Why do we use unique key

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Stud ID can have a unique constraint where entries in Stud ID column can be unique because each student of a university must have a unique ID number. In case, if student is changing the university, in that case, he or she would not have any stud ID. The entry may have a null value as only one null is allowed in the unique key constraint. Skip to content. The purpose of the primary key is to enforce entity integrity on the other hand the purpose of unique key is to enforce unique data.

In primary key, default Index is clustered whereas in unique key, default index is not-clustered Primary key does not allow null columns whereas unique allows null columns.

In the primary key, duplicate keys are not allowed while in a unique key, if one or more key parts are null, then duplicate keys are allowed. Report a Bug. Previous Prev. Next Continue. Home Testing Expand child menu Expand. In my book, every table should have a primary key.

It provides the main way primary to identify each row. This becomes a great way to ensure data validation. In our example, we used the GovernmentNumber, but many others occur. In fact, you often see a pattern with the primary key is a synthetic value such as an identity value sequence , therefore, an naturally occurring unique values, such as Account Numbers become candidates for unique keys. Kris Wenzel has been working with databases over the past 30 years as a developer, analyst, and DBA.

Kris has written hundreds of blog articles and many online courses. He loves helping others learn SQL. Nothing is worse than, being excited to learn a new tool but not knowing where to start, wasting time learning the wrong features, and being overwhelmed. November 1, Good Question! Share 0.

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