOREIGN KEY requirement is utilised to forestall activities that would crush joins between tables.

To assign a foreign key, it is important to mention it while creating the table. It can be assigned by invoking the FOREIGN KEY query. Something like this:

FOREIGN KEY (Any_ID) REFERENCES Table_to_reference(Any_ID)

Write a query for a column addition in MySQL

2021-12-18T08:13:00.005-08:00

For this, an ALTER TABLE query is required. Once invoked, simply mention the column and its definition. Something like this:

ALTER TABLE cars

ADD COLUMN engine VARCHAR(80) AFTER colour;

What are the tables in MySQL? Explain the types.

2021-12-18T08:11:00.003-08:00

This is a must-know MySQL interview question. Let’s see the answer-

MySQL stores everything in logical tables. Tables can be thought of as the core storage structure of MySQL. And hence tables are also known as storage engines. Here are the storage engines provided by MySQL:

· MyISAM – MyISAM is the default storage engine for MySQL. It extends the former ISAM storage engine. MyISAM offers big storage, up to 256TB! The tables can also be compressed to get extra storage. MyISAM tables are not transaction-safe.

· MERGE – A MERGE table is a virtual table that consolidates different MyISAM tables that have a comparable structure to one table. MERGE tables use the indexes of the base tables, as they do not have indexes of their own.

· ARCHIVE – As the name suggests, Archive helps in archiving the tables by compressing them, in-turn reducing the storage space. Hence, you can store a lot of records with the Archive. It uses the compression-decompression procedure while writing and reading the table records. It is done using the Zlib library.

· CSV – This is more like a storage format. CSV engine stores the values in the Comma-separated values (CSV) format. This engine makes it easier to migrate the tables into a non-SQL pipeline.

· InnoDB – InnoDB is the most optimal while choosing an engine to drive performance. InnoDB is a transaction-safe engine. Hence it is ACID-compliant and can efficiently restore your database to the most stable state in case of a crash.

· Memory– Memory tables were formerly known as HEAP. With memory tables, there can be a performance boost as the tables are stored in the memory. But it does not work with large data tables due to the same reason.

· Federated – Federated tables allow accessing remote MySQL server tables. It can be done without any third-party integration or cluster technology.

How To Delete Duplicate Rows in MySQL

2021-09-15T10:21:00.003-07:00

Summary: in this tutorial, you will learn various ways to delete duplicate rows in MySQL.

In the previous tutorial, we have shown you how to find duplicate values in a table. Once the duplicates rows are identified, you may want to delete them to clean up your data.

Prepare sample data

The following script creates table contacts and inserts sample data into the contacts table for the demonstration.

DROP TABLE IF EXISTS contacts;

CREATE TABLE contacts (
    id INT PRIMARY KEY AUTO_INCREMENT,
    first_name VARCHAR(50) NOT NULL,
    last_name VARCHAR(50) NOT NULL, 
    email VARCHAR(255) NOT NULL
);

INSERT INTO contacts (first_name,last_name,email) 
VALUES ('Carine ','Schmitt','carine.schmitt@verizon.net'),
       ('Jean','King','jean.king@me.com'),
       ('Peter','Ferguson','peter.ferguson@google.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Jonas ','Bergulfsen','jonas.bergulfsen@mac.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Zbyszek ','Piestrzeniewicz','zbyszek.piestrzeniewicz@att.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com'),
       ('Julie','Murphy','julie.murphy@yahoo.com'),
       ('Kwai','Lee','kwai.lee@google.com'),
       ('Jean','King','jean.king@me.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com');
Code language: SQL (Structured Query Language) (sql)

Note that you can execute this script to recreate test data after you execute a DELETE statement.

This query returns data from the contacts table:

SELECT * FROM contacts
ORDER BY email;
Code language: SQL (Structured Query Language) (sql)

The following query returns the duplicate emails in the contacts table:

SELECT 
    email, COUNT(email)
FROM
    contacts
GROUP BY 
    email
HAVING 
    COUNT(email) > 1;
Code language: SQL (Structured Query Language) (sql)

As you can see, we have four rows with duplicate emails.

A) Delete duplicate rows using `DELETE JOIN` statement

MySQL provides you with the DELETE JOIN statement that allows you to remove duplicate rows quickly.

The following statement deletes duplicate rows and keeps the highest id:

DELETE t1 FROM contacts t1
INNER JOIN contacts t2 
WHERE 
    t1.id < t2.id AND 
    t1.email = t2.email;
Code language: SQL (Structured Query Language) (sql)

This query references the contacts table twice, therefore, it uses the table alias t1 and t2.

The output is:

Query OK, 4 rows affected (0.10 sec)
Code language: CSS (css)

It indicated that four rows have been deleted. You can execute the query that find duplicate emails again to verify the delete:

SELECT 
    email, 
    COUNT(email)
FROM
    contacts
GROUP BY 
    email
HAVING 
    COUNT(email) > 1;
Code language: SQL (Structured Query Language) (sql)

The query returns an empty set, which means that the duplicate rows have been deleted.

Let’s verify data from the contacts table:

SELECT * FROM contacts;
Code language: SQL (Structured Query Language) (sql)

The rows with id 2, 4, 7, and 9 have been deleted.

In case you want to delete duplicate rows and keep the lowest id, you can use the following statement:

DELETE c1 FROM contacts c1
INNER JOIN contacts c2 
WHERE
    c1.id > c2.id AND 
    c1.email = c2.email;
Code language: SQL (Structured Query Language) (sql)

Note that you can execute the script for creating contacts table again and test this query. The following output shows the data of the contacts table after removing duplicate rows.

B) Delete duplicate rows using an intermediate table

The following shows the steps for removing duplicate rows using an intermediate table:

Create a new table with the structure the same as the original table that you want to delete duplicate rows.
Insert distinct rows from the original table to the immediate table.
Drop the original table and rename the immediate table to the original table.

The following queries illustrate the steps:

Step 1. Create a new table whose structure is the same as the original table:

CREATE TABLE source_copy LIKE source;
Code language: SQL (Structured Query Language) (sql)

Step 2. Insert distinct rows from the original table to the new table:

INSERT INTO source_copy
SELECT * FROM source
GROUP BY col; -- column that has duplicate values
Code language: SQL (Structured Query Language) (sql)

Step 3. drop the original table and rename the immediate table to the original one

DROP TABLE source;
ALTER TABLE source_copy RENAME TO source;
Code language: SQL (Structured Query Language) (sql)

For example, the following statements delete rows with duplicate emails from the contacts table:

-- step 1
CREATE TABLE contacts_temp 
LIKE contacts;

-- step 2
INSERT INTO contacts_temp
SELECT * 
FROM contacts 
GROUP BY email;


-- step 3
DROP TABLE contacts;

ALTER TABLE contacts_temp 
RENAME TO contacts;
Code language: SQL (Structured Query Language) (sql)

C) Delete duplicate rows using the `ROW_NUMBER()` function

Note that the ROW_NUMBER() function has been supported since MySQL version 8.02 so you should check your MySQL version before using the function.

The following statement uses the ROW_NUMBER() function to assign a sequential integer number to each row. If the email is duplicate, the row number will be greater than one.

SELECT 
	id, 
    email, 
    ROW_NUMBER() OVER ( 
		PARTITION BY email 
        ORDER BY email
	) AS row_num 
FROM contacts;
Code language: SQL (Structured Query Language) (sql)

The following statement returns id list of the duplicate rows:

SELECT 
	id 
FROM (
	SELECT 
		id,
		ROW_NUMBER() OVER (
			PARTITION BY email
			ORDER BY email) AS row_num
	FROM 
		contacts
) t
WHERE 
	row_num > 1;
Code language: SQL (Structured Query Language) (sql)

And you just delete the duplicate rows from the contacts table using the DELETE statement with a subquery in the WHERE clause:

DELETE FROM contacts 
WHERE 
	id IN (
	SELECT 
		id 
	FROM (
		SELECT 
			id,
			ROW_NUMBER() OVER (
				PARTITION BY email
				ORDER BY email) AS row_num
		FROM 
			contacts
		
	) t
    WHERE row_num > 1
);
Code language: SQL (Structured Query Language) (sql)

MySQL issued the following message:

4 row(s) affected

In this tutorial, you have learned how to delete duplicate rows in MySQL by using the the DELETE JOIN statement or an intermediate table.

How To Find Duplicate Values in MySQL

2021-09-15T10:18:00.006-07:00

Summary: in this tutorial, you will learn how to find duplicate values of one or more columns in MySQL.

Data duplication happens because of many reasons. Finding duplicate values is one of the important tasks that you must deal with when working with the databases.

Setting up a sample table

First, create a table named contacts with four columns: id, first_name, last_name, and email.

CREATE TABLE contacts (
    id INT PRIMARY KEY AUTO_INCREMENT,
    first_name VARCHAR(50) NOT NULL,
    last_name VARCHAR(50) NOT NULL,
    email VARCHAR(255) NOT NULL
);
Code language: SQL (Structured Query Language) (sql)

Second, inserts rows into the contacts table:

INSERT INTO contacts (first_name,last_name,email) 
VALUES ('Carine ','Schmitt','carine.schmitt@verizon.net'),
       ('Jean','King','jean.king@me.com'),
       ('Peter','Ferguson','peter.ferguson@google.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Jonas ','Bergulfsen','jonas.bergulfsen@mac.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Zbyszek ','Piestrzeniewicz','zbyszek.piestrzeniewicz@att.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com'),
       ('Julie','Murphy','julie.murphy@yahoo.com'),
       ('Kwai','Lee','kwai.lee@google.com'),
       ('Jean','King','jean.king@me.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com');
Code language: SQL (Structured Query Language) (sql)

Third, query data from the contacts table:

SELECT * FROM contacts
ORDER BY email;
Code language: SQL (Structured Query Language) (sql)

In the contacts table, we have some rows that have duplicate values in the first_name, last_name, and email columns. Let’s learn how to find them.

Find duplicate values in one column

The find duplicate values in on one column of a table, you use follow these steps:

First, use the GROUP BY clause to group all rows by the target column, which is the column that you want to check duplicate.
Then, use the COUNT() function in the HAVING clause to check if any group have more than 1 element. These groups are duplicate.

The following query illustrates the idea:

SELECT 
    col, 
    COUNT(col)
FROM
    table_name
GROUP BY col
HAVING COUNT(col) > 1;
Code language: SQL (Structured Query Language) (sql)

By using this query template, you can to find rows that have duplicate emails in the contacts table as follows:

SELECT 
    email, 
    COUNT(email)
FROM
    contacts
GROUP BY email
HAVING COUNT(email) > 1;
Code language: SQL (Structured Query Language) (sql)

This picture shows the output of the query that shows the duplicate emails:

Find duplicate values in multiple columns

Sometimes, you want to find duplicate rows based on multiple columns instead of one. In this case, you can use the following query:

SELECT 
    col1, COUNT(col1),
    col2, COUNT(col2),
    ...

FROM
    table_name
GROUP BY 
    col1, 
    col2, ...
HAVING 
       (COUNT(col1) > 1) AND 
       (COUNT(col2) > 1) AND 
       ...
Code language: SQL (Structured Query Language) (sql)

Rows are considered duplicate only when the combination of columns are duplicate therefore we used the AND operator in the HAVING clause.

For example, to find rows in the contacts table with duplicate values in first_name, last_name, and email column, you use the following query:

SELECT 
    first_name, COUNT(first_name),
    last_name,  COUNT(last_name),
    email,      COUNT(email)
FROM
    contacts
GROUP BY 
    first_name , 
    last_name , 
    email
HAVING  COUNT(first_name) > 1
    AND COUNT(last_name) > 1
    AND COUNT(email) > 1;
Code language: SQL (Structured Query Language) (sql)

The following illustrates the output of the query:

In this tutorial, you have learned how to find duplicate rows based on value of one or more columns in MySQL.

What does a MySQL database contain?

2021-09-02T05:54:00.001-07:00

A MySQL database contains one or more tables, each of which contains records or rows. Within these rows are various columns or fields that contain the data itself.

What does SQL in MySQL stand for?

2021-09-02T05:52:00.009-07:00

The SQL in MySQL stands for Structured Query Language. This language is also used in other databases such as Oracle and Microsoft SQL Server. One can use commands such as the following to send requests from a database:

SELECT title FROM publications WHERE author = ' J. K. Rowling’;

Note that SQL is not case sensitive. However, it is a good practice to write

the SQL keywords in CAPS and other names and variables in a small case.

What do you mean by ‘databases’?

2021-09-02T05:51:00.002-07:00

A database is a structured collection of data stored in a computer system and organized in a way to be quickly searched. With databases, information can be rapidly retrieved.

What are some of the advantages of using MySQL?

2021-09-02T05:50:00.001-07:00

Flexibility: MySQL runs on all operating systems
Power: MySQL focuses on performance
Enterprise-Level SQL Features: MySQL had for some time been lacking in advanced features such as subqueries, views, and stored procedures.
Full-Text Indexing and Searching
Query Caching: This helps enhance the speed of MySQL greatly
Replication: One MySQL server can be duplicated on another, providing numerous advantages
Configuration and Security

What is MySQL?

2021-08-31T08:36:00.002-07:00

MySQL is a database management system for web servers. It can grow with the website as it is highly scalable. Most of the websites today are powered by MySQL.

What is Member Variable and Member function?

2021-08-31T08:24:00.001-07:00

Member Variable − These are the variables defined inside a class. This data will be invisible to the outside of the class and can be accessed via member functions. These variables are called attribute of the object once an object is created.

Member function − These are the function defined inside a class and are used to access object data.

SQL Info

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Difference between Abstract class and Interfaces.

Interface:

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Introduction

Interface Class

Abstract Class

Conclusion

What is a foreign key? Write a query to implement the same in MySQL.

Write a query for a column addition in MySQL

What are the tables in MySQL? Explain the types.

How To Delete Duplicate Rows in MySQL

Prepare sample data

A) Delete duplicate rows using DELETE JOIN statement

B) Delete duplicate rows using an intermediate table

C) Delete duplicate rows using the ROW_NUMBER() function

How To Find Duplicate Values in MySQL

Setting up a sample table

Find duplicate values in one column

Find duplicate values in multiple columns

What does a MySQL database contain?

What does SQL in MySQL stand for?

What do you mean by ‘databases’?

What are some of the advantages of using MySQL?

What is MySQL?

What is Member Variable and Member function?

A) Delete duplicate rows using `DELETE JOIN` statement

C) Delete duplicate rows using the `ROW_NUMBER()` function