Oracle Class VII More on Exception Sequence Triggers RowID & Rownum Views

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Emery Jennings
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1 Oracle Class VII More on Exception Sequence Triggers RowID & Rownum Views

2 Sequence An Oracle sequence is an Oracle database object that can be used to generate unique numbers. You can use sequences to automatically generate primary key values. After a sequence has been created, you can access its values in SQL statements with these pseudocolumns:. CURRVAL returns the current value of the sequence.. NEXTVAL increments the sequence and returns the new value We can use a maxvalue or minvalue to indicate the maximum value a sequence can have CYCLE is used to indicate, if maxvalue is reached, then the values will be cycled (NOCYCLE by default) Example of sequence: CREATE SEQUENCE trig_seq START WITH 1; CREATE SEQUENCE trig_seq1 START WITH 1 INCREMENT BY 1; CREATE SEQUENCE trig_seq2 START WITH 1 INCREMENT BY 1 maxvalue 255; Example of using a sequence: INSERT INTO test01 VALUES (trig_seq.nextval);

3 Triggers A database trigger is a named PL/SQL block stored in a database and executed implicitly when a triggering event occurs. The act of executing a trigger is called firing the trigger. Four types of triggering events :- A DML statement (such as INSERT, UPDATE, or DELETE) executed against a database table. Such a trigger can fire before or after a triggering event. A DDL statement (such as CREATE or ALTER) executed either by a particular user against a schema or by any user. A system event such as startup or shutdown of the database. A user event such as logon and logoff

4 Trigger continued CREATE [OR REPLACE] TRIGGER Ttrigger_name {BEFORE AFTER} Triggering_event ON table_name [FOR EACH ROW] [FOLLOWS another_trigger] CREATE OR REPLACE TRIGGER student_bi BEFORE INSERT ON students FOR EACH ROW DECLARE sid int; BEGIN select max(stid)+1 into sid from students; dbms_output.put_line('hello'); :new.stid:=sid; END; / FOR EACH ROW WHEN (NVL(NEW.ZIP, ' ') <> OLD.ZIP)

5 RowId & Rownum For each row in the database, the ROWID pseudocolumn returns the address of the row. Use of Rowid: They are the fastest way to access a single row. They can show you how the rows in a table are stored. They are unique identifiers for rows in a table. For each row returned by a query, the ROWNUM pseudocolumn returns a number indicating the order in which Oracle selects the row from a table or set of joined rows. The first row selected has a ROWNUM of 1, the second has 2, and so on.

6 Views Use the CREATE VIEW statement to define a view, which is a logical table based on one or more tables or views. A view contains no data itself. The tables upon which a view is based are called base tables. Views have two purposes: Simplify issuing complex SQL queries Restrict users access to sensitive data Types of View : -Simple View -Complex View -Inline View -Materialized View

7 Views create view emp10 As select * from emp where deptno=10; View can be created using join on multiple table create view empbonus (ename,sal,job,comm,deptno) as select a.ename,a.sal,a.job,b.comm, a.deptno from emp a inner join bonus b on a.ename=b.ename /

Assignments Modify the procedure that takes salary and returns the name, handle the exception so that if user gives a nonnumeric value this exits gracefully.

8 Assignments Modify the procedure that takes salary and returns the name, handle the exception so that if user gives a nonnumeric value this exits gracefully. Create a table books, which has columns as bookid and bookname. Insert two rows with booknames database fundamentals and database technologies. Use a sequence to populate the bookid for both the rows. There is a circular from federal govt. no employees can get salary less than 1000$. So in case some employees are being inserted in the table with salary less than 1000 automatically this should be updated to 1000, Write a trigger for the same. Do it first by a just changing the below in insert before, and also by an update command after insert is done. Create a trigger on emp table such that when a new employee is inserted with a deptno that is new, first insert that record in department and then insert the same in emp to avoid foreign key error. Create a view, which hides the salary column from the user. Cerate a view which has ename and dname respectively, check if you can update/insert into the views, justify your results

9 Exception Differentiate error and exception More example : EXCEPTION WHEN ZERO_DIVIDE THEN DBMS_OUTPUT.PUT_LINE ('A number cannot be divided by zero.'); VALUE_ERROR :- When there is conversion or type mismatch error. TOO_MANY_ROWS: This exception is raised when a SELECT INTO statement returns more than one row. You can declare your own exception DECLARE e_exception1 EXCEPTION; BEGIN RAISE e_exception1

D a t a b a s e M a n a g e m e n t S y s t e m L a b Institute of Engineering and Management

D a t a b a s e M a n a g e m e n t S y s t e m L a b Institute of Engineering and Management Institute of Engineering and Management Department of Computer Science and Engineering Database Management Laboratory (CS 691) Lab Manual Syllabus: Database Management System Lab Code: CS691 Contact: 3P