汇总 MySQL 中两个表之间的列

Question

我有一个应用程序，学生可以从列表中选择第 1、第 2 和第 3 个选项。

表 1 是他们可以选择的列表：

id	day
1	Monday
2	Thusday
3	Wednesday

表2是他们填写的选项：

id	first_choice	second_choice	third_choice
12345	1	3	2
23456	3	2	1
34567	2	1	3
45678	1	2	3

我正在努力的是我想计算每天的选择和优先级以获得这样的列表：

id	first_choice	second_choice	third_choice
Monday	2	1	1
Thusday	1	2	1
Wednesday	1	1	2

SELECT a.day, count(b.first_choice), count(c.second_choice), count(d.third_choice) FROM table1 a LEFT JOIN table2 b ON a.id = b.first_choice LEFT JOIN table2 c ON a.id = c.second_choice LEFT JOIN table2 d ON a.id = d.third_choice GROUP BY a.day

但是，这样做我最终得到了这个

id	first_choice	second_choice	third_choice
Monday	2	2	2
Thusday	2	2	2
Wednesday	2	2	2

谁能帮我查询一下？提前致谢

Answer 1

在那些table结构中，我通常使用子查询而不是连接。


SELECT
    a.day,
    (SELECT COUNT(*) FROM Table2 WHERE first_choice = a.id) AS first_choice,
    (SELECT COUNT(*) FROM Table2 WHERE second_choice = a.id) AS second_choice,
    (SELECT COUNT(*) FROM Table2 WHERE third_choice = a.id) AS third_choice
FROM Table1 a

Answer 2

您可以使用单个 INNER JOIN 并使用条件 IF 语句计算：

SELECT  
    Table1.id, 
    Table1.day,
    COUNT(IF(Table2.first_choice=Table1.id, Table2.first_choice, NULL)) AS first_choice,
    COUNT(IF(Table2.second_choice=Table1.id, Table2.second_choice, NULL)) AS second_choice,
    COUNT(IF(Table2.third_choice=Table1.id, Table2.third_choice, NULL)) AS third_choice
FROM
    Table1
INNER JOIN 
    Table2
ON 
    Table1.id = Table2.first_choice 
OR 
    Table1.id = Table2.second_choice 
OR 
    Table1.id = Table2.third_choice 
GROUP BY 
    Table1.id,
    Table1.day

参考这个fiddle：https://dbfiddle.uk/?rdbms=mysql_5.7&fiddle=eea27fe9364c0fe4b96a846c9441f723

汇总 MySQL 中两个表之间的列

Sum up columns between two tables in MySQL

mysql

join

count