验证mysql查询是否为NULL

是否可以在mysql中查询?

如果查询返回null，则set x = false else set x = true

SET @s1 = (SELECT * FROM `wo_ticket_details_replay` 
WHERE message NOT LIKE "[ SYSTEM%" AND `sv_number`='0715201569998')
CASE WHEN @s1 = NULL THEN
  x = 'false'
ELSE
  x = 'true'
END CASE;