Выражение case в sql

Обновлено: 21.11.2024

Evaluates a list of conditions and returns one of multiple possible result expressions.

The CASE expression has two formats:

The simple CASE expression compares an expression to a set of simple expressions to determine the result.

The searched CASE expression evaluates a set of Boolean expressions to determine the result.

Both formats support an optional ELSE argument.

CASE can be used in any statement or clause that allows a valid expression. For example, you can use CASE in statements such as SELECT, UPDATE, DELETE and SET, and in clauses such as select_list, IN, WHERE, ORDER BY, and HAVING.

Syntax

To view Transact-SQL syntax for SQL Server 2014 and earlier, see Previous versions documentation.

Arguments

input_expression
Is the expression evaluated when the simple CASE format is used. input_expression is any valid expression.

WHEN when_expression
Is a simple expression to which input_expression is compared when the simple CASE format is used. when_expression is any valid expression. The data types of input_expression and each when_expression must be the same or must be an implicit conversion.

THEN result_expression
Is the expression returned when input_expression equals when_expression evaluates to TRUE, or Boolean_expression evaluates to TRUE. result expression is any valid expression.

ELSE else_result_expression
Is the expression returned if no comparison operation evaluates to TRUE. If this argument is omitted and no comparison operation evaluates to TRUE, CASE returns NULL. else_result_expression is any valid expression. The data types of else_result_expression and any result_expression must be the same or must be an implicit conversion.

WHEN Boolean_expression
Is the Boolean expression evaluated when using the searched CASE format. Boolean_expression is any valid Boolean expression.

To view Transact-SQL syntax for SQL Server 2014 and earlier, see Previous versions documentation.

Return Types

Returns the highest precedence type from the set of types in result_expressions and the optional else_result_expression. For more information, see Data Type Precedence (Transact-SQL).

Return Values

Simple CASE expression:

The simple CASE expression operates by comparing the first expression to the expression in each WHEN clause for equivalency. If these expressions are equivalent, the expression in the THEN clause will be returned.

Allows only an equality check.

In the order specified, evaluates input_expression = when_expression for each WHEN clause.

Returns the result_expression of the first input_expression = when_expression that evaluates to TRUE.

If no input_expression = when_expression evaluates to TRUE, the SQL Server Database Engine returns the else_result_expression if an ELSE clause is specified, or a NULL value if no ELSE clause is specified.

Searched CASE expression:

Evaluates, in the order specified, Boolean_expression for each WHEN clause.

Returns result_expression of the first Boolean_expression that evaluates to TRUE.

If no Boolean_expression evaluates to TRUE, the Database Engine returns the else_result_expression if an ELSE clause is specified, or a NULL value if no ELSE clause is specified.

Remarks

SQL Server allows for only 10 levels of nesting in CASE expressions.

The CASE expression cannot be used to control the flow of execution of Transact-SQL statements, statement blocks, user-defined functions, and stored procedures. For a list of control-of-flow methods, see Control-of-Flow Language (Transact-SQL).

The CASE expression evaluates its conditions sequentially and stops with the first condition whose condition is satisfied. In some situations, an expression is evaluated before a CASE expression receives the results of the expression as its input. Errors in evaluating these expressions are possible. Aggregate expressions that appear in WHEN arguments to a CASE expression are evaluated first, then provided to the CASE expression. For example, the following query produces a divide by zero error when producing the value of the MAX aggregate. This occurs prior to evaluating the CASE expression.

You should only depend on order of evaluation of the WHEN conditions for scalar expressions (including non-correlated sub-queries that return scalars), not for aggregate expressions.

Examples

A. Using a SELECT statement with a simple CASE expression

Within a SELECT statement, a simple CASE expression allows for only an equality check; no other comparisons are made. The following example uses the CASE expression to change the display of product line categories to make them more understandable.

B. Using a SELECT statement with a searched CASE expression

Within a SELECT statement, the searched CASE expression allows for values to be replaced in the result set based on comparison values. The following example displays the list price as a text comment based on the price range for a product.

C. Using CASE in an ORDER BY clause

The following examples uses the CASE expression in an ORDER BY clause to determine the sort order of the rows based on a given column value. In the first example, the value in the SalariedFlag column of the HumanResources.Employee table is evaluated. Employees that have the SalariedFlag set to 1 are returned in order by the BusinessEntityID in descending order. Employees that have the SalariedFlag set to 0 are returned in order by the BusinessEntityID in ascending order. In the second example, the result set is ordered by the column TerritoryName when the column CountryRegionName is equal to 'United States' and by CountryRegionName for all other rows.

D. Using CASE in an UPDATE statement

The following example uses the CASE expression in an UPDATE statement to determine the value that is set for the column VacationHours for employees with SalariedFlag set to 0. When subtracting 10 hours from VacationHours results in a negative value, VacationHours is increased by 40 hours; otherwise, VacationHours is increased by 20 hours. The OUTPUT clause is used to display the before and after vacation values.

E. Using CASE in a SET statement

The following example uses the CASE expression in a SET statement in the table-valued function dbo.GetContactInfo . In the AdventureWorks2012 database, all data related to people is stored in the Person.Person table. For example, the person may be an employee, vendor representative, or a customer. The function returns the first and last name of a given BusinessEntityID and the contact type for that person.The CASE expression in the SET statement determines the value to display for the column ContactType based on the existence of the BusinessEntityID column in the Employee , Vendor , or Customer tables.

F. Using CASE in a HAVING clause

The following example uses the CASE expression in a HAVING clause to restrict the rows returned by the SELECT statement. The statement returns the maximum hourly rate for each job title in the HumanResources.Employee table. The HAVING clause restricts the titles to those that are held by men with a maximum pay rate greater than 40 dollars or women with a maximum pay rate greater than 42 dollars.

Examples: Azure Synapse Analytics and Analytics Platform System (PDW)

G. Using a SELECT statement with a CASE expression

Within a SELECT statement, the CASE expression allows for values to be replaced in the result set based on comparison values. The following example uses the CASE expression to change the display of product line categories to make them more understandable. When a value does not exist, the text "Not for sale' is displayed.

H. Using CASE in an UPDATE statement

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