查詢超過3個表時，左連接性能太慢

Question

我需要查詢7個表，每個表都有district_id （區域有自己的id作為id，它是自動增量字段）。

表名是：

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

這是我用於測試的查詢語法。

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左連接，但到第四個它運行查詢但永遠不會停止。

目標 ：我嘗試解析所有區域並從塊，panchayats，居住地，村庄，分區和子分區獲取所有匹配的district_id計數數據。

期待結果 ：

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

我對SQL不太滿意，請提前感謝您的時間和幫助。

表結構：

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是主鍵，在phpMyAdmin我做district_id每個表中的索引。

Answer 1

您擁有它的查詢可能會創建一個巨大的結果集，然后對該結果集應用4個不同的不同計數。 在這種情況下，您可能最好使用子查詢。 不可否認，也不是一個很好的解決方案......

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

你試過了嗎：

GROUP BY d.id, d.district_name

在您的原始查詢？

我認為MySQL允許以你所做的方式編寫GROUP BY ，但是其他數據庫不會以這種方式編寫它。