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