如何在 MySQL 查询中有条件地连接表

Question

嗨，

我有 3 张表，即Accounts 、 Companies和Freelancers 。 accounts 表包含email 、 password和account_type字段，如果account_type值为ENUM('companies','freelancers') 。 我想知道如果account_type字段等于companies ，我如何编写一个加入Companies表的查询，否则应该加入Freelancers

Answer 1

如果你想把所有数据放到一张表中，

1. 您可以简单地使用union来绑定从连接 2 个表中获得的 2 个结果集。

    SELECT * FROM users Accounts a LEFT JOIN Companies c ON c.account_id = a.id WHERE a.account_type = 'companies' UNION SELECT * FROM users Accounts a LEFT JOIN Freelancers f ON f.account_id = a.id WHERE a.account_type = 'freelancers'

2. 您可以通过将 id 获取到一张表来根据 id 加入表。

    SELECT Accounts.id, Accounts.type, Companies.account_id AS id2, Freelancers.account_id AS id3 FROM Accounts LEFT JOIN Companies ON Accounts.id = Companies.account_id AND Accounts.type = 'companies' LEFT JOIN Freelancers ON Freelancers.account_id = Accounts.id AND Accounts.type = 'freelancers'

我希望这将帮助您解决您的问题。

Answer 2

例如

SELECT Accounts.column1, 
       COALESCE(Companies.column2, Freelancers.column3), -- common column
       Companies.column4,                                -- specific column
       Freelancers.column5                               -- specific column
FROM Accounts
LEFT JOIN Companies ON Companies.account_id = Accounts.id 
                   AND Accounts.account_type = 'companies'
LEFT JOIN Freelancers ON Freelancers.account_id = Accounts.id 
                   AND Accounts.account_type = 'freelancers'