Question

我有一个关于如何在SQL中使用IF语句的问题，所以我有3个表

用户
mechanics
交流

每个表中的字段如下

 user        mechanics      exchanges
------      ----------      ---------
name         name           id_user
id_user      id_mecha       id_mecha 
                            message

我想使用类似以下条件的条件，如果他们user，我将选择mechanic或message及其对应id的名称}匹配exchanges为其中每一个user或mechanic）

SELECT CASE  
         WHEN mechanics.id_mecha = exchanges.id_mecha  
            THEN  mechanics.name, exchanges.message
         WHEN users.id_user = exchanges.id_user 
            THEN users.name, exchanges.message
FROM users 
JOIN mechanics 
JOIN exchanges

Answer 1

双JOIN将在机制和用户之间产生完整的交叉产品。这可能不是你想要的。您应该对每个表使用单个连接，然后将它们与UNION结合使用。

SELECT m.name, e.message
FROM mechanics AS m
JOIN exchanges AS e ON m.id_mecha = e.id_mecha

UNION

SELECT u.name, e.message
FROM users AS u
JOIN exchanges AS e ON m.id_user = e.id_user

Answer 2

如果我理解正确，您需要END SELECT ON JOIN，SELECT CASE WHEN mechanics.id_mecha = exchanges.id_mecha THEN mechanics.name WHEN users.id_user = exchanges.id_user THEN users.name END, exchanges.message FROM users u JOIN mechanics m ON users.id_user = exchanges.id_user JOIN exchanges e ON mechanics.id_mecha = exchanges.id_mecha需要{{1}}，如下所示

{{1}}

Answer 3

实际上你需要一个带有JOIN语句的case（假设一个特定的消息只能用于一个用户或一个特定的机制 - 而不是两个同时在一行中）：< / p>

SELECT case 
       when e.id_user is null then m.name
       when e.id_mecha is null then u.name
       else null end,
       e.message
from exchanges e join users u on e.id_user=u.id_user
     join mechanics m on e.id_mecha=m.id_mecha

在SQL中使用IF / CASE调节

3 个答案: