一个关于having max的问题
sql1:
SELECT a.* FROM td_send a,
(SELECT ID FROM td_send GROUP BY ID HAVING MAX(STEPNOW)=2) b
WHERE a.ID=b.id;

sql2:
SELECT a.* FROM td_send a where STEPNOW=2;

以上两个SQL有什么不同吗？
[解决办法]
第一个是以ID为筛选列，第二个是以STEPNOW为筛选列，能一样么

WITH t AS 
(select 1 ID ,0 step from dual UNION 
select 1 ID ,1 step from dual UNION 
select 1 ID ,2 step from dual UNION 
select 2 ID ,0 step from dual UNION
select 2 ID ,1 step from dual  )
/*SELECT ID FROM t GROUP BY ID HAVING MAX(STEP)=2*/
/*SELECT a.* FROM t a where step=2;*/
SELECT a.* FROM t a,
(SELECT ID FROM t GROUP BY ID HAVING MAX(STEP)=2) b 
WHERE a.ID=b.id;

[解决办法]

引用:

sql1:
SELECT a.* FROM td_send a,
(SELECT ID FROM td_send GROUP BY ID HAVING MAX(STEPNOW)=2) b
WHERE a.ID=b.id;

sql2:
SELECT a.* FROM td_send a where STEPNOW=2;

以上两个SQL有什么不同吗？

一样与否关键在于ID是不是唯一的（或主键）
如果ID是主键，那么结果完全一致
如果ID不是主键，结果会有不同

前一个sql的意思是：查询出同一ID中相应的STEPNOW最大值为2的所有记录
后一个sql的意思是：查出STEPNOW=2的所有记录