Question

我需要从一个表（tbl_b）获取用户结果，并获取另一个表（tbl_a）的ID计数。

tbl_b:

ID       NAME       LAST_NAME         UNTIL_DATE
================================================
1        Sharon     Alexander         2016-04-01
2        David      Lynn              2014-04-01
3        Jason      Rominsky          2015-04-01


tbl_a:

ID      
==
1
2
3
4
5
6

（显然是错误的）查询：

SELECT tbl_b.ID, tbl_b.NAME, tbl_b.LAST_NAME, COUNT(tbl_a.ID) as TOTAL
FROM tbl_a, tbl_b
WHERE tbl_b.UNTIL_DATE>=NOW()

我需要从tbl_b中选择一个人员列表以及来自tbl_a的总登录数（每个登录名都有一个ID）。如果tbl_b为空或UNTIL_DATE小于NOW()，则TOTAL的结果将等于0，这是错误的。（结果必须是正数）

Answer 1

使用相关的子查询来计算：

GROUP BY

或者使用SELECT tbl_b.ID, tbl_b.NAME, tbl_b.LAST_NAME, COUNT(tbl_a.ID) as TOTAL FROM tbl_b LEFT JOIN tbl_a ON tbl_a.id = tbl_b.id WHERE tbl_b.UNTIL_DATE >= NOW() GROUP BY tbl_b.ID, tbl_b.NAME, tbl_b.LAST_NAME执行左外连接：

dictionary1 = {1: '5', 2: '8', 3: '4'} 
dictionary2 = {1: '3', 2: '5', 3: '7'}
dictionary3 = {1: '1', 2: '2', 3: '3'}

Answer 2

试试这个，我使用MSSQL

SELECT tbl_b.ID, tbl_b.NAME, tbl_b.LAST_NAME, ISNULL(Logins.TOTAL_LOGINS, 0) as TOTAL
FROM tbl_b
LEFT JOIN
(SELECT ID, COUNT(*)  as TOTAL_LOGINS FROM tbl_a GROUP BY ID) as Logins
ON tbl_b.ID = Logins.ID WHERE tbl_b.UNTIL_DATE >= NOW ()

需要将另一个表中的COUNT添加到select中

2 个答案: