= (Equals) (Transact-SQL)

Άρθρο
11/19/2024

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW) SQL analytics endpoint in Microsoft Fabric Warehouse in Microsoft Fabric SQL database in Microsoft Fabric

Compares the equality of two expressions (a comparison operator) in SQL Server.

Transact-SQL syntax conventions

Syntax

expression = expression

Arguments

expression
Is any valid expression. If the expressions are not of the same data type, the data type for one expression must be implicitly convertible to the data type of the other. The conversion is based on the rules of data type precedence.

Result Types

Boolean

Remarks

When you compare using a NULL expression, the result depends on the ANSI_NULLS setting:

If ANSI_NULLS is set to ON, the result of any comparison with NULL is UNKNOWN, following the ANSI convention that NULL is an unknown value and cannot be compared with any other value, including other NULLs.
If ANSI_NULLS is set to OFF, the result of comparing NULL to NULL is TRUE, and the result of comparing NULL to any other value is FALSE.

For more information, see SET ANSI_NULLS (Transact-SQL).

A boolean expression resulting in UNKNOWN behaves similarly to FALSE in most, but not all cases. See NULL and UNKNOWN (Transact-SQL) and NOT (Transact-SQL) for more information.

Examples

A. Using = in a simple query

The following example uses the Equals operator to return all rows in the HumanResources.Department table in which the value in the GroupName column is equal to the word 'Manufacturing'.

-- Uses AdventureWorks  
  
SELECT DepartmentID, Name  
FROM HumanResources.Department  
WHERE GroupName = 'Manufacturing';

Here's the result set.

  
DepartmentID Name  
------------ --------------------------------------------------  
7            Production  
8            Production Control  
  
(2 row(s) affected)

B. Comparing NULL and non-NULL values

The following example uses the Equals (=) and Not Equal To (<>) comparison operators to make comparisons with NULL and nonnull values in a table. The example also shows that IS NULL is not affected by the SET ANSI_NULLS setting.

-- Create table t1 and insert 3 rows.  
CREATE TABLE dbo.t1 (a INT NULL);  
INSERT INTO dbo.t1 VALUES (NULL),(0),(1);  
GO  
  
-- Print message and perform SELECT statements.  
PRINT 'Testing default setting';  
DECLARE @varname int;   
SET @varname = NULL;  
  
SELECT a  
FROM t1   
WHERE a = @varname;  
  
SELECT a   
FROM t1   
WHERE a <> @varname;  
  
SELECT a   
FROM t1   
WHERE a IS NULL;  
GO  
  
-- SET ANSI_NULLS to ON and test.  
PRINT 'Testing ANSI_NULLS ON';  
SET ANSI_NULLS ON;  
GO  
DECLARE @varname int;  
SET @varname = NULL  
  
SELECT a   
FROM t1   
WHERE a = @varname;  
  
SELECT a   
FROM t1   
WHERE a <> @varname;  
  
SELECT a   
FROM t1   
WHERE a IS NULL;  
GO  
  
-- SET ANSI_NULLS to OFF and test.  
PRINT 'Testing SET ANSI_NULLS OFF';  
SET ANSI_NULLS OFF;  
GO  
DECLARE @varname int;  
SET @varname = NULL;  
SELECT a   
FROM t1   
WHERE a = @varname;  
  
SELECT a   
FROM t1   
WHERE a <> @varname;  
  
SELECT a   
FROM t1   
WHERE a IS NULL;  
GO  
  
-- Drop table t1.  
DROP TABLE dbo.t1;

Here's the result set.

Testing default setting  
a  
-----------  
NULL  
  
(1 row(s) affected)  
  
a  
-----------  
0  
1  
  
(2 row(s) affected)  
  
a  
-----------  
NULL  
  
(1 row(s) affected)  
  
Testing ANSI_NULLS ON  
a  
-----------  
  
(0 row(s) affected)  
  
a  
-----------  
  
(0 row(s) affected)  
  
a  
-----------  
NULL  
  
(1 row(s) affected)  
  
Testing SET ANSI_NULLS OFF  
a  
-----------  
NULL  
  
(1 row(s) affected)  
  
a  
-----------  
0  
1  
  
(2 row(s) affected)  
  
a  
-----------  
NULL  
  
(1 row(s) affected)

Κοινή χρήση μέσω