SQL 服务器：交叉应用 - 为 NULL 结果记录 0

Question

T1 是 table 公司及其（多个用户），T2 是 table 注册用户。我计算了，对于 T1 中的每个公司，他们有多少用户在 T2 中，但需要 c3 出现在结果中 table with #regUser == 0:

T1:

company         user
c1               u1
c1               u2
c2               u2
c2               u3
c3               u4
c3               u1

T2:

user
u2
u3

因此结果 table 应该如下所示：

company         #regUser
c1                 1
c2                 2
c3                 0

使用以下代码，我只获得非空公司的结果：

select t1s.company, count(1)
from (select * from t1) t1s
  cross apply (select *
               from t2 t2s
               where t2s.reguser = t1s.[user]) t12s
group by t1s.company

谢谢

Answer 1

只需使用左连接

   select t1.company,count(t2.user)
     from t1 left join t2 on t1.user=t2.user
group by t1.company

根据您的要求不需要子查询

但是如果你想使用 apply 那么你需要像下面的查询

select t1s.company, count(t12s.Users)
from (select * from t1) t1s
  outer apply (select Users
               from t2 t2s
               where t2s.Users = t1s.[Users]) t12s
group by t1s.company

产出

company     #regUser
c1          1
c2          2
c3          0

demolink

Answer 2

您可以使用 LEFT JOIN 获取左侧 table 的所有信息以及右侧 table 的匹配信息。通过使用 GROUP BY，您可以按公司对行进行分组，并获得每个公司的 COUNT 注册用户：

SELECT t1.company, COUNT(t2.[user]) AS regUser 
FROM t1 LEFT JOIN t2 ON t1.[user] = t2.[user]
GROUP BY t1.company
ORDER BY t1.company ASC

你也可以使用CROSS APPLY来解决这个问题：

SELECT t1.company, SUM(CASE WHEN t1.[user] = t2.[user] THEN 1 ELSE 0 END) AS regUser 
FROM t1 CROSS APPLY t2
GROUP BY t1.company
ORDER BY t1.company ASC

demo on dbfiddle.uk

Answer 3

你只需要一个左连接:

select company,count(t2.[user])
from t1 left outer join t2 on t1.[user]=t2.[user]
group by company

问题的查询过于复杂。例如，from (select * from t1) t1s 等同于 from t1 as t1s。

Answer 4

只是 LEFT JOIN 和 SUM()

CREATE TABLE T1(
  Company VARCHAR(20),
  Users VARCHAR(20)
);

CREATE TABLE T2(
  Users VARCHAR(20)
);

INSERT INTO T1 VALUES
('c1', 'u1'),
('c1', 'u2'),
('c2', 'u2'),
('c2', 'u3'),
('c3', 'u4'),
('c3', 'u1');

INSERT INTO T2 VALUES
('u2'),
('u3');

SELECT T1.Company,
       SUM(CASE WHEN T2.Users IS NULL THEN 0 ELSE 1 END) Cnt
FROM T1 LEFT JOIN T2
ON T1.Users = T2.Users
GROUP BY T1.Company;

Returns:

+---------+-----+
| Company | Cnt |
+---------+-----+
| c1      |   1 |
| c2      |   2 |
| c3      |   0 |
+---------+-----+

Live Demo

SQL 服务器：交叉应用 - 为 NULL 结果记录 0

SQL Server: Cross-apply - record 0 for NULL results

sql-server

cross-apply