Comparison Of The Where Clause And The Link Statement - Tandem ENFORM 058057 Reference Manual

LINKS and the LINK OPTIONAL Statement Rules
How ENFORM Defines a LINK
C–4
The converted form of this WHERE clause contains two terms:
apple.color = "RED"
AND banana.color = "YELLOW"
Since neither term references two record descriptions, the WHERE clause does not
establish a link.

Comparison of the WHERE Clause and the LINK Statement

A WHERE clause differs from a LINK statement as follows:
A WHERE clause can link more than two record descriptions. A single LINK
statement can link only two record descriptions.
A WHERE clause can indicate that the record descriptions are to be linked where
the values of linking fields are not equal. A LINK statement indicates that the
values of the linking fields must be equal.
You cannot specify OPTIONAL in a WHERE clause.
Consider the WHERE clause in the following query:
OPEN apple, orange, banana;
LIST ...
WHERE apple.seeds = orange.seeds
OR orange.skin <> banana.skin;
The converted WHERE clause contains one term:
apple.seeds = orange.seeds OR orange.skin <> banana.skin
Since this term references three record descriptions (apple, orange, and banana), the
preceding WHERE clause establishes three two-directional links between apple,
orange, and banana.
Like a LINK statement, a WHERE clause establishes a multiple dependency condition
between the linked record descriptions. A two-directional link established by a
WHERE clause can be sketched as:
record-description-1
where the arrow heads point at the dependent record descriptions.
058057 Tandem Computers Incorporated
first term
second term
record-description-2