Null (SQL) - Outer Joins

Outer Joins

SQL outer joins, including left outer joins, right outer joins, and full outer joins, automatically produce Nulls as placeholders for missing values in related tables. For left outer joins, for instance, Nulls are produced in place of rows missing from the table appearing on the right-hand side of the LEFT OUTER JOIN operator. The following simple example uses two tables to demonstrate Null placeholder production in a left outer join.

The first table (Employee) contains employee ID numbers and names, while the second table (PhoneNumber) contains related employee ID numbers and phone numbers, as shown below.

Employee ID LastName FirstName 1 Johnson Joe 2 Lewis Larry 3 Thompson Thomas 4 Patterson Patricia

PhoneNumber ID Number 1 555-2323 3 555-9876

The following sample SQL query performs a left outer join on these two tables.

SELECT e.ID, e.LastName, e.FirstName, pn.NUMBER FROM Employee e LEFT OUTER JOIN PhoneNumber pn ON e.ID = pn.ID;

The result set generated by this query demonstrates how SQL uses Null as a placeholder for values missing from the right-hand (PhoneNumber) table, as shown below.

Query result

ID	LastName	FirstName	Number
1	Johnson	Joe	555-2323
2	Lewis	Larry	NULL
3	Thompson	Thomas	555-9876
4	Patterson	Patricia	NULL