SQL Case Expression Syntax?

What is the complete and correct syntax for the SQL Case expression?

Answer

The complete syntax depends on the database engine you’re working with:

For SQL Server:

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

or:

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

expressions, etc:

case-expression    - something that produces a value
when-expression-x  - something that is compared against the case-expression
value-1            - the result of the CASE statement if:
                         the when-expression == case-expression
                      OR the boolean-when-expression == TRUE
boolean-when-exp.. - something that produces a TRUE/FALSE answer

Link: CASE (Transact-SQL)

Also note that the ordering of the WHEN statements is important. You can easily write multiple WHEN clauses that overlap, and the first one that matches is used.

Note: If no ELSE clause is specified, and no matching WHEN-condition is found, the value of the CASE expression will be NULL.