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NULL Statement
The NULL statement clearly specifies in action; it does nothing other than to pass control to the next statement. It can, though, improve the readability. In a construct allowing the substitute actions, the NULL statements serve as a placeholder. It tells the readers that the related alternative has not been overlooked, but required that no action is necessary. In the following illustration, the NULL statement shows that no action is taken for the unnamed exceptions:EXCEPTIONWHEN ZERO_DIVIDE THENROLLBACK;WHEN VALUE_ERROR THENINSERT INTO errors VALUES...COMMIT;WHEN OTHERS THENNULL;END;Each of the clause in an IF statement should contain at least one executable statement. TheNULL statement is executable; therefore you can use it in clauses that correspond to the circumstances in which no action is taken. In the following illustration, the NULL statements emphasize that only top-rated employees get bonus:IF rating > 90 THENcompute_bonus(emp_id);ELSENULL;END IF;The NULL statement is also a handy way to create stubs when designing the applications from the top down. A stub is dummy subprogram that permits you to defer the definition of a procedure or function till you test and debug the main program. In the following illustration, the NULL statement meets the requirement that at least one statement should appear in the executable part of a subprogram:PROCEDURE debit_account (acct_id INTEGER, amount REAL) ISBEGINNULL;END debit_account;
Other monadic - SQL In 2VL there are just 4 (2 2 ) monadic operators, of which negation is really the only "useful" one. When a third truth value is introduced we have 27 (3 3
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Adding Table Constraints ALTER TABLE ENROLMENT ADD CONSTRAINT NameNotNull CHECK (Name IS NOT NULL) ; ALTER TABLE ENROLMENT ADD CONSTRAINT PK_StudentId_CourseId PRIM
Semidifference via EXCEPT and JOIN - SQL SELECT * FROM (SELECT StudentId FROM IS_CALLED WHERE Name = 'Devinder' EXCEPT DISTINCT CORRESPONDING SELECT StudentId
write the program for traffic control system with 10 second, 15 secod, and 20 second delay
Effect of Anonymous Columns Now, recall that a VALUES expression denotes a table with undefined column names. If an initial value is to be specified when a base table is creat
Example of GROUPBY Operator Example: How many students sat each exam, using GROUP BY, NATURAL LEFT JOIN, and COALESCE SELECT CourseId, COALESCE (n, 0) AS n FROM COURS
Literals A literal is an explicit numeric, string, character, or Boolean value not represented by an identifier. Numeric literal 147 and the Boolean literal FALSE are some of
Cursor Attributes The Cursors and cursor variables have 4 attributes which give you helpful information about the execution of a data manipulation statement. Syntax:
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