Web Reference: Nov 18, 2025 · You can create a unique constraint in SQL Server by using SQL Server Management Studio or Transact-SQL to ensure no duplicate values are entered in specific columns that don't participate in a primary key. Creating a unique constraint automatically creates a corresponding unique index. What you're looking for is indeed part of the ANSI standards SQL:92, SQL:1999 and SQL:2003, i.e., a UNIQUE constraint must disallow duplicate non-NULL values but accept multiple NULL values. In Microsoft SQL Server, however, a single NULL is allowed, but multiple NULLs are not. The UNIQUE constraint ensures that all values in a column are unique. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
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