Comparison operators test whether two expressions are the same. Comparison operators can be used on all expressions except expressions of the text, ntext, or image data types. The following table lists the Transact\-SQL comparison operators.
| Operator | Meaning |
|---|---|
| = (Equals) | Equal to |
| > (Greater Than) | Greater than |
| < (Less Than) | Less than |
| >= (Greater Than or Equal To) | Greater than or equal to |
| <= (Less Than or Equal To) | Less than or equal to |
| <> (Not Equal To) | Not equal to |
| != (Not Equal To) | Not equal to (not ISO standard) |
| !< (Not Less Than) | Not less than (not ISO standard) |
| !> (Not Greater Than) | Not greater than (not ISO standard) |
The result of a comparison operator has the Boolean data type. This has three values: TRUE, FALSE, and UNKNOWN. Expressions that return a Boolean data type are known as Boolean expressions.
Unlike other SQL Server data types, a Boolean data type cannot be specified as the data type of a table column or variable, and cannot be returned in a result set.
When SET ANSI_NULLS is ON, an operator that has one or two NULL expressions returns UNKNOWN. When SET ANSI_NULLS is OFF, the same rules apply, except for the equals (=) and not equals (<>) operators. When SET ANSI_NULLS is OFF, these operators treat NULL as a known value, equivalent to any other NULL, and only return TRUE or FALSE (never UNKNOWN).
Expressions with Boolean data types are used in the WHERE clause to filter the rows that qualify for the search conditions and in control-of-flow language statements such as IF and WHILE, for example:
-- Uses AdventureWorks
DECLARE @MyProduct int;
SET @MyProduct = 750;
IF (@MyProduct <> 0)
SELECT ProductID, Name, ProductNumber
FROM Production.Product
WHERE ProductID = @MyProduct;