mysql避免子查询

Question

如果我有一个包含三列的表

userid,eventid,entrytime
 

和价值观一样

userid,eventid,entrytime
  1      1      NULL
  2      1    2012-01-01
  1      2    2012-01-01
  2      2    2012-01-01
  3      1      NULL
  3      2      NULL

选择入口时间始终为NULL的用户标识的最佳方法是什么

Answer 1

由于COUNT(entrytime)聚合将消除NULL值，因此您可以使用它来确定哪个userid没有entrytime的非空值，方法是将其与{{1}中的0进行比较}。clause。

HAVING

Here is a live demonstration（结果为SELECT userid, COUNT(entrytime) FROM yourtable GROUP BY userid HAVING COUNT(entrytime) = 0 = 3）

Answer 2

试试这个：

select userid from
(select userid,entrytime,count(*) from yourtable 
group by userid,entrytime)a
group by userid
having count(*)=1 and max(entrytime) is null

SQl Fiddle Demo

Answer 3

SELECT DISTINCT t1.UserId
FROM <TableName> t1
    LEFT JOIN <TableName> t2 ON t1.UserID = t2.UserID AND t2.EntryTime IS NOT NULL
WHERE t2.UserID IS NULL

Answer 4

    select userid from table_name  where entrytime is null