使用不同的值两次连接同一个表

Question

我正在尝试使用 ID 将“transfers.pickup_areas_group_id”和“transfers.drop_areas_group_id”的值替换为表“areas_group”中的值我正在使用这个查询：SELECT transfers.id AS transfer_id, transfers.name AS transfer_name,  transfers.pickup_areas_group_id AS transfer_pickup_areas_group_id,transfers.drop_areas_group_id AS transfer_drop_areas_group_id, transfers_pricing.vehicle_id AS vehicle_id, transfers_pricing.date_start AS date_start, transfers_pricing.date_end AS date_end, transfers_pricing.price AS price FROM transfersINNER JOIN transfers_pricing ON transfers_pricing.transfer_id = transfers.id我尝试了一个额外的 INNER JOIN 来替换第一个值“transfers.pickup_areas_group_id”，但我找不到替换第二个值“transfers.drop_areas_group_id”的方法我试过这个查询：SELECT transfers.id AS transfer_id, transfers.name AS transfer_name,  transfers.pickup_areas_group_id AS transfer_pickup_areas_group_id,areas_group.area_id AS pickup_area_ids,transfers.drop_areas_group_id AS transfer_drop_areas_group_id, transfers_pricing.vehicle_id AS vehicle_id, transfers_pricing.date_start AS date_start, transfers_pricing.date_end AS date_end, transfers_pricing.price AS price FROM transfersINNER JOIN transfers_pricing ON transfers_pricing.transfer_id = transfers.idINNER JOIN areas_group ON areas_group.id = transfers.pickup_areas_group_id谢谢，

Answer 1

基本上，您需要另一个加入areas_group；要消除对同一个表的两个引用的歧义，您需要使用表别名。

实际上，对在查询中起作用的所有表使用表别名是一个很好的做法：这使得查询更易于读写。

SELECT 

    t.id AS transfer_id, 

    t.name AS transfer_name,  

    t.pickup_areas_group_id AS transfer_pickup_areas_group_id,

    ag1.area_id AS pickup_area_ids,

    t.drop_areas_group_id AS transfer_drop_areas_group_id, 

    ag2.area_id AS drop_area_ids

    tp.vehicle_id AS vehicle_id, 

    tp.date_start AS date_start, 

    tp.date_end AS date_end, 

    tp.price AS price 

FROM transfers t

INNER JOIN transfers_pricing tp ON tp.transfer_id = t.id

INNER JOIN areas_group ag1 ON ag1.id = t.pickup_areas_group_id

INNER JOIN areas_group ag2 ON ag2.id = t.drop_areas_group_id