mysql - 当查询超过 3 个表时，Left Join 性能太慢

Question

我有 7 个表需要查询，每个表都有 district_id(district 有它自己的 id 作为 id 并且它是自动递增字段)。

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

它在 3 Left Join 之前工作正常，但是当涉及到第四个时，它运行查询但从不停止。

目标:我尝试解析所有地区并从街区、panchayats、居住地、村庄、分区和子分区中获取所有匹配 district_id 的计数数据。

预期结果:

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

我不太擅长 SQL，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 都是主键，在 phpMyAdmin 上，我在每个表索引中都设置了 district_id。

Answer 1

您所拥有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，您最好使用子查询。不可否认，这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