Set or Change the Column Collation

Artikkeli
02/28/2023

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

You can override the database collation for char, varchar, text, nchar, nvarchar, and ntext data by specifying a different collation for a specific column of a table and using one of the following:

The COLLATE clause of CREATE TABLE and ALTER TABLE, as seen in the examples below.

In-place conversion. Consider one of the existing tables defined below:

-- NVARCHAR column is encoded in UTF-16 because a supplementary character enabled collation is used
CREATE TABLE dbo.MyTable (CharCol NVARCHAR(50) COLLATE Latin1_General_100_CI_AI_SC);

-- VARCHAR column is encoded the Latin code page and therefore is not Unicode capable
CREATE TABLE dbo.MyTable (CharCol VARCHAR(50) COLLATE Latin1_General_100_CI_AI);

To convert the column in-place to use UTF-8, run an ALTER COLUMN statement that sets the required data type and a UTF-8 enabled collation:

ALTER TABLE dbo.MyTable 
ALTER COLUMN CharCol VARCHAR(50) COLLATE Latin1_General_100_CI_AI_SC_UTF8

This method is easy to implement, however it's a possibly blocking operation which may become an issue for large tables and busy applications.

Copy and replace. Consider one of the existing tables defined below:

-- NVARCHAR column is encoded in UTF-16 because a supplementary character enabled collation is used
CREATE TABLE dbo.MyTable (CharCol NVARCHAR(50) COLLATE Latin1_General_100_CI_AI_SC);
GO

-- VARCHAR column is encoded using the Latin code page and therefore is not Unicode capable
CREATE TABLE dbo.MyTable (CharCol VARCHAR(50) COLLATE Latin1_General_100_CI_AI);
GO

To convert the column to use UTF-8, copy the data to a new table where the target column is already the required data type and a UTF-8 enabled collation, and then replace the old table:

CREATE TABLE dbo.MyTableNew (CharCol VARCHAR(50) COLLATE Latin1_General_100_CI_AI_SC_UTF8);
GO
INSERT INTO dbo.MyTableNew 
SELECT * FROM dbo.MyTable;
GO
DROP TABLE dbo.MyTable;
GO
EXEC sp_rename 'dbo.MyTableNew', 'dbo.MyTable';
GO

This method is much faster than in-place conversion, but handling complex schemas with many dependencies (FKs, PKs, Triggers, DFs) and syncing the tail of the table (if the database is in use) requires more planning.

For more information, see Collation and Unicode Support.

SQL Server Management Studio. For more information, see Modify Columns (Database Engine).
Using the Column.Collation property in SQL Server Management Objects (SMO).

You cannot change the collation of a column that is currently referenced by any one of the following:

A computed column
An index
Distribution statistics, either generated automatically or by the CREATE STATISTICS statement
A CHECK constraint
A FOREIGN KEY constraint

When you work with tempdb, the COLLATE clause includes a database_default option to specify that a column in a temporary table uses the collation default of the current user database for the connection instead of the collation of tempdb.

Collations and text Columns

You can insert or update values in a text column whose collation is different from the code page of the default collation of the database. SQL Server implicitly converts the values to the collation of the column.

Collations and tempdb

The tempdb database is built every time SQL Server is started and has the same default collation as the model database. This is typically the same as the default collation of the instance. If you create a user database and specify a different default collation than model, the user database has a different default collation than tempdb. All temporary stored procedures or temporary tables are created and stored in tempdb. This means that all implicit columns in temporary tables and all coercible-default constants, variables, and parameters in temporary stored procedures have collations that are different from comparable objects created in permanent tables and stored procedures.

This could lead to problems with a mismatch in collations between user-defined databases and system database objects. For example, an instance of SQL Server uses the Latin1_General_CS_AS collation and you execute the following statements:

CREATE DATABASE TestDB COLLATE Estonian_CS_AS;  
USE TestDB;  
CREATE TABLE TestPermTab (PrimaryKey int PRIMARY KEY, Col1 nchar );

In this system, the tempdb database uses the Latin1_General_CS_AS collation with code page 1252, and TestDB and TestPermTab.Col1 use the Estonian_CS_AS collation with code page 1257. For example:

USE TestDB;  
GO  
-- Create a temporary table with the same column declarations  
-- as TestPermTab  
CREATE TABLE #TestTempTab (PrimaryKey int PRIMARY KEY, Col1 nchar );  
INSERT INTO #TestTempTab  
         SELECT * FROM TestPermTab;  
GO

With the previous example, the tempdb database uses the Latin1_General_CS_AS collation, and TestDB and TestTab.Col1 use the Estonian_CS_AS collation. For example:

SELECT * FROM TestPermTab AS a INNER JOIN #TestTempTab on a.Col1 = #TestTempTab.Col1;

Because tempdb uses the default server collation and TestPermTab.Col1 uses a different collation, SQL Server returns this error: "Cannot resolve collation conflict between 'Latin1_General_CI_AS_KS_WS' and 'Estonian_CS_AS' in equal to operation."

To prevent the error, you can use one of the following alternatives:

Specify that the temporary table column use the default collation of the user database, not tempdb. This enables the temporary table to work with similarly formatted tables in multiple databases, if that is required of your system.
```
CREATE TABLE #TestTempTab  
   (PrimaryKey int PRIMARY KEY,  
    Col1 nchar COLLATE database_default  
   );  
```

Specify the correct collation for the #TestTempTab column:

CREATE TABLE #TestTempTab  
   (PrimaryKey int PRIMARY KEY,  
    Col1 nchar COLLATE Estonian_CS_AS  
   );

Jaa

Set or Change the Column Collation

Collations and text Columns

Collations and tempdb

See Also

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