Added P8 trigger (PLSQL)

Practical/Practical Exam/PL-SQL/P8 - Trigger.md

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+# P8 - Trigger
+
+**Problem Statement:** Write a Row Level Before and After Trigger on Library table. The System should keep track of the records that are being updated or deleted. The old value of updated or deleted records should be added in Library_Audit table.
+
+## Creating tables
+
+```plsql
+CREATE TABLE Library(
+    id NUMBER(12),
+    title VARCHAR(255),
+    dateofissue DATE,
+    author VARCHAR(255)
+);
+
+CREATE TABLE Library_Audit(
+    id NUMBER(12),
+    title VARCHAR(255),
+    dateofaction DATE,
+    author VARCHAR(255),
+    status VARCHAR(255)
+);
+
+```
+
+## Inserting values
+
+```plsql
+INSERT INTO Library VALUES (1, 'Berserk', TO_DATE('2024-07-28','YYYY-MM-DD'), 'Prashant');
+INSERT INTO Library VALUES (2, 'Dark', TO_DATE('2024-07-15','YYYY-MM-DD'), 'Rajendra');
+INSERT INTO Library VALUES (3, 'Hannibal', TO_DATE('2024-07-20','YYYY-MM-DD'), 'Manoj');
+INSERT INTO Library VALUES (4, 'AOT', TO_DATE('2024-07-30','YYYY-MM-DD'), 'Rajesh');
+INSERT INTO Library VALUES (5, 'GOT', TO_DATE('2024-07-19','YYYY-MM-DD'), 'Anil');
+
+```
+
+## Trigger
+
+```sql
+CREATE OR REPLACE TRIGGER library_action
+AFTER INSERT OR UPDATE OR DELETE
+ON Library
+FOR EACH ROW
+BEGIN
+	IF INSERTING THEN
+    	INSERT INTO Library_Audit (id, title, dateofaction, author, status) VALUES (:NEW.id, :NEW.title, current_timestamp, :NEW.author, 'Insert');
+	ELSIF UPDATING THEN
+        INSERT INTO Library_Audit (id, title, dateofaction, author, status) VALUES (:OLD.id, :OLD.title, current_timestamp, :OLD.author, 'Update');
+	ELSIF DELETING THEN
+        INSERT INTO Library_Audit (id, title, dateofaction, author, status) VALUES (:OLD.id, :OLD.title, current_timestamp, :OLD.author, 'Delete');
+	END IF;
+END;
+/
+
+```
+
+## Testing trigger
+
+### Insert operation
+
+```sql
+INSERT INTO Library VALUES (15, 'CREW', TO_DATE('2024-07-22','YYYY-MM-DD'), 'Ramesh');
+INSERT INTO Library VALUES (14, 'Ninteen Eighty Four', TO_DATE('2024-07-01','YYYY-MM-DD'), 'Omkar');
+
+SELECT * FROM Library;
+SELECT * FROM Library_Audit;
+
+```
+### Update operation
+
+```sql
+UPDATE Library SET id = 6, title = 'Sherlock', author = 'Deepak' where id = 3;
+UPDATE Library SET id = 7, title = 'MR. ROBOT', author = 'Varad' where id = 4;
+
+SELECT * FROM Library;
+SELECT * FROM Library_Audit;
+
+```
+
+### Delete operation
+
+```sql
+DELETE FROM Library WHERE id = 1;
+DELETE FROM Library WHERE id = 5;
+
+SELECT * FROM Library;
+SELECT * FROM Library_Audit;
+
+```
+
+---
+