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This article mainly introduces the corresponding relationship between NULL and the only constraint UNIQUE is, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let the editor with you to understand.

The correspondence between NULL and unique constraint UNIQUE

In the database, NULL indicates that the column value is empty. The unique constraint UNIQUE states that the values of the specified column must be unique and cannot be the same between values. At this point, a question arises as to whether NULL and NULL are the same or different. Because the SQL syntax is not clearly defined, each database vendor handles it differently.

(1) PostgreSQL and Oracle believe that NULL represents no specified data, so there are many possibilities for data, so any NULL is allowed in the UNIQUE constraint column.

(2) Informix and SQL Server think that NULL means null and is the only state. So there can be only one NULL in the UNIQUE constraint column.

(3) DB2, SQLAnyWhere and InterBase believe that NULL is uncertain and cannot be compared, so NULL is not allowed in UNIQUE constraint columns.

SQLite, on the other hand, follows the policies of PostgreSQL and Oracle, allowing any NULL in the UNIQUE constraint column.

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