如何在 MySQL 查询中有条件地连接表

Question

Hie,

I have 3 tables namely Accounts , Companies and Freelancers . The accounts table holds the email , password and account_type fields where if account_type values is an ENUM('companies','freelancers') . I want to know how I can write a query that Joins with Companies Table if account_type fields is equal to companies else should Join with Freelancers

Answer 1

If you want to get all the data to one table,

1. you can simply use a union to bind 2 results sets that you get from joining 2 tables.

    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. You can join the tables according to the ids by getting the ids to one table.

    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'

I hope this will help you to resolve your issue.

Answer 2

For example

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'