What is an inner JOIN, and an outer JOIN ?

Inner Join: combine information from two or more tables by comparing all values that meet the search criteria in the designated column or columns of one table with the entire clause in corresponding columns of the other table or tables. This kind of join which involve a match in both columns are called inner joins.

Outer join: is one in which you want both matching and non-matching rows to be returned. DB2 has no specific operator for outer joins; it can be simulated by combining a join and a correlated sub query with a UNION.