Understanding SQL Relationships and How to Define Them in SQL Server

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Defining relationships in SQL Server is essential for maintaining data integrity, particularly through referential integrity. While relationships can be visually established in a database management tool, they can also be asserted using SQL statements, specifically through the use of foreign keys. This ensures that a primary key in one table corresponds to a foreign key in another, preventing orphaned records and maintaining consistent data.The purpose of defining these relationships goes beyond just facilitating JOIN operations in queries. They enforce rules that prevent actions like deleting a record that is still referenced in another table, which could lead to data inconsistencies and application errors. For example, if a task is deleted without a defined relationship, it could result in issues if related details still exist in another table. Thus, establishing relationships is crucial for robust database design and operational reliability.
FrankJ777
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I'm trying to learn SQL, specifically for SQL server. One question I have is about defining relationships. I know how to build a relationship graphically (showing that the primary key from one table is the foreign key of another), but I'm not sure how to assert the relationship with an equivalent SQL statement. Is this possible?

Also, what is the purpose of defining a relationship? It seems that for any query I could accomplish the same thing using a JOIN operation.

Any attempt to enlighten me would be most appreciated.
Thanks
 
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FrankJ777 said:
I'm trying to learn SQL, specifically for SQL server. One question I have is about defining relationships. I know how to build a relationship graphically (showing that the primary key from one table is the foreign key of another), but I'm not sure how to assert the relationship with an equivalent SQL statement. Is this possible?

This should help:
http://msdn.microsoft.com/en-us/library/aa933118(SQL.80).aspx

Also, what is the purpose of defining a relationship? It seems that for any query I could accomplish the same thing using a JOIN operation.

To enforce http://en.wikipedia.org/wiki/Referential_integrity" .
 
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Couldn't understand the first question.. I don't know too much of sql (just enough to get done simple tasks)

FrankJ777 said:
Also, what is the purpose of defining a relationship? It seems that for any query I could accomplish the same thing using a JOIN operation.
From experience at work:
Say, I have two tables
Tasks
TaskDetails (contains taskId and more details about task)

If they have no relationship:
One of the records in task get accidentally deleted, SQL wouldn't complain .. and in future either this would cause some trouble or my program that uses this database would crash :(

With relationship:
If I attempt to delete task I would get an error that it cannot be deleted because it is referenced in TaskDetails.
 
Thanks for the succinct answers. I've been thumbing through several books tryong to find those answers!
 
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