Aliases are used where a table is joined to itself and referred to more than once in a SELECT statement. In this case, the different uses of a table may be followed by an 'alias' name, which can be used instead of the table name to distinguish which instance of the table is meant. For example, suppose you wanted to know which employees have grades higher than Michael Holmes:
SELECT x.name, y.name, y.grade
FROM employee x, employee y
WHERE y.grade > x.grade AND x.name='Michael Holmes'
ORDER BY y.grade, y.name
Here X and Y are used as aliases for the two copies of the EMPLOYEE table which are being joined.