A qualifier that designates a specific object table is called a table designator. The clause that identifies the object tables also establishes the table designators for them.
One reason for qualifying a column name is to designate the object from which the column comes.
Table designators: A qualifier that designates a specific object table is called a table designator. The clause that identifies the object tables also establishes the table designators for them. For example, the object tables of an expression in a SELECT statement are named in the FROM clause that follows it, as in the following statement:
This example show how ambiguity is resolved for single source.
SELECT * FROM http://www.delven.io/test/index-01.html
WHERE css('#title') IN ('Title 1', 'Title 2', 'Title 3')
SELECT * FROM http://www.delven.io/test/index-01.html AS T1
WHERE T1.css('#title') IN ('Title 1', 'Title 2', 'Title 3')
SELECT * FROM
(
SELECT css("#title") as title
FROM http://www.delven.io/test/index-01.html
) AS T1,
(
SELECT css("#title") as title
FROM http://www.delven.io/test/index-02.html
) AS T2
WHERE T1.title IN ('Title 1', 'Title 2', 'Title 3')