SQL PRIMARY KEY Constraint The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields). Introduction to SQL Server PRIMARY KEY constraint A primary key is a column or a group of columns that uniquely identifies each row in a table. You create a primary key for a table by using the PRIMARY KEY constraint. If the primary key consists of only one column, you can define use PRIMARY KEY constraint as a column constraint.
APPLIES TO: SQL Server 2016 and later Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse
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You can define a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique clustered index, or a nonclustered index if specified as such.
Before You BeginLimitations and Restrictions
![]() SecurityPermissions
Creating a new table with a primary key requires CREATE TABLE permission in the database and ALTER permission on the schema in which the table is being created.
Creating a primary key in an existing table requires ALTER permission on the table.
Using SQL Server Management StudioTo create a primary key
Caution
If you want to redefine the primary key, any relationships to the existing primary key must be deleted before the new primary key can be created. A message will warn you that existing relationships will be automatically deleted as part of this process.
Microsoft Sql Generate Primary Key Examples
A primary key column is identified by a primary key symbol in its row selector.
If a primary key consists of more than one column, duplicate values are allowed in one column, but each combination of values from all the columns in the primary key must be unique.
If you define a compound key, the order of columns in the primary key matches the order of columns as shown in the table. However, you can change the order of columns after the primary key is created. For more information, see Modify Primary Keys.
Using Transact-SQLTo create a primary key in an existing table
The following example creates a primary key on the column
TransactionID in the AdventureWorks database.
To create a primary key in a new table
The following example creates a table and defines a primary key on the column
TransactionID in the AdventureWorks database.
![]() To create a primary key with clustered index in a new table
The following example creates a table and defines a primary key on the column
CustomerID and a clustered index on TransactionID in the AdventureWorks database.
See Also-->
Learn about table constraints in Synapse SQL pool, including primary key, foreign key, and unique key.
Table constraints
Synapse SQL pool supports these table constraints:
Microsoft Sql Generate Primary Key 2017
For syntax, check ALTER TABLE and CREATE TABLE.
FOREIGN KEY constraint is not supported in Synapse SQL pool.
Remarks
Having primary key and/or unique key allows Synapse SQL pool engine to generate an optimal execution plan for a query. All values in a primary key column or a unique constraint column should be unique.
Ms Sql Insert Primary Key
After creating a table with primary key or unique constraint in Synapse SQL pool, users need to make sure all values in those columns are unique. A violation of that may cause the query to return inaccurate result. This example shows how a query may return inaccurate result if the primary key or unique constraint column includes duplicate values.
Examples
Create a Synapse SQL pool table with a primary key:
Create a Synapse SQL pool table with a unique constraint:
Next steps
After creating the tables for your Synapse SQL pool, the next step is to load data into the table. For a loading tutorial, see Loading data to Synapse SQL pool.
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