左連接不返回左表中的所有記錄

Question

我想通過對兩個表進行join來獲得計數，我正在做group by和left join 。

但如果該記錄與另一個表不匹配，我不會得到所有行。

在給定的查詢中：

create table UserTable (
  Id integer not null,
  Name varchar(12) not null
);

insert into UserTable  values (1,  'A B');
insert into UserTable  values (2,  'A C');
insert into UserTable  values (3,  'A C A C');
insert into UserTable  values (4,  'A C C');
insert into UserTable  values (5,  'A C B');
insert into UserTable  values (6,  'A C C');
insert into UserTable  values (7,  'A C D');
insert into UserTable  values (8,  'A C E');
insert into UserTable  values (9,  'A C F');


create table LogTable (
  LogId integer not null,
  Username varchar(12) not null,
  Event varchar(12) not null
);

insert into LogTable  values (1, 'A C A C', 'Read');
insert into LogTable  values (2, 'A C F', 'Write');
insert into LogTable  values (3, 'A C F', 'Read');
insert into LogTable  values (4, 'A C C', 'Update');
insert into LogTable  values (5,'A C C', 'Read');
insert into LogTable  values (6,'A C F', 'Read');
insert into LogTable  values (7,'A C F', 'Update');
insert into LogTable  values (7,'A C F', 'Write');
insert into LogTable  values (7,'A C E','Update');
insert into LogTable  values (7,'A C F', 'Delete');
insert into LogTable  values (10,'A C B', 'Delete');
insert into LogTable  values (11, 'A C F','Copy');
insert into LogTable  values (12, 'A C B','Read');
insert into LogTable  values (13, 'A C F','Update');
insert into LogTable  values (14, 'A C F','Copy');
insert into LogTable  values (15, 'A C F','Read');
insert into LogTable  values (16, 'A C F','Update');
insert into LogTable  values (17, 'A C F','Copy');
insert into LogTable  values (18, 'A C C','Read');
insert into LogTable  values (19, 'A C D','Update');

我在做：

SELECT COUNT(*) as Read,UT.Name, UT.Id 
FROM UserTable UT 
LEFT JOIN LogTable LT ON LT.Username = UT.Name 
WHERE LT.Event = 'Read'
GROUP by UT.Name, UT.Id

我想要的是從UserTable獲取所有記錄，如果LogTable中不存在的話

如果不存在， ReadCount為 0。

為什么 Left Joining 不返回此查詢中UserTable的所有記錄？

所需 Output：

Id  |   Name    |   ReadCount
-----------------------------
1   |   A B     |   0
2   |   A C     |   0
3   |   A C A C |   1
4   |   A C C   |   2
5   |   A C B   |   1
6   |   A C C   |   2
7   |   A C D   |   0
8   |   A C E   |   0
9   |   A C F   |   3

Answer 1

您需要將條件移動到ON子句。 但是，您不需要COUNT(*) ，因為您想要計算匹配項的數量。 那將是：

SELECT UT.Name, UT.Id , COUNT(LT.UserName) as num_read
FROM UserTable UT LEFT JOIN
     LogTable LT
     ON LT.Username = UT.Name AND LT.Event = 'Read'
GROUP by UT.Name, UT.Id;

也就是說，這種類型的查詢通常使用相關子查詢更快：

select ut.*,
       (select count(*)
        from logtable lt
        where lt.username = ut.name and lt.event = 'Read'
       ) as num_read
from usertable ut;

特別是，這可以利用logtable(username, event)上的索引。

Answer 2

SELECT COUNT(*) as Read,UT.Name, UT.Id 
FROM UserTable UT 
LEFT JOIN LogTable LT ON LT.Username = UT.Name 
WHERE LT.Event = 'Read'
GROUP by UT.Name, UT.Id

您使用WHERE LT.Event = 'Read'這使得它成為內部連接，這就是為什么您沒有獲得左表的所有行

SELECT COUNT(*) as Read,UT.Name, UT.Id 
FROM UserTable UT 
LEFT JOIN LogTable LT ON LT.Username = UT.Name 
 and  LT.Event = 'Read'

GROUP by UT.Name, UT.Id

移動 on 子句中的條件