A TableExpression specifies a table, view, or
function in a FROM clause. It is the source from which
a SelectExpression selects
a result.
A correlation name can be applied to a table in a TableExpression so that its columns can be qualified with that name.
If you do not supply a correlation name, the table name qualifies the column
name. When you give a table a correlation name, you cannot use the table name
to qualify columns. You must use the correlation name when qualifying column
names.
No two items in the FROM clause can have the same correlation
name, and no correlation name can be the same as an unqualified table name
specified in that FROM clause.
In addition, you can give the columns
of the table new names in the AS clause. Some situations in which this is
useful:
- When a VALUES expression is used as a TableSubquery, since there is no other way to name
the columns of a VALUES expression.
- When column names would otherwise be the same as those of columns in other
tables; renaming them means you don't have to qualify them.
The Query in a TableSubquery appearing in a FromItem can contain
multiple columns and return multiple rows. See TableSubquery.
For information about the optimizer overrides you
can specify, see Tuning Derby.
Example
-- SELECT from a JOIN expression
SELECT E.EMPNO, E.LASTNAME, M.EMPNO, M.LASTNAME
FROM EMPLOYEE E LEFT OUTER JOIN
DEPARTMENT INNER JOIN EMPLOYEE M
ON MGRNO = M.EMPNO
ON E.WORKDEPT = DEPTNO
