Check out example codes for "psql join". It will help you in understanding the concepts better.

Code Example 1

SELECT *
    FROM weather INNER JOIN cities ON (weather.city = cities.name);

Code Example 2

-- This is in postgreSQL

-- We have tableA and tableB, x is primary key in tableA and also
-- a foreign key in tableB as y
SELECT * FROM tableA JOIN tableB ON tableA.x = tableB.y;

-- This will select all the columns from both tables that share a key

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