MySql中 is Null段判断无效和IFNULL()失效的解决方案

select a,b,c from table group by a,b,c //这时候a,b,c 如果判断null不管用
select IFNULL(a,0),IFNULL(b,0),IFNULL(3,0) 
from (select a,b,c from table group by a,b,c )  //这样就好啦

SELECT
			PID,
			MAX(CASE WHEN SID = 1 THEN IFNULL(PNUM ,0) END) AS SID1,
			MAX(CASE WHEN SID = 2 THEN IFNULL(PNUM ,0) END) AS SID2,
			MAX(CASE WHEN SID = 3 THEN IFNULL(PNUM ,0) END) AS SID3
		FROM
			(
				SELECT
					PID,
					SUM(PNUM) PNUM,
					SID
				FROM
					A
				GROUP BY
					PID
			) temp
		GROUP BY
			temp.PID

SELECT
	PID,IFNULL(SID1,0) SID1,IFNULL(SID2,0) SID2,IFNULL(SID3,0) SID3
FROM
	(
		SELECT
			PID,
			MAX(CASE WHEN SID = 1 THEN PNUM END) AS SID1,
			MAX(CASE WHEN SID = 2 THEN PNUM END) AS SID2,
			MAX(CASE WHEN SID = 3 THEN PNUM END) AS SID3
		FROM
			(
				SELECT
					PID,
					SUM(PNUM) PNUM,
					SID
				FROM
					A
				GROUP BY
					PID
			) temp
		GROUP BY
			temp.PID
	) temp1

IFNULL(a,b) c

IFNULL(a,b) c

if(a ='' or a  is null,b,a) c