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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;
Effects of NULL for UNIQUE Specification When a UNIQUE specification u for base table t includes a column c that is not subject to a NOT NULL constraint, the appearance of sev
Initializing and Referencing Collections Until you initialize a collection, a nested table or varray is automatically null (i.e. the collection itself is null, not its elements)
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SQL Functions The PL/SQL uses all the SQL functions involving the following aggregate functions that summarize the whole columns of the Oracle data: GROUPING, AVG, COUNT, STDDE
Using FIRST and LAST FIRST and LAST return the first and last (minimum and maximum) index numbers in a collection. When the collection is empty, the FIRST and LAST return NULL
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Parameter SELF in pl/sql The MEMBER methods recognize a built-in parameter named SELF that is an instance of the object type. Whether declared explicitly or implicitly, it is
Example of WHEN or THEN Constraints A concrete example showing how SQL supports WHEN/THEN constraints CREATE TABLE SAL_HISTORY (EmpNo CHAR (6), Salary INTEGER NOT NULL,
Projection in SQL - correct version Student StudentId is enrolled on some course. SELECT DISTINCT StudentId FROM IS_ENROLLED_ON In more complicated examples it is someti
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