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CASE Expression

Posted By: Sarita Patel     Category: SQL Server     Views: 11430

This article explains about CASE expression in sql server with examples.

CASE expression enables many forms of conditional processing to be placed into a SQL statement. By using CASE, more logic can be placed into SQL statements instead of being expressed in a host language or 4GL program.

CASE is a deterministic i.e They return same value every time they are called with a specific set of values.

CASE expression has 2 formates as listed below.
  • Simple CASE function compares  an expression to a set of simple expressions.
  • Searched CASE function evaluates a set of boolean expression .  


Syntax of Simple CASE Expression :

CASE input_expression 
    WHEN when_expression THEN result_expression 
     [ ...n ] 
     [ 
        ELSE else_result_expression 
     ] 
END

input_expression is any valid sql server expression evaluated when using the simple .

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

n is a placeholder indicating that multiple WHEN when_expression THEN result_expression clauses, or multiple WHEN Boolean_expression THEN result_expression clauses can be used.

THEN result_expression is a expression returned when input_expression equals when_expression evaluates to TRUE, or Boolean_expression evaluates to TRUE. result expression is any valid SQL Server expression.

ELSE else_result_expression is a 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 SQL Server expression. The data types of else_result_expression and any result_expression must be the same or must be an implicit conversion.

Syntax of Searched CASE Expression :

CASE
    WHEN Boolean_expression THEN result_expression 
     [ ...n ] 
     [ 
        ELSE else_result_expression 
     ] 
END

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

n is a placeholder indicating that multiple WHEN when_expression THEN result_expression clauses, or multiple WHEN Boolean_expression THEN result_expression clauses can be used.

THEN result_expression is a expression returned when input_expression equals when_expression evaluates to TRUE, or Boolean_expression evaluates to TRUE. result expression is any valid SQL Server expression.

ELSE else_result_expression is a 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 SQL Server expression. The data types of else_result_expression and any result_expression must be the same or must be an implicit conversion.




Examples of CASE Expression :

Example 1 : Use of simple CASE expression function in select clause 

SELECT 
             CASE SUBSTRING('Sintax-Example',1,2)
                        WHEN 'Si' THEN 'Please correct your spelling. Don''t use si instead of sy. It is Syntax-                                                                       Example.'
                        WHEN 'Se' THEN 'Please correct your spelling. Don''t use se instead of sy. It is Syntax-                                                                     Example.'
                        ELSE 'Syntax-Example'
  END
Output
Please correct your spelling. Don't use si instead of sy. It is Syntax-Example.

Above example compares first 2 characters specified string and displays output based on comparision expression evaluates to true.  




Example 2 : Use of searched CASE expression function in select clause


SELECT 
             CASE 
                      WHEN SUBSTRING('Sintax-Example',1,2) = 'Si' THEN 'Please correct your spelling.                                                                Don''t use si instead of sy. It is Syntax-Example.'
                      WHEN SUBSTRING('Sintax-Example',1,2) = 'Se' THEN 'Please correct your spelling.                                                                Don''t use se instead of sy. It is Syntax-Example.'
                      ELSE 'Syntax-Example'
             END
Output
Please correct your spelling. Don't use si instead of sy. It is Syntax-Example.

Above example compares first 2 characters specified string and displays output based on comparision expression evaluates to true.  
  
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Sarita Patel
Sarita Patel author of CASE Expression is from United States.
 
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