How Does SQL CASE Expression Syntax Vary Across Database Engines?

SQL CASE expression syntax

The full syntax of a SQL CASE expression varies depending on the database engine used. The following is the syntax for SQL Server:

CASE syntax using case-expression:

<code class="language-sql">CASE case-expression
    WHEN when-expression-1 THEN value-1
  [ WHEN when-expression-n THEN value-n ... ]
  [ ELSE else-value ]
END</code>

CASE syntax using boolean-when-expression:

<code class="language-sql">CASE
    WHEN boolean-when-expression-1 THEN value-1
  [ WHEN boolean-when-expression-n THEN value-n ... ]
  [ ELSE else-value ]
END</code>

The following are the terms and concepts used in grammar:

• case-expression: An expression that produces a value.

• when-expression-x: An expression to compare with case-expression.

• boolean-when-expression: An expression that produces a TRUE/FALSE result.

• value-x: The result of a CASE statement if:

  • when-expression == case-expression
  • boolean-when-expression == TRUE

• else-value: The result of the CASE statement if no matching WHEN condition is found.

You need to pay attention to the following points:

• The order of the WHEN statements is critical because the first matching condition will be used.
• If no ELSE clause is provided and no matching WHEN conditions are met, the result will be NULL.

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