Question

我有3个包含以下架构的表

Table1 - plan
id | count

Table2 - subscription
id | plan_id

Table3 - users
id | subscription_id

我必须从计划中选择count，其plan.id与subscription.plan_id匹配，users.subscription_id与subscription.id和users.id ='5'

匹配

我如何在这里使用JOIN？

我尝试使用没有JOIN，如下所示：

SELECT count 
    FROM users, subscription, plan 
    WHERE users.id = '5' and users.subscription_id = subscription.id and subscription.plan_id = plan.id;

Answer 1

尽量避免使用implicit sql连接，而是使用explicit连接（e.g. inner join,left join..）以保持可读性。

更简洁的方法如下：

SELECT
    count
FROM
    users
INNER JOIN  subscription ON users.subscription_id = subscription.id
INNER JOIN  plan ON subscription.plan_id = plan.id
WHERE   users.id = '5'

Explicit vs implicit SQL joins

Postgres SQL - 从3个表中连接数据以选择一列

1 个答案: