Add trigger and audit logging example to PostgreSQL advanced concepts

psql-query.md

@@ -1,124 +1,110 @@
 ```sql
--- Create a table named department
+-- 1. Create a table named department
 CREATE TABLE department (
     dept_name VARCHAR(100) PRIMARY KEY,
     building VARCHAR(120) NOT NULL,
-    budget NUMERIC(12,2) NOT NULL DEFAULT 'undecided',
+    budget NUMERIC(12,2) NOT NULL DEFAULT 0.00,
     num_staff INT
 );
 
--- Insert values into the department table
+-- 2. Insert unique values into the department table
 INSERT INTO department (dept_name, building, budget, num_staff)
-VALUES ('cse', 'ma wazed', 12.20, 10),
-       ('cse', 'ma wazed', 12.20, 10),
-       ('cse', 'ma wazed', 12.20, 10);
+VALUES ('CSE', 'MA Wazed', 12000.00, 10),
+       ('EEE', 'Main Building', 15000.00, 8),
+       ('ME', 'Workshop Block', 13000.00, 12);
 
--- Add a column named dept_code to the existing department table
-ALTER TABLE department ADD COLUMN dept_code VARCHAR(6) NOT NULL;
+-- 3. Add new columns with default values
+ALTER TABLE department ADD COLUMN dep_code VARCHAR(15) DEFAULT 'NA';
 
--- Add a column named dep_code to the existing department table after the dept_name column
-ALTER TABLE department ADD COLUMN dep_code VARCHAR(6) NOT NULL AFTER dept_name;
-
--- Remove the dept_code column from the department table
-ALTER TABLE department DROP COLUMN dept_code;
-
--- Modify the data type of the dept_code column to VARCHAR(10) in the department table
-ALTER TABLE department ALTER COLUMN dept_code TYPE VARCHAR(10);
-
--- Delete the department table along with its structure
-DROP TABLE department;
-
--- Delete all data from the department table, keeping the table structure
-DELETE FROM department;
--- OR
-TRUNCATE department;
-
--- Insert values into a table (TABLE2) from another table (TABLE1) based on a condition
-INSERT INTO TABLE2 (employeeId, employeeName)
-SELECT id, firstName FROM TABLE1 WHERE CONDITION;
+-- 4. Insert values into another table from a table
+-- Make sure employee_archive exists
+INSERT INTO employee_archive (employeeId, employeeName)
+SELECT emp_id, first_name
+FROM employee
+WHERE salary > 50000;
 
--- Update the salary column of the instructor table by increasing it by 5% for instructors with salary below the average salary
+-- 5. Update instructor salary below average by 5%
 UPDATE instructor
 SET salary = salary * 1.05
 WHERE salary < (SELECT AVG(salary) FROM instructor);
 
--- Update the salary column of the instructor table using a case statement
+-- 6. Update salary using CASE
 UPDATE instructor
 SET salary = CASE
     WHEN salary <= 100000 THEN salary * 1.05
     ELSE salary * 1.03
 END;
 
--- Retrieve records from the instructor table and order the results by dept_name in descending order and salary in ascending order
+-- 7. Select instructors ordered by dept_name DESC, salary ASC
 SELECT *
 FROM instructor
 ORDER BY dept_name DESC, salary ASC;
 
--- Retrieve a limited number of rows from a table
+-- 8. Retrieve limited rows
 SELECT *
-FROM TABLE_NAME
-LIMIT n;
+FROM instructor
+LIMIT 5;  -- first 5 rows
 
--- Retrieve a limited number of rows from a table, starting from the mth row
+-- 9. Retrieve rows with offset
 SELECT *
-FROM TABLE_NAME
-LIMIT n OFFSET m;
+FROM instructor
+LIMIT 5 OFFSET 10;  -- rows 11-15
 
--- Retrieve records from the instructor table where the name is not 'pranto' or 'Zahid'
+-- 10. Select where name not in list
 SELECT *
 FROM instructor
-WHERE name NOT IN ('pranto', 'Zahid');
+WHERE name NOT IN ('Pranto', 'Zahid');
 
--- Retrieve records from the instructor table where the id is within a range
+-- 11. Select where id is within a subquery
 SELECT *
 FROM instructor
 WHERE id IN (SELECT id FROM instructor WHERE id < 500);
 
--- Retrieve records from the instructor table where the salary is between 5000 and 10000
+-- 12. Select salary between 5000 and 10000
 SELECT sname
 FROM instructor
 WHERE salary BETWEEN 5000 AND 10000;
 
--- Retrieve records from the instructor table where the salary is NULL
+-- 13. Select rows where salary is NULL
 SELECT *
 FROM instructor
 WHERE salary IS NULL;
 
--- Retrieve records from the instructor table where the id ends with '5' followed by any character
+-- 14. Select id ending with '5' and any character
 SELECT *
 FROM instructor
-WHERE id LIKE '%5_';
+WHERE id::TEXT LIKE '%5_';
 
--- Find the average salary of instructors in the CSE department
-SELECT AVG(salary)
+-- 15. Average salary of CSE department
+SELECT AVG(salary) AS avg_salary
 FROM instructor
 WHERE dept_name = 'CSE';
 
--- Find the number of departments in the instructor relation
-SELECT COUNT(DISTINCT dept_name)
+-- 16. Number of distinct departments
+SELECT COUNT(DISTINCT dept_name) AS num_departments
 FROM instructor;
 
--- Find the number of tuples (rows) in the instructor relation
+-- 17. Number of rows in instructor table
 SELECT COUNT(*) AS num_rows
 FROM instructor;
 
--- Retrieve the department name and the average salary of instructors for each department
+-- 18. Department name and average salary
 SELECT dept_name, AVG(salary) AS avg_salary
 FROM instructor
 GROUP BY dept_name;
 
--- Retrieve the department name and the number of instructors in each department
+-- 19. Department name and number of instructors
 SELECT dept_name, COUNT(id) AS num_of_teacher
 FROM instructor
 GROUP BY dept_name;
 
--- Perform a natural join between the employee and branch tables
-SELECT branch_name, first_name, last_name
+-- 20. Join example: LEFT JOIN employee and branch
+SELECT branch.branch_name, employee.first_name, employee.last_name
 FROM employee
-[LEFT|RIGHT] JOIN branch
+LEFT JOIN branch
 ON employee.emp_id = branch.mgr_id;
 
--- Perform a nested query to retrieve employee names based on their IDs in the works_with table
+-- 21. Nested query example
 SELECT employee.first_name, employee.last_name
 FROM employee
 WHERE employee.emp_id IN (
@@ -127,7 +113,7 @@ WHERE employee.emp_id IN (
     WHERE total_sales > 30000
 );
 
--- Create a trigger that inserts values into another table when a new row is inserted into table1
+-- 22. Trigger example: insert into table2 when a row is inserted in table1
 CREATE OR REPLACE FUNCTION triggertest()
 RETURNS TRIGGER AS $$
 BEGIN
@@ -141,36 +127,30 @@ BEFORE INSERT ON table1
 FOR EACH ROW
 EXECUTE FUNCTION triggertest();
 
--- Create a trigger with conditional statements that inserts values into another table based on the sex column in the employee table
-CREATE OR REPLACE FUNCTION my_trigger()
+-- 23. Trigger with conditional logic
+CREATE OR REPLACE FUNCTION sex_trigger()
 RETURNS TRIGGER AS $$
 BEGIN
     IF NEW.sex = 'M' THEN
         INSERT INTO trigger_test VALUES ('added male employee');
     ELSIF NEW.sex = 'F' THEN
-        INSERT INTO trigger_test VALUES ('added female');
+        INSERT INTO trigger_test VALUES ('added female employee');
     ELSE
         INSERT INTO trigger_test VALUES ('added other employee');
     END IF;
     RETURN NEW;
 END;
 $$ LANGUAGE plpgsql;
 
-CREATE TRIGGER my_trigger
+CREATE TRIGGER my_sex_trigger
 BEFORE INSERT ON employee
 FOR EACH ROW
-EXECUTE FUNCTION my_trigger();
-
--- Example of a complex query involving multiple tables and joins
-SELECT t.eName
-FROM (
-    SELECT *
-    FROM employee
-    NATURAL JOIN manages
-) AS t
-INNER JOIN employee AS e
-ON t.mName = e.eName
-AND t.city = e.city
-AND t.street = e.street;
-
-```
+EXECUTE FUNCTION sex_trigger();
+
+-- 24. Complex query: explicit JOIN instead of NATURAL JOIN
+SELECT e1.eName
+FROM manages m
+INNER JOIN employee e1 ON m.emp_id = e1.emp_id
+INNER JOIN employee e2 ON m.mName = e2.eName
+    AND m.city = e2.city
+    AND m.street = e2.street;

psql/advance.md

@@ -57,7 +57,7 @@ CREATE TABLE example_table (
 );
 ```
 
-Constraints can be defined when a table is created or added later using the ALTER TABLE statement. Constraints can also be dropped using tthe ALTER TABLE statement.
+Constraints can be defined when a table is created or added later using the ALTER TABLE statement. Constraints can also be dropped using the ALTER TABLE statement.
 
 ## Joins
 
@@ -198,25 +198,48 @@ In the above syntax, the trigger function trigger_function_name is created using
 
 Example:
 ```sql
--- Example of creating a trigger
+-- 1. Create target table
+CREATE TABLE target_table (
+    id SERIAL PRIMARY KEY,
+    name TEXT,
+    age INT
+);
+
+-- 2. Create audit table
+CREATE TABLE audit_table (
+    audit_id SERIAL PRIMARY KEY,
+    event_type TEXT NOT NULL,
+    event_time TIMESTAMP NOT NULL DEFAULT NOW(),
+    table_name TEXT NOT NULL,
+    old_data JSONB,
+    new_data JSONB
+);
+
+-- 3. Create trigger function
 CREATE OR REPLACE FUNCTION log_changes()
 RETURNS TRIGGER AS $$
 BEGIN
     IF TG_OP = 'INSERT' THEN
         INSERT INTO audit_table (event_type, event_time, table_name, new_data)
-        VALUES ('INSERT', NOW(), TG_TABLE_NAME, NEW);
+        VALUES ('INSERT', NOW(), TG_TABLE_NAME, to_jsonb(NEW));
+        RETURN NEW;
+
     ELSIF TG_OP = 'UPDATE' THEN
         INSERT INTO audit_table (event_type, event_time, table_name, old_data, new_data)
-        VALUES ('UPDATE', NOW(), TG_TABLE_NAME, OLD, NEW);
+        VALUES ('UPDATE', NOW(), TG_TABLE_NAME, to_jsonb(OLD), to_jsonb(NEW));
+        RETURN NEW;
+
     ELSIF TG_OP = 'DELETE' THEN
         INSERT INTO audit_table (event_type, event_time, table_name, old_data)
-        VALUES ('DELETE', NOW(), TG_TABLE_NAME, OLD);
+        VALUES ('DELETE', NOW(), TG_TABLE_NAME, to_jsonb(OLD));
+        RETURN OLD;
     END IF;
 
-    RETURN NEW;
+    RETURN NULL;
 END;
 $$ LANGUAGE plpgsql;
 
+-- 4. Attach trigger to target table
 CREATE TRIGGER audit_trigger
 AFTER INSERT OR UPDATE OR DELETE
 ON target_table