Joins

Equi Join: An equi-join links two tables on common values in a column in table 1 with a column in table 2. The restriction conforms to the following syntax:
Table1.column_a = Table2.column_a
In a normalized database the columns used in an equi-join are usually the primary key from one table and the foreign key in the other

Theta Join: A theta join is a join that links tables based on a relationship other than equality between two columns. A theta join could use any operator other than the “equal” operator.

Outer Join: An outer join is a join that links two tables, one of which has rows that do not match those in the common column of the other table. You define an outer join by specifying which table is the outer table in the original equi-join. The outer table contains the column for which you want to return all values, even if they are unmatched.

Shortcut join:A shortcut join is a join that provides an alternative path between two tables.
Shortcut joins improve the performance of a query by not taking into account intermediate tables, and so shortening a normally longer join path.

Self restricting joins: A self restricting join is not really a join at all, but a self restriction on a single table. You can use a self restricting join to restrict the results returned by a table values using a fixed value.