FROM Clause¶

Specification of the origin view is carried out using the FROM clause. In said clause the name of the relation - or relations - from which data are to be extracted is indicated. It is possible to specify aliases for the relations in the FROM clause. Aliases can be used in the other clauses in the SELECT statement and will facilitate the creation of Join conditions. If an alias is indicated for a relation in the FROM clause, the name of the relation should not be used in the rest of the SELECT statement to prefix fields of same; the alias should always be used.

It is possible to use subqueries in the FROM clause. The subquery must be included between brackets.

Example The following statement uses a subquery that carries out a UNION operation between the internet_inc and phone_inc views:

SELECT *
FROM (
  SELECT *
  FROM internet_inc

  UNION

  SELECT *
  FROM phone_inc
)
WHERE taxid = 'B78596011'

If several relations are listed in the FROM clause without separating them from the JOIN clause, then their cross product will be performed. The following subsection deals with the different join operations available.

The FROM clause may also contain calls to Denodo stored procedures. The results returned by the calling up of a procedure will be dealt with in this case like the tuples of a view. See section Stored Procedures for more details.