MySQL 使用 3 个表进行左连接、计数和分组的查询

Question

我有 3 个 table 试图放置如下所示的连接查询。

下面是包含 3 个部分的 table 部分。

Section
*****************************
* section_id * section_name *
*****************************
*  1         * A            *
*  2         * B            *
*  3         * C            *
*****************************

下面是section_subjecttable。第一节包含 2 个科目，第二节包含 2 个科目，第三节包含 3 个科目。

Section_Subject
***********************************
* ss_id * section_id * subject_id *
***********************************
* 1     * 1          * 8          *
* 2     * 1          * 9          *
* 3     * 2          * 6          *
* 4     * 2          * 5          *
* 5     * 3          * 2          *
* 6     * 3          * 3          *
* 7     * 3          * 4          *
***********************************

下面是section_batchtable。仅第 3 部分包含 2 个批次

Section_Batch
*********************************
* sb_id * section_id * batch_id *
*********************************
* 1     * 3          * 6        *
* 2     * 3          * 7        *
*********************************

我希望查询产生以下结果

**************************************************************
* section_id * section_name * count_subjects * count_batches *
**************************************************************
* 1          * A            * 2              * 0             *
* 2          * B            * 2              * 0             *
* 3          * C            * 3              * 2             *
**************************************************************

我知道我们可以做一些子查询来达到上面的结果。但是如何使用left join和group query得到结果呢？

Answer 1

您可以将 count 与 distinct 一起使用：

select t1.section_id
     , t1.section_name
     , count(distinct t2.subject_id) as count_subjects
     , count(distinct t3.batch_id) as count_batches
from Section t1
left join Section_Subject t2 on t1.section_id = t2.section_id
left join Section_Batch t3 on t1.section_id = t3.section_id
group by t1.section_id
       , t1.section_name

SQLFiddle

Answer 2

您需要分别执行 left join 和 group by table 以获得计数，然后在它们之间进行连接

SQL Fiddle: http://www.sqlfiddle.com/#!9/ea4ee/12

select T1.section_id,
       T1.section_name,
       T1.subjects_count,
       T2.batch_count
FROM (
select S.section_id, 
       S.section_name,
       COUNT(SS.subject_id) as subjects_count
from Section S
LEFT JOIN Section_Subject SS
on S.section_id = SS.section_id
group by S.section_id, S.section_name )T1
LEFT JOIN (
select S.section_id, 
       S.section_name,
       COUNT(SB.batch_id ) as batch_count
from Section S
LEFT JOIN Section_Batch SB
on S.section_id = SB.section_id
group by S.section_id, S.section_name
  ) T2
 on T1.section_id = T2.section_id

Answer 3

我相信使用 count(distinct) 可以满足您的需求。您必须使用 distinct，因为连接具有乘数效应，其中一个部分有多个主题和多个批次。

select
    s.section_id,
    min(t1.section_name) as section_name,
    count(distinct ss.subject_id) as subject_count,
    count(distinct sb.batch_id) as batch_count,
from
    Section as s
    left join Section_Subject as ss on ss.section_id = s.section_id
    left join Section_Batch as sb on sb.section_id = s.section_id
group by
    s.section_id

顺便说一句，我认为左连接可能是内部连接。

MySQL 使用 3 个表进行左连接、计数和分组的查询

MySQL query with left join, count and group with 3 tables

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