How Does the SQL CASE Expression Work and What is its Syntax?

SQL CASE expression syntax

SQL CASE expressions, also known as "CASE WHEN statements", allow you to evaluate multiple conditions and return different values ​​based on the results. Its syntax varies depending on the database engine used.

SQL Server syntax:

• Case expression syntax:

CASE case-expression
    WHEN when-expression-1 THEN value-1
  [ WHEN when-expression-n THEN value-n ... ]
  [ ELSE else-value ]
END

• Boolean expression syntax:

CASE
    WHEN boolean-when-expression-1 THEN value-1
  [ WHEN boolean-when-expression-n THEN value-n ... ]
  [ ELSE else-value ]
END

Among them:

• case-expression: Evaluates a value or Boolean condition.
• when-expression-x: Evaluates a value or Boolean condition that is compared to the case-expression.
• value-1: Return this value if when-expression is equal to case-expression, or if boolean when-expression is TRUE.
• boolean-when-expression: evaluates to TRUE or FALSE.
• else-value: This value is returned if no match is found in the WHEN condition and no ELSE clause is specified.

Note that the order of the WHEN clauses is important and the first match is used. If no ELSE clause is provided and no matching WHEN condition is met, the result will be NULL.

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