MySQL联接和COUNT()在多个表上 [英] MySQL join and COUNT() on multiple tables

问题描述

我正在尝试在一个查询中对多个表进行 COUNT()，但是无法正常工作.这是我到目前为止的内容:

I'm trying to COUNT() on multiple tables in one query, but I can't get it to work. Here's what I have so far:

表格:

table1
---------------------
id | name
---------------------
 1 | test
 2 | test2


table2
---------------------
id | table1_id
---------------------
 1 | 1
 2 | 1
 3 | 1


table3
---------------------
id | table2_id
---------------------
 1 | 1


table4
---------------------
id | size | table3_id
---------------------
 1 | 1024 | 1
 1 | 200  | 1

SQL:

SELECT
    table1.name,
    COUNT(table2.table1_id) AS table2_count,
    COUNT(table3.table2_id) AS table3_count,
    COUNT(table4.table3_id) AS table4_count,
    SUM(table4.size) AS table4_size
FROM
    table1
LEFT JOIN table2
    ON table1.id = table2.table1_id
LEFT JOIN table3
    ON table2.id = table3.table2_id
LEFT JOIN table4
    ON table3.id = table4.table3_id
WHERE
    table1.id = 1

我从上述查询中得到的结果:

Results I'm getting from the above query:

name | table2_count | table3_count | table4_count | table4_size
---------------------------------------------------------------
test |      4       |      2       |      2       |    1224

我应该得到的结果:

name | table2_count | table3_count | table4_count | table4_size
---------------------------------------------------------------
test |      3       |      1       |      2       |    1224

推荐答案

您将需要使用 DISTINCT ，但是您还需要计算ID，而不是外键:

You will need to use DISTINCT, but also you need to count the IDs, not the foreign keys:

SELECT
    table1.name,
    COUNT(DISTINCT table2.id) AS table2_count,
    COUNT(DISTINCT table3.id) AS table3_count,
    COUNT(DISTINCT table4.id) AS table4_count,
    SUM(table4.size) AS table4_size
FROM table1
LEFT JOIN table2 ON table1.id = table2.table1_id
LEFT JOIN table3 ON table2.id = table3.table2_id
LEFT JOIN table4 ON table3.id = table4.table3_id
WHERE table1.id = 1

这里是小提琴.

说明: DISTINCT 关键字可消除所有重复值，从而产生唯一值列表.

Explanation: The DISTINCT key word eliminates all duplicate values resulting in a list of unique values.

如果在没有 COUNT()和 SUM()的情况下运行查询，则会得到:

If you run your query without the COUNT() and SUM(), you get:

name  table1_id  table2_id  table3_id  size
test  1          1          1          1024 
test  1          1          1          200 
test  1          (null)     (null)     (null) 
test  1          (null)     (null)     (null) 

因此，如果添加 COUNT()和 SUM()，您显然会得到:

So if you add the COUNT() and SUM(), you obviously get:

name  table1_id  table2_id  table3_id  size
test  4          2          2          1224 

但是，在查询中使用 DISTINCT 并没有帮助，因为您可以清楚地看到重复的值，这将导致:

However, using DISTINCT with your query won't help because you can clearly see the duplicate values, which will result in:

name  table1_id  table2_id  table3_id  size
test  1          1          1          1224 

现在，如果在没有 COUNT()和 SUM()的情况下运行我的查询，则会得到:

Now, if you run my query without the COUNT() and SUM(), you get:

name  table1_id  table2_id  table3_id  size
test  1          1          1          1024 
test  1          1          2          200 
test  2          (null)     (null)     (null) 
test  3          (null)     (null)     (null) 

如果添加 COUNT()和 SUM()，则会得到与查询完全相同的结果:

If you add the COUNT() and SUM(), you get exactly the same results like your query:

name  table1_id  table2_id  table3_id  size
test  4          2          2          1224 

但是，由于这次您有不同的值(即，并非所有值都是1)，所以现在，如果您使用 DISTINCT 来计算唯一值，则会得到:

However, because this time you have different values (i.e. not all are 1), so now if you count the unique values using DISTINCT, you get:

name  table1_id  table2_id  table3_id  size
test  3          1          2          1224 

这篇关于MySQL联接和COUNT()在多个表上的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！