查询某张表中时间最近的一条数据

SELECT
	  a1.id          
FROM
	a a1         
	LEFT JOIN b b1 ON a1.cid = b1.cid          
WHERE
	NOT EXISTS 
    ( 
      SELECT 1 FROM a d1 LEFT  JOIN b c1 ON d1.cid = c1.cid          
      WHERE
	  d1.pid = a1.pid          
	  AND b1.start_time < c1.start_time 
     )