How to Create a Unique Constraint in SQL Server 2005?

Ensuring Data Integrity with Unique Constraints in SQL Server 2005

Data integrity is paramount in database management. One crucial aspect is guaranteeing the uniqueness of specific values within a table. This is readily achieved by implementing a unique constraint. This guide explains how to create these constraints in SQL Server 2005.

Using T-SQL

The Transact-SQL (T-SQL) command provides a direct method for adding unique constraints:

<code class="language-sql">ALTER TABLE <tablename> ADD CONSTRAINT <constraintname> UNIQUE NONCLUSTERED (<columnname>)</code>

For instance:

<code class="language-sql">ALTER TABLE Employee ADD CONSTRAINT UniqueEmployeeID UNIQUE NONCLUSTERED (EmployeeID)</code>

This code adds a unique constraint named UniqueEmployeeID to the Employee table, ensuring that the EmployeeID column contains only unique values.

Graphical Approach via Database Diagram

Alternatively, you can create unique constraints using the database diagram interface:

1. Right-click the target table and select "Indexes/Keys."
2. Click "Add" to create a new index.
3. In the index properties:
   • Select the relevant column(s).
   • Set "Is Unique" to "Yes."
   • Provide a meaningful index name.

By employing either method, you effectively prevent duplicate entries in the specified column(s), maintaining data accuracy and consistency.

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