postgresql - 关键字USING在PostgreSQL中如何工作？

Question

我对USING关键字感到困惑，该关键字用于连接postgres中的两个表。我首先在另一个post Compare two tables in postgres中看到了它。我检查了手册中的postgres 2.6. Joins Between Tables。
我不明白postgres如何识别要加入的user_id(在SO帖子中)

Answer 1

如PostgreSQL documentation所述:

The USING clause is a shorthand that allows you to take advantage of the specific situation where both sides of the join use the same name for the joining column(s). It takes a comma-separated list of the shared column names and forms a join condition that includes an equality comparison for each one. For example, joining T1 and T2 with USING (a, b) produces the join condition ON T1.a = T2.a AND T1.b = T2.b.

简而言之，在这种情况下，两个表都具有 user_id列，并基于它们进行联接。