<database> A less commonly used variant of the inner join
relational database operation. An inner join selects rows
from two tables such that the value in one column of the
first table also appears in a certain column of the second
table. For an outer join, the result also includes all rows
from the first operand ("left outer join"), or the second
operand ("right outer join"), or both ("full outer join"). A
field in a result row will be null if the corresponding input
table did not contain a matching row.
For example, if we want to list all employees and their
employee number, but not all employees have a number, then we
could say (in SQL-92 syntax, as used by Microsoft SQL
Server):
SELECT employee.name, empnum.number
FROM employee
LEFT JOIN empnum ON employee.id = empnum.id
or, in Sybase syntax:
SELECT employee.name, empnum.number
FROM employee, empnum
WHERE employee.id *= empnum.id
The "*" on the left means "left outer join". "*=*" would be a
full outer join.
In Oracle syntax:
SELECT employee.name, empnum.number
FROM employee, empnum
WHERE employee.id = empnum.id (+)
Note that the "(+)" on the right means "left outer join".
These all mean that all rows from the left-hand "employee"
table will appear in the result, even if there is no match for
their ID in the empnum table. Where there is no empnum.id
equal to a given employee.id, a result row is output anyway
but with all result columns from the empnum table null
(empnum.number in this case).
