SQL從聯接表獲取條目的總和

Question

我在mysql中越深入，我迷失的就越多，而現在我迷失了。 所以我有一些桌子：

主表

id|message|name
-----------------
1 |test   |OP
2 |test2  |jim
3 |test3  |ted

表格1

id|likes
---------
2 | 1
3 | 0

表2

id|likes
---------
2 | 1

表3

id|likes
---------
1 | 1
2 | 1
3 | 0

我想要做的是獲取一列中每個ID的likes總數（其中一個等於1），這樣我就可以計算一條消息的總數（帶有其各自的ID）。

到目前為止，我已經成功地加入了我的表，所以最后我得到了一個likes專欄：

SELECT id,Table1.likes,Table2.likes,Table3.likes 
FROM MainTable
LEFT JOIN Table1.id ON MainTable.id = Table1.id LEFT JOIN Table2.id ON MainTable.id = Table2.id LEFT JOIN Table3.id ON MainTable.id = Table3.id

首先，有可能嗎？ 我知道我的代碼不是很好，但至少是一個開始！

謝謝！

Answer 1

我猜您正在尋找其中之一...

SELECT
    id
    ,SUM(Table1.likes)
    ,SUM(Table2.likes)
    ,SUM(Table3.likes)
FROM MainTable
LEFT JOIN Table1 ON MainTable.id = Table1.id
LEFT JOIN Table2 ON MainTable.id = Table2.id
LEFT JOIN Table3 ON MainTable.id = Table3.id
GROUP BY MainTable.id

SELECT
    id
    ,SUM(Table1.likes)+SUM(Table2.likes)+SUM(Table3.likes)
FROM MainTable
LEFT JOIN Table1 ON MainTable.id = Table1.id
LEFT JOIN Table2 ON MainTable.id = Table2.id
LEFT JOIN Table3 ON MainTable.id = Table3.id
GROUP BY MainTable.id

Answer 2

這是查詢，為MainTable中的每一行計算非零贊數：

SELECT 
    MainTable.id,
    MainTable.name,
    MainTable.message,
    COUNT(Table1.likes) + COUNT(Table2.likes) 
    + COUNT(Table3.likes) AS n_likes
FROM 
    MainTable
LEFT JOIN 
    Table1 ON MainTable.id = Table1.id
AND
    Table1.likes=1
LEFT JOIN
    Table2 ON MainTable.id = Table2.id
AND
    Table2.likes=1
LEFT JOIN
    Table3
ON
    MainTable.id = Table3.id
AND
    Table3.likes=1
GROUP BY
    MainTable.id;

小提琴

當心，您的SQL語法有錯誤：

LEFT JOIN Table1.id

聯接時，必須寫表名，不帶列：

LEFT JOIN Table1

Answer 3

您可以使用UNION ALL來解決此問題。

請參閱以下測試。

CREATE  TABLE `test`.`items` (
  `id` INT NOT NULL ,
  `message` VARCHAR(45) NULL ,
  `name` VARCHAR(45) NULL ,
  PRIMARY KEY (`id`) );

insert into items values
(1, "Message 1", "OP"),
(1, "Message 2", "jim"),
(1, "Message 3", "ted");


CREATE  TABLE `test`.`table1` (
  `id` INT NOT NULL ,
  `likes` INT(10),
  PRIMARY KEY (`id`) );

CREATE  TABLE `test`.`table2` (
  `id` INT NOT NULL ,
  `likes` INT(10),
  PRIMARY KEY (`id`) );

CREATE  TABLE `test`.`table3` (
  `id` INT NOT NULL ,
  `likes` INT(10),
  PRIMARY KEY (`id`) );

insert into table1 values 
( 2, 1),
( 3, 0);

insert into table2 values 
( 2, 1),
( 3, 0);

insert into table3 values
(1,1),
(2,1),
(3,0);


select source.id, sum(likes)
from (
select id, likes from table1 as t1
UNION ALL
select id, likes from table2 as t2
UNION ALL
select id, likes from table3 as t3
) as source group by id;

輸出量

1, 1
2, 3
3, 0