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Merge pull request #29376 from WilliamDAssafMSFT/20231220-jzabroski-patch-25
20231220 jzabroski patch 25
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‎docs/relational-databases/tables/create-unique-constraints.md‎

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description: "This article shows you how to create unique constraints using SQL Server Management Studio and Transact-SQL."
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author: WilliamDAssafMSFT
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ms.author: wiassaf
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ms.date: "09/27/2023"
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ms.date: 12/20/2023
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ms.service: sql
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ms.subservice: table-view-index
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ms.topic: conceptual
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[!INCLUDE [sqlserver2016-asdb-asdbmi](../../includes/applies-to-version/sqlserver2016-asdb-asdbmi.md)]
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You can create a unique constraint in [!INCLUDE[ssnoversion](../../includes/ssnoversion-md.md)] by using [!INCLUDE[ssManStudioFull](../../includes/ssmanstudiofull-md.md)] or [!INCLUDE[tsql](../../includes/tsql-md.md)] 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.
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You can create a unique constraint in [!INCLUDE [ssnoversion](../../includes/ssnoversion-md.md)] by using [!INCLUDE [ssManStudioFull](../../includes/ssmanstudiofull-md.md)] or [!INCLUDE [tsql](../../includes/tsql-md.md)] 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.
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> [!NOTE]
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> For information on unique constraints in Azure Synapse Analytics, see [Primary key, foreign key, and unique key in Azure Synapse Analytics](/azure/sql-data-warehouse/sql-data-warehouse-table-constraints).
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## <a name="Security"></a><a name="Permissions"></a> Permissions
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## <a id="Security"></a> <a name="Permissions"></a> Permissions
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Requires ALTER permission on the table.
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## <a name="SSMSProcedure"></a> Use SQL Server Management Studio (SSMS)
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## <a id="SSMSProcedure"></a> Use SQL Server Management Studio (SSMS)
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### To create a unique constraint using SSMS
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### <a id="to-create-a-unique-constraint-using-ssms"></a> Create a unique constraint using SSMS
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1. In **Object Explorer**, right-click the table to which you want to add a unique constraint, and select **Design**.
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2. On the **Table Designer** menu, select **Indexes/Keys**.
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1. On the **Table Designer** menu, select **Indexes/Keys**.
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3. In the **Indexes/Keys** dialog box, select **Add**.
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1. In the **Indexes/Keys** dialog box, select **Add**.
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4. In the grid under **General**, select **Type** and choose **Unique Key** from the drop-down list box to the right of the property, and then select **Close**.
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1. In the grid under **General**, select **Type** and choose **Unique Key** from the dropdown list box to the right of the property, and then select **Close**.
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5. On the **File** menu, select **Save** _table name_.
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1. On the **File** menu, select **Save _table name_**.
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## <a name="TsqlExample"></a><a name="TsqlProcedure"></a> Use Transact-SQL
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## <a id="TsqlExample"></a> <a name="TsqlProcedure"></a> Use Transact-SQL
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### To create a unique constraint using Transact-SQL
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### <a id="to-create-a-unique-constraint-using-transact-sql"></a> Create a unique constraint using Transact-SQL
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1. In **Object Explorer**, connect to an instance of [!INCLUDE[ssDE](../../includes/ssde-md.md)].
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1. In **Object Explorer**, connect to an instance of [!INCLUDE [ssDE](../../includes/ssde-md.md)].
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2. On the Standard bar, select **New Query**.
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1. On the **Standard** bar, select **New Query**.
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3. Copy and paste the following example into the query window and select **Execute**. The example creates the table `TransactionHistoryArchive4` and creates a unique constraint on the column `TransactionID`.
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1. Copy and paste the following example into the query window and select **Execute**. The example creates the table `TransactionHistoryArchive4` and creates a unique constraint on the column `TransactionID`.
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```sql
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USE AdventureWorks2022;
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GO
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```
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### To create a unique constraint on an existing table
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### <a id="to-create-a-unique-constraint-on-an-existing-table"></a> Create a unique constraint on an existing table
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1. In **Object Explorer**, connect to an instance of [!INCLUDE[ssDE](../../includes/ssde-md.md)].
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1. In **Object Explorer**, connect to an instance of [!INCLUDE [ssDE](../../includes/ssde-md.md)].
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2. On the Standard bar, select **New Query**.
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1. On the **Standard** bar, select **New Query**.
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3. Copy and paste the following example into the query window and select **Execute**. The example creates a unique constraint on the columns `PasswordHash` and `PasswordSalt` in the table `Person.Password`.
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1. Copy and paste the following example into the query window and select **Execute**. The example creates a unique constraint on the columns `PasswordHash` and `PasswordSalt` in the table `Person.Password`.
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```sql
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USE AdventureWorks2022;
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```
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### To create a unique constraint on a new table
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### <a id="to-create-a-unique-constraint-on-a-new-table"></a> Create a unique constraint on a new table
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1. In **Object Explorer**, connect to an instance of [!INCLUDE[ssDE](../../includes/ssde-md.md)].
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1. In **Object Explorer**, connect to an instance of [!INCLUDE [ssDE](../../includes/ssde-md.md)].
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2. On the Standard bar, select **New Query**.
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1. On the **Standard** bar, select **New Query**.
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3. Copy and paste the following example into the query window and select **Execute**. The example creates a table and defines a unique constraint on the column `TransactionID`.
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1. Copy and paste the following example into the query window and select **Execute**. The example creates a table and defines a unique constraint on the column `TransactionID`.
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```sql
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USE AdventureWorks2022;
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CONSTRAINT AK_TransactionID UNIQUE(TransactionID)
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);
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GO
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```
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### Create a unique constraint on a nullable column
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1. In **Object Explorer**, connect to an instance of [!INCLUDE [ssDE](../../includes/ssde-md.md)].
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1. On the **Standard** bar, select **New Query**.
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1. Copy and paste the following example into the query window and select **Execute**. The example creates a [filtered](../indexes/create-filtered-indexes.md) unique constraint using the `CREATE UNIQUE INDEX` syntax, only enforcing uniqueness on non-`NULL` values.
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```sql
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USE AdventureWorks2022;
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GO
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CREATE UNIQUE INDEX UQ_AdventureWorksDWBuildVersion
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ON dbo.AdventureWorksDWBuildVersion (DBVersion)
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WHERE (DBVersion IS NOT NULL);
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GO
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```
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## Next steps
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- [ALTER TABLE &#40;Transact-SQL&#41;](../../t-sql/statements/alter-table-transact-sql.md)
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- [CREATE TABLE &#40;Transact-SQL&#41;](../../t-sql/statements/create-table-transact-sql.md)
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- [table_constraint &#40;Transact-SQL&#41;](../../t-sql/statements/alter-table-table-constraint-transact-sql.md)
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## Related content
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- [ALTER TABLE (Transact-SQL)](../../t-sql/statements/alter-table-transact-sql.md)
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- [CREATE TABLE (Transact-SQL)](../../t-sql/statements/create-table-transact-sql.md)
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- [table_constraint (Transact-SQL)](../../t-sql/statements/alter-table-table-constraint-transact-sql.md)

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