MySQL CASE...WHERE...THEN statements

Add ELSE NULL or something before the END of the case. See http://dev.mysql.com/doc/refman/5.0/en/case-statement.html for more info.

It isn't that the CASE must have more than one, WHEN...THEN, it's that it must handle all the data you give it.

If you removed one of the clauses, you leave a hole. e.g.

UPDATE partsList SET quantity =  
CASE
  WHEN partFK = 1 THEN 4
END
WHERE buildFK = 1;

With this update statement, if parkFK is 2, then the update fails because the CASE can't handle the input.

You can either limit your source data by adding another line to your where-clause (e.g. AND partFK in (1,2)), or you could add an ELSE to the case expression.

UPDATE partsList SET quantity =  
CASE
  WHEN partFK = 1 THEN 4
  WHEN partFK = 2 THEN 8
  ELSE 12 
END
WHERE buildFK = 1;

However, based on the SQL statement you've shown, there is probably a better way. Presumably, partFK is a foreign-key to some other table. Can you pull the value for quantity from there?