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.
Note
Virtual DataPort allows executing SELECT
statements
without the FROM
clause. E.g. SELECT 2+2