Typically, you create joins that return rows only if they satisfy join conditions; these are called inner joins, and are the default join used when querying. However, sometimes you may want to preserve all the rows in one table. To do this, you use an outer join.

A left or right outer join of two tables preserves all the rows in one table, and supplies nulls for the other table when it does not meet the join condition. A left outer join preserves every row in the left-hand table, and a right outer join preserves every row in the right-hand table. In a full outer join, all rows from both tables are preserved.

The table expressions on either side of a left or right outer join are referred to as preserved and null-supplying. In a left outer join, the left-hand table expression is preserved and the right-hand table expression is null-supplying.

For information about creating outer joins with Transact-SQL syntax, see Transact-SQL outer joins (*= or =*).

Example

The following statement includes all customers. If a particular customer has not placed an order, each column in the result that corresponds to order information contains the NULL value.

SELECT Surname, OrderDate, City
FROM Customers LEFT OUTER JOIN SalesOrders
   ON Customers.ID = SalesOrders.CustomerID
WHERE Customers.State = 'NY'
ORDER BY OrderDate;

You can interpret the outer join in this statement as follows. Note that this is a conceptual explanation, and does not represent how SQL Anywhere actually computes the result set.