Misc. Triggers Views Roles Sequences - Synonyms. Eng. Mohammed Alokshiya. Islamic University of Gaza. Faculty of Engineering

Transcription

1 Islamic University of Gaza Faculty of Engineering Computer Engineering Dept. Database Lab (ECOM 4113) Lab 9 Misc. Triggers Views Roles Sequences - Synonyms Eng. Mohammed Alokshiya December 7, 2014

2 Views A view is a stored query. You can create a new view using the following syntax: View s Syntax CREATE [OR REPLACE] VIEW VIEW_NAME AS SELECT_STATEMENT; Example CREATE OR REPLACE VIEW VIEW_TOP_SALARIES AS SELECT FIRST_NAME, LAST_NAME, SALARY, DEPARTMENT_NAME FROM EMPLOYEES E LEFT OUTER JOIN DEPARTMENTS D ON E.DEPARTMENT_ID = D.DEPARTMENT_ID WHERE SALARY >= ALL ( SELECT SALARY FROM EMPLOYEES EE WHERE E.DEPARTMENT_ID = EE.DEPARTMENT_ID ) ORDER BY 3 DESC; Now you can use it as a table (it still a query not a table): Select Values From a View SELECT * FROM VIEW_TOP_SALARIES; Caution: when you create a view that select an expression, then you have to name this expression with a column alias. Example: create a view to select full name of all employees. Using Alias in a View Definition CREATE OR REPLACE VIEW VIEW_FULL_NAME AS SELECT FIRST_NAME ' ' LAST_NAME "Full Name" FROM EMPLOYEES; Then you can select values from the view using alias name: Select Values Using Alias SELECT "Full Name" FROM VIEW_FULL_NAME; 2

3 Note that the View code is stored in the database, in USER_VIEWS view. To display it, use this statement: Select Values Using Alias SELECT * FROM USER_VIEWS WHERE VIEW_NAME = 'VIEW_TOP_SALARIES'; Create a Table from Select Statement It is useful when you want to back up a table. Syntax CREATE TABLE TABLE_NAME AS SELECT_STATEMENT; CREATE TABLE EMPLOYEES_BACKUP AS SELECT * FROM EMPLOYEES; Example The previous statement will create a new table EMPLOYEES_BACKUP and fill it the rows that return from the select statement. Note that the new table is independent on the main table Employees. SELECT * FROM EMPLOYEES_BACKUP; Select Values From The New Table 3

4 Sequence A sequence is a schema object that can generate unique sequential values. These values are often used for primary and unique keys. You can refer to sequence values in SQL statements with these pseudocolumns: CURRVAL: Returns the current value of a sequence NEXTVAL: Increments the sequence and returns the next value You must qualify CURRVAL and NEXTVAL with the name of the sequence: sequence.currval sequence.nextval To create a sequence, use the following syntax: Syntax CREATE SEQUENCE SEQUENCE_NAME [START WITH N] [INCREMENT BY M]; By default, the sequence starts with 1 and the increment value for it is 1. Example CREATE SEQUENCE SEQ; The full syntax of sequence creating is: 4

5 How to Use Sequence Values When you create a sequence, you can define its initial value and the increment between its values. The first reference to NEXTVAL returns the initial value of the sequence. Subsequent references to NEXTVAL increment the sequence value by the defined increment and return the new value. Any reference to CURRVAL always returns the current value of the sequence, which is the value returned by the last reference to NEXTVAL. Before you use CURRVAL for a sequence in your session, you must first initialize the sequence with NEXTVAL. Within a single SQL statement containing a reference to NEXTVAL, Oracle increments the sequence once. Example: Using Sequence Values SELECT SEQ.NEXTVAL FROM DUAL; -- RETURN 1 SELECT SEQ.NEXTVAL FROM DUAL; -- RETURN 2 SELECT SEQ.NEXTVAL FROM DUAL; -- RETURN 3 SELECT SEQ.CURRVAL FROM DUAL; -- RETURN 3 SELECT SEQ.CURRVAL FROM DUAL; -- RETURN 3 SELECT SEQ.NEXTVAL FROM DUAL; -- RETURN 4 5

6 Synonyms Synonyms is an alternative name for a table, view, sequence, operator, procedure, stored function, package, materialized view, Java class schema object, user-defined object type, or another synonym. To create a synonym for any database object, use the following syntax: Synonym s Syntax CREATE [OR REPLACE] SYNONYM SYNONYM_NAME FOR ORIGINAL_OBJECT; Then use the original object with the new name. Example: Create Synonym EMP_SYN CREATE OR REPLACE SYNONYM EMP_SYN FOR EMPLOYEES; Then use it to select rows from EMPLOYEES table: Using Synonym EMP_SYN SELECT * FROM EMP_SYN; 6

7 Roles A role is a set of privileges that can be granted to users or to other roles. You can use roles to administer database privileges. You can add privileges to a role and then grant the role to a user. The user can then enable the role and exercise the privileges granted by the role. Role Syntax CREATE ROLE ROLE_NAME; Example: create a role named data_entry Create a new Role CREATE ROLE DATA_ENTRY; Then you can grant privileges to the role, using GRANT keyword; you can also grant other roles to a role. Grant Privileges and Roles to a Role GRANT SELECT ON EMPLOYEES TO DATA_ENTRY; GRANT UPDATE ON EMPLOYEES TO DATA_ENTRY; GRANT ALL ON COUNTRIES TO DATA_ENTRY; -- GRANT SELECT, UPDATE, DELETE AND INSERT ON COUNTRIES GRANT EXECUTE ON QUERY_EMP TO DATA_ENTRY; -- GRANT EXECUTE ON A PROCEDURE GRANT CONNECT, RESOURCE TO DATA_ENTRY; -- GRANT OTHER ROLES TO DATA_ENTRY ROLE Then you can grant the role to a user. When you grant a role to a user, then he has all privileges and roles included in the granted role. Grant Role to a User GRANT DATA_ENTRY TO USER_NAME; Note that REVOKE keyword is the opposite of GRANT keyword. It is used to revoke roles or privileges from a user or another role. Revoke Role from a User REVOKE DATA_ENTRY FROM USER_NAME; 7

8 Triggers A trigger is a PL/SQL block or a PL/SQL procedure associated with a table, view, schema, or database. Executes implicitly whenever a particular event takes place. Syntax is: Trigger s Syntax CREATE [OR REPLACE] TRIGGER TRIGGER_NAME TIMING EVENT1 [OR EVENT2 OR EVENT3] ON OBJECT_NAME [ [REFERENCING OLD AS old NEW AS new] FOR EACH ROW [WHEN (CONDITION)] ] TRIGGER_BODY The trigger type determines whether the body executes for each row or only once for the triggering statement. - A statement trigger: Executes once for the triggering event Is the default type of trigger Fires once even if no rows are affected at all - A row trigger: Executes once for each row affected by the triggering event Is not executed if the triggering event does not affect any rows Is indicated by specifying the FOR EACH ROW clause Trigger Timing: When should the trigger fire? BEFORE: Execute the trigger body before the triggering DML event on a table. AFTER: Execute the trigger body after the triggering DML event on a table. INSTEAD OF: Execute the trigger body instead of the triggering statement. This is used for views that are not otherwise modifiable. 8

9 A trigger event: Determines which DML statement causes the trigger to execute. Types are: INSERT UPDATE [OF column] DELETE A trigger body: Is a PL/SQL block or a CALL to a procedure. It determines what action is performed. Example: write a trigger that is fire when someone is trying to modify an employee s salary. The trigger will insert the values of the old salary and the new salary into an AUDIT table named AUDIT_EMP. 1. Create AUDIT_EMP table. CREATE TABLE AUDIT_EMP ( EMPLOYEE_ID NUMBER(6), PREV_SAL NUMBER(8, 2), NEXT_SAL NUMBER(8, 2), MODIFY_DATE DATE ); Create Table AUDIT_EMP 2. Create trigger TRACK_SAL_CHANGES Create Trigger TRACK_SAL_CHANGES CREATE OR REPLACE TRIGGER TRACK_SAL_CHANGES BEFORE UPDATE ON EMPLOYEES FOR EACH ROW WHEN (OLD.SALARY <> NEW.SALARY) DECLARE BEGIN INSERT INTO AUDIT_EMP VALUES(:NEW.EMPLOYEE_ID, :OLD.SALARY, :NEW.SALARY, SYSDATE); END; 9

10 Now try to issue some UPDATE statements and notice the changes on table AUDIT_EMP Update Records UPDATE EMPLOYEES SET SALARY = 5000 WHERE EMPLOYEE_ID = 200; UPDATE EMPLOYEES SET SALARY = 5000 WHERE EMPLOYEE_ID = 100; Result: 10