2个表的联合的SQL查询

Question

我陷入了疑问。 我有两个表，并且两个表都有一个共同的列，但是两个表中的数据可能不同：

标签1

+---------+------------+
|   Sub   | student_id |
+---------+------------+
| Math    | 0033       |
| Science | 0034       |
| Geom    | 0035       |
| Math    | 0034       |
+---------+------------+

标签2

+---------+----------+
|   Sub   | class_id |
+---------+----------+
| Science | 001      |
| Geom    | 002      |
| Geom    | 001      |
| Civics  | 004      |
+---------+----------+

现在，我需要通过以下方式获得结果：

+---------+----------------+--------------+
|   Sub   | Student(count) | Class(count) |
+---------+----------------+--------------+
| Math    | 2              | 0            |
| Science | 1              | 1            |
| Geom    | 1              | 2            |
| Civics  | 0              | 1            |
+---------+----------------+--------------+

我想我必须使用union，并且可以在两个不同的查询中获得结果，但是无法显示上述最终表。

这就是我所拥有的：

select sub,count(*)as student_id from tab1
group by sub
union
select sub,count(*)as class_id from tab2
group by sub

但是我得到的是2列而不是3列。

任何帮助将不胜感激。

Answer 1

试试这个

SELECT AllSubs.Sub as Sub,
       COUNT(DISTINCT tab1.Student_Id) as "Student(Count)",
       COUNT(DISTINCT tab2.Class_Id) as "Class(Count)"
FROM
(SELECT DISTINCT Sub FROM tab1
UNION
SELECT DISTINCT Sub FROM tab2)AllSubs
LEFT JOIN tab1 ON tab1.Sub = AllSubs.Sub
LEFT JOIN tab2 ON tab2.Sub = AllSubs.Sub
GROUP BY AllSubs.Sub

sqlFiddle

该查询从tab1和tab2中选择所有不同的主题，然后与tab1和tab2左连接，然后计算不同的studdent_id并按主题对不同的class_id进行计数。

更新

SELECT AllSubs.Sub as Sub,
       COUNT(DISTINCT tab1.Student_Id) as "Student(Count)",
       COUNT(DISTINCT tab2.Class_Id) as "Class(Count)",
       COUNT(DISTINCT tab1.Student_Id) - COUNT(DISTINCT tab2.Class_Id) as difference
FROM
(SELECT DISTINCT Sub FROM tab1
UNION
SELECT DISTINCT Sub FROM tab2)AllSubs
LEFT JOIN tab1 ON tab1.Sub = AllSubs.Sub
LEFT JOIN tab2 ON tab2.Sub = AllSubs.Sub
GROUP BY AllSubs.Sub
ORDER BY difference DESC

sqlFiddle

如果只需要第一行，只需在查询末尾添加LIMIT 1 。