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.
Simple CASE expression: CASE input_expression WHEN when_expression THEN result_expression [ ...n ] [ ELSE else_result_expression ] END Searched CASE expression: CASE WHEN Boolean_expression THEN result_expression [ ...n ] [ ELSE else_result_expression ] END
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).
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.
Evaluates input_expression, and then 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.
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 statement evaluates its conditions sequentially and stops with the first condition whose condition is satisfied. In some situations, an expression is evaluated before a CASE statement 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 statement are evaluated first, then provided to the CASE statement. 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.
WITH Data (value) AS ( SELECT 0 UNION ALL SELECT 1 ) SELECT CASE WHEN MIN(value) <= 0 THEN 0 WHEN MAX(1/value) >= 100 THEN 1 END FROM Data ;
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.
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.
USE AdventureWorks2008R2; GO SELECT ProductNumber, Name, "Price Range" = CASE WHEN ListPrice = 0 THEN 'Mfg item - not for resale' WHEN ListPrice < 50 THEN 'Under $50' WHEN ListPrice >= 50 and ListPrice < 250 THEN 'Under $250' WHEN ListPrice >= 250 and ListPrice < 1000 THEN 'Under $1000' ELSE 'Over $1000' END FROM Production.Product ORDER BY ProductNumber ; GO
C. Using CASE to replace the IIf function that is used in Microsoft Access
CASE provides functionality that is similar to the IIf function in Microsoft Access. The following example shows a simple query that uses IIf to provide an output value for the TelephoneInstructions column in an Access table that is named db1.ContactInfo.
SELECT FirstName, LastName, TelephoneNumber, IIf(IsNull(TelephoneInstructions),"Any time", TelephoneInstructions) AS [When to Contact] FROM db1.ContactInfo;
The following example uses CASE to provide an output value for the TelephoneSpecialInstructions column in the AdventureWorks2008R2 view Person.vAdditionalContactInfo.
D. 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 EmployeeID in descending order. Employees that have the SalariedFlag set to 0 are returned in order by the EmployeeID 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.
E. 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.
USE AdventureWorks2008R2; GO UPDATE HumanResources.Employee SET VacationHours = ( CASE WHEN ((VacationHours - 10.00) < 0) THEN VacationHours + 40 ELSE (VacationHours + 20.00) END ) OUTPUT Deleted.BusinessEntityID, Deleted.VacationHours AS BeforeValue, Inserted.VacationHours AS AfterValue WHERE SalariedFlag = 0;
F. 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 AdventureWorks2008R2 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.
USE AdventureWorks2008R2; GO CREATE FUNCTION dbo.GetContactInformation(@BusinessEntityID int) RETURNS @retContactInformation TABLE ( BusinessEntityID int NOT NULL, FirstName nvarchar(50) NULL, LastName nvarchar(50) NULL, ContactType nvarchar(50) NULL, PRIMARY KEY CLUSTERED (BusinessEntityID ASC) ) AS -- Returns the first name, last name and contact type for the specified contact. BEGIN DECLARE @FirstName nvarchar(50), @LastName nvarchar(50), @ContactType nvarchar(50); -- Get common contact information SELECT @BusinessEntityID = BusinessEntityID, @FirstName = FirstName, @LastName = LastName FROM Person.Person WHERE BusinessEntityID = @BusinessEntityID; SET @ContactType = CASE -- Check for employee WHEN EXISTS(SELECT * FROM HumanResources.Employee AS e WHERE e.BusinessEntityID = @BusinessEntityID) THEN 'Employee' -- Check for vendor WHEN EXISTS(SELECT * FROM Person.BusinessEntityContact AS bec WHERE bec.BusinessEntityID = @BusinessEntityID) THEN 'Vendor' -- Check for store WHEN EXISTS(SELECT * FROM Purchasing.Vendor AS v WHERE v.BusinessEntityID = @BusinessEntityID) THEN 'Store Contact' -- Check for individual consumer WHEN EXISTS(SELECT * FROM Sales.Customer AS c WHERE c.PersonID = @BusinessEntityID) THEN 'Consumer' END; -- Return the information to the caller IF @BusinessEntityID IS NOT NULL BEGIN INSERT @retContactInformation SELECT @BusinessEntityID, @FirstName, @LastName, @ContactType; END; RETURN; END; GO SELECT BusinessEntityID, FirstName, LastName, ContactType FROM dbo.GetContactInformation(2200); GO SELECT BusinessEntityID, FirstName, LastName, ContactType FROM dbo.GetContactInformation(5);
G. 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 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.
USE AdventureWorks2008R2; GO SELECT JobTitle, MAX(ph1.Rate)AS MaximumRate FROM HumanResources.Employee AS e JOIN HumanResources.EmployeePayHistory AS ph1 ON e.BusinessEntityID = ph1.BusinessEntityID GROUP BY JobTitle HAVING (MAX(CASE WHEN Gender = 'M' THEN ph1.Rate ELSE NULL END) > 40.00 OR MAX(CASE WHEN Gender = 'F' THEN ph1.Rate ELSE NULL END) > 42.00) ORDER BY MaximumRate DESC;