Set operators follow this order of precedence unless they are overridden by parentheses in the expression(s): INTERSECT is evaluated first. OUTER UNION, UNION, and EXCEPT have the same level of ...

You can also surround CASE in double quotes ("CASE") and set the PROC SQL option DQUOTE=ANSI. The keywords AS, ON, FULL, JOIN, LEFT, FROM, WHEN, WHERE, ORDER, GROUP, RIGHT, INNER, OUTER, UNION, EXCEPT ...