tag:blogger.com,1999:blog-3083863313051828992024-09-27T21:35:20.296-07:00Sandeep Pandithttp://www.blogger.com/profile/15814853000578906100noreply@blogger.comBlogger1125tag:blogger.com,1999:blog-308386331305182899.post-64102517003651355002010-01-08T03:48:00.000-08:002010-01-10T20:21:19.283-08:00Relationships between two entities may be classified as being either "identifying" or "non-identifying". Identifying relationships exist when the primary key of the parent entity is included in the primary key of the child entity. On the other hand, a non-identifying relationship exists when the primary key of the parent entity is included in the child entity but not as part of the child entity's primary key. In addition, non-identifying relationships may be further classified as being either "mandatory" or "non-mandatory". A mandatory non-identifying relationship exists when the value in the child table cannot be null. On the other hand, a non-mandatory non-identifying relationship exists when the value in the child table can be null.<br />Here's a simple example of an identifying relationship:<br />Parent<br />------<br />ID (PK)<br />Name<br /><br />Child<br />-----<br />ID (PK)<br />ParentID (PK, FK to Parent.ID) -- notice PK<br />Name<br />Here's a corresponding non-identifying relationship:<br />Parent<br />------<br />ID (PK)<br />Name<br /><br />Child<br />-----<br />ID (PK)<br />ParentID (FK to Parent.ID) -- notice no PK<br />Name<br /><br /><br />Existence Dependency (Strong vs. Weak Entity). ?<br /><br />•The MOVIE and MOVIE_COPY example also is a case of existence dependency. <br />•A Strong Entity is one that exists on its own, independent of other entities. <br />•A Weak Entity is one whose existence depends on another entity. This means an occurrence of one entity cannot exist unless there is an occurrence of a related entity. <br />•This modeling situation usually occurs for binary 1:N relationships where the cardinality is mandatory-one for the weak entity. <br />•The Strong Entity is usually referred to as the Owner or Parent entity, while the Weak Entity is referred to as the Dependent or Child entity. <br />•Weak entities often do not have a natural identifier (candidate key). <br />•The primary key of the owner entity is used as part of the primary key of the dependent child entity. <br />•Note the primary key of the MOVIE_COPY entity is a composite key consisting of the attributes MovieNumber and CopyNumber. <br />•This situation is also called an identifying relationship. Benefits include: <br />o1. Data integrity of the existence dependent entity are enforced. <br />o2. Ease of access for related dependent entities via part of the composite key.Sandeep Pandithttp://www.blogger.com/profile/15814853000578906100noreply@blogger.com3