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Substitution and Instantiation - SQL
It shows how NULL might appear in substitution for a parameter of a predicate and how it might thus participate in instantiation of that predicate to yield a proposition. Now consider instantiations of the dyadic predicate a < b. As well as instantiations such as 5 < 10 (a true one) and 9 < 6 (a false one), we now have to entertain the possibility of instantiations such as 5 < NULL, NULL < 6, and NULL < NULL.
In SQL these comparisons evaluate to that intrusive truth value, unknown. It goes on to explain that the extension of a predicate consists exactly of those instantiations of it that evaluate to true, from which we can conclude, of every instantiation that does not appear in the extension, that it is false, in which case it must appear instead in the extension of the negation of that predicate. In SQL, then, the instantiation 5 < NULL, for example, cannot be considered to appear in either the extension of a < b or NOT (a < b). Or so it would appear.
Parameter Default Values As the illustration below shows, you can initialize the IN parameters to the default values. In that way, you can pass various numbers of actual par
ALTER TABLE bb_basketitem ADD CONSTRAINT bitems_qty_ck CHECK (quantity BEGIN INSERT INTO bb_basketitem VALUES (88,8,10.8,21,16,2,3); END; Brewbean’s wants to add a check
Assignment of Variable - Updating a Variable Syntax: SET SN = SID ('S2'); This can obviously be read as "set the variable SN to be equal in value to SID ( 'S2' )".
Conditionals - SQL At first sight SQL does not appear to have a single operator for expressing logical implication. In this respect it would be in common with most programming
what are ER diagrams
Case Sensitivity Similar to all the identifiers, the variables, the names of constants, and parameters are not case sensitive. For illustration, PL/SQL considers the following n
CLOSE Statement The CLOSE statement allows the resources held by a cursor variable or open cursor to be reused. No more rows can be fetched from the cursor variable or closed
%ISOPEN The %ISOPEN yields TRUE if its cursor or cursor variable is open; or else, the %ISOPEN yields FALSE. In the illustration, you use the %ISOPEN to select an action:
Using Aliases The Select-list items fetched from a cursor related with the %ROWTYPE should have simple names or, if they are expressions, should have aliases. In the example bel
Declaring Subprograms You can declare subprograms in any PL/SQL subprogram, block, or package. But, you should declare subprograms at the end of the declarative part after a
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