I am confused with the USING keyword which is used to join two tables in postgres. I first saw it in another SO post Compare two tables in postgres. I checked the manual for postgres 2.6. Joins Between Tables.
I can't understand how postgres identified the user_id to be joined (in the SO post).
As the PostgreSQL documentation states:
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.
So simply said, in this case both tables have the column user_id and the join is done based on them.