An RDBMS uses joins to match rows from one table with rows from another table.  For example, we can use joins to match sales with customers or books with authors.  Without joins, we might have a list of sales and customers or books and authors, but we would have no way to determine which customers bought which items or which authors wrote which books.

We can join two tables explicitly by writing a query that lists both tables in the FROM clause.  We can also join two tables by using a variety of different sub-queries.  Finally, SQL Server may introduce joins for a variety of purposes into a query plan during optimization.

These are:
 •Inner join
 •Outer join
 •Cross join
 •Cross apply
 •Semi-join
 •Anti-semi-join


Types of Joins