There are 5 types of joins in SQL: 1. outer join 2. inner join 3. left join 4. right join 5. cross join example: table: t_a ID : 1, 2, 3, 4, 5 table: t_b ID: 3, 4, 5, 6, 7 ----------------------------- OUTER JOIN : keep both tables' data if there is no match, fill with NULL ----------------------------- select t_a.ID as IDA, t_b.ID as IDB from t_a, t_b where t_a.ID OUTER_JOIN t_b.ID result ==> IDA IDB 1 NULL 2 NULL 3 3 4 4 5 5 NULL 6 NULL 7 ----------------------------- INNER JOIN : if there is no match, don't return it ----------------------------- select t_a.ID as IDA, t_b.ID as IDB from t_a, t_b where t_a.ID INNER_JOIN t_b.ID -- or t_a.ID = t_b.ID result ==> IDA IDB 3 3 4 4 5 5 ----------------------------- LEFT JOIN : keep left table's data if there is no match, fill with NULL ----------------------------- select t_a.ID as IDA, t_b.ID as IDB from t_a, t_b where t_a.ID LEFT_JOIN t_b.ID result ==> IDA IDB 1 NULL 2 NULL 3 3 4 4 5 5 ----------------------------- RIGHT JOIN : keep right tables' data if there is no match, fill with NULL ----------------------------- select t_a.ID as IDA, t_b.ID as IDB from t_a, t_b where t_a.ID RIGHT_JOIN t_b.ID result ==> IDA IDB 3 3 4 4 5 5 NULL 6 NULL 7 ----------------------------- CROSS JOIN : basically it is a cross product very expensive ----------------------------- select t_a.ID as IDA, t_b.ID as IDB from t_a, t_b where t_a.ID CROSS_JOIN t_b.ID result ==> IDA IDB 1 3 1 4 1 5 1 6 1 7 2 3 2 4 2 5 2 6 2 7 3 3 3 4 3 5 3 6 3 7 4 3 4 4 4 5 4 6 4 7 5 3 5 4 5 5 5 6 5 7