查询超过3个表时，左连接性能太慢

Question

我需要查询7个表，每个表都有district_id （区域有自己的id作为id，它是自动增量字段）。

表名是：

districts, blocks, panchayats, habitations, villages, divisions, sub_divisions

这是我用于测试的查询语法。

SELECT 
d.id, d.district_name, 
COUNT(DISTINCT b.id) AS total_block, 
COUNT(DISTINCT g.id) AS gp_total,
COUNT(DISTINCT v.id) AS vi_total,
COUNT(DISTINCT h.id) AS h_total
FROM districts d 
LEFT JOIN blocks b ON b.district_id = d.id
LEFT JOIN panchayats g ON g.district_id = d.id
LEFT JOIN villages v ON v.district_id = d.id
LEFT JOIN habitations h ON h.district_id = d.id
GROUP BY district_name

它工作正常，直到3左连接，但到第四个它运行查询但永远不会停止。

目标 ：我尝试解析所有区域并从块，panchayats，居住地，村庄，分区和子分区获取所有匹配的district_id计数数据。

期待结果 ：

districts | blocks | panchayats | habitations | villages | division | sub_divisions
-----------------------------------------------------------------------------------
ABC       | 6      | 20         | 10          | 10       | 3         | 2
-----------------------------------------------------------------------------------
DEF       | 3      | 12         | 4           | 7        | 7         | 4

我对SQL不太满意，请提前感谢您的时间和帮助。

表结构：

Districts
------------------------------------------------------------------------------
id | district_name | district_code | last_update(TIMESTAMP) | created_on(DATE)
------------------------------------------------------------------------------

Blocks 
------------------------------------------------------------------------------
id | block_name | block_code | district_id | last_update | created_on
------------------------------------------------------------------------------

Panchayats
------------------------------------------------------------------------------
id | panchayat_name | panchayat_code | district_id | block_id | last_update | created_on
------------------------------------------------------------------------------

Habitations 
------------------------------------------------------------------------------
id  |  name  |  code  |  district_id  |  block_id  |  panchayat_id | village_id
------------------------------------------------------------------------------

Villages
------------------------------------------------------------------------------
id |  village_name  |  village_code  |  district_id  |  block_id | panchayat_id 
------------------------------------------------------------------------------

Division
------------------------------------------------------------------------------
id | name | district_id .....
------------------------------------------------------------------------------

Sub Division
------------------------------------------------------------------------------
id | name | district_id
------------------------------------------------------------------------------

所有的ID是主键，在phpMyAdmin我做district_id每个表中的索引。

Answer 1

您拥有它的查询可能会创建一个巨大的结果集，然后对该结果集应用4个不同的不同计数。 在这种情况下，您可能最好使用子查询。 不可否认，也不是一个很好的解决方案......

SELECT
    d.id,
    d.district_name,
    (SELECT COUNT(b.id) FROM blocks b WHERE b.district_id = d.id) total_block,
    (SELECT COUNT(p.id) FROM panchayats p WHERE p.district_id = d.id) gp_total,
    (SELECT COUNT(v.id) FROM villages v WHERE v.district_id = d.id) vi_total,
    (SELECT COUNT(h.id) FROM habitations h WHERE h.district_id = d.id) h_total
FROM districts d

Answer 2

你试过了吗：

GROUP BY d.id, d.district_name

在您的原始查询？

我认为MySQL允许以你所做的方式编写GROUP BY ，但是其他数据库不会以这种方式编写它。