支不支持 if exists()？

[icemanpro]

像这样的，在 mysql 如何写？
IF EXISTS(SELECT * FROM shares WHERE file_id='1' AND user_id='4') THEN
SELECT * FROM shares WHERE file_id='1' AND user_id='4'
ELSE
SELECT * FROM shares WHERE file_id='2' AND user_id='5'
END IF

[ElmerZhang]

SELECT * FROM shares WHERE ( file_id='1' AND user_id='4' ) OR (file_id='2' AND user_id='5') ORDER BY file_id ASC LIMIT 1;