Specification of the origin view is carried out using the
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
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
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.
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.
Virtual DataPort allows executing
FROM clause. E.g.