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 SQL Interview Preparation 1. What is the difference between INNER JOIN and OUTER JOIN  Assuming you're joining on columns with no duplicates, which is a very common case: An inner join of A and B gives the result of A intersect B, i.e. the inner part of a  Venn diagram  intersection. An outer join of A and B gives the results of A union B, i.e. the outer parts of a Venn diagram union. Eg. Suppose you have two tables, with a single column each, and data as follows: A B - - 1 3 2 4 3 5 4 6 Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B. Inner join An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common. select * from a INNER JOIN b on a.a = b.b; select a. * , b. * from a,b where a.a = b.b; a | b --+-- 3 | 3 4 | 4 Left outer join A left outer join will give all rows in A, plus any common rows in B. select * from ...