This repository contains a collection of PostgreSQL commands for various database management tasks.

This repository provides a set of PostgreSQL commands and examples for managing databases, tables, and data. Each section covers a specific aspect of database management, and the commands are presented with explanations for better understanding.

Grant the necessary permissions to users for performing specific actions within the database.

• Grant the "CONNECT" permission to the "test" database for the "postgres" user:

  GRANT CONNECT ON DATABASE test TO postgres;
  

• Grant the permission to create databases to the "postgres" user:

  ALTER USER postgres CREATEDB;
  

• Create a new database named "test2":

  CREATE DATABASE test2;
  

• Rename the database "test2" to "test3":

  ALTER DATABASE test2 RENAME TO test3;
  

• Delete the "test3" database:

  DROP DATABASE test3;
  

• Create a "student" table with columns for student information:

  CREATE TABLE student (
    student_id INT,
    first_name VARCHAR(15),
    last_name VARCHAR(15),
    cgpa NUMERIC(1, 2) );
  

• Rename the "student" table to "learner":

  ALTER TABLE student RENAME TO learner;
  

• Delete the "learner" table:

  DROP TABLE learner;
  

• Create a "users" table with columns for user information and constraints:

  CREATE TABLE users (
    user_id SERIAL PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(50) NOT NULL,
    age INT DEFAULT 18,
    UNIQUE(username, email));
  

• Remove all data from the "users" table while keeping its structure:

  TRUNCATE TABLE users;
  

• Insert user data into the "users" table using various methods:

• Method 1:

  INSERT INTO users(user_id, username, email, age)
  VALUES ('sam', 'sam@gmail.com', 21);
  

• Method 2:

  INSERT INTO users(username, email, age)
  VALUES ('clark', 'clark@gmail.com', 35),
       ('clark2', 'clark2@gmail.com', 35);
  

• Method 3:

  INSERT INTO users
  VALUES (1, 'persian', 'persian@gmail.com'),
       (2, 'persian2', 'persian2@gmail.com');
  

• Add a "password" column to the "users" table with a default value:

  ALTER TABLE users
  ADD COLUMN password VARCHAR(255) DEFAULT 'admin123' NOT NULL;
  

• Delete the "age" column from the "users" table:

  ALTER TABLE users DROP COLUMN age;
  

• Add and modify columns in the "users" table:

  ALTER TABLE users ADD COLUMN demo INT;
  ALTER TABLE users ALTER COLUMN demo TYPE TEXT;
  ALTER TABLE users ALTER COLUMN country SET DEFAULT 'bangladesh';
  ALTER TABLE users ALTER COLUMN demo DROP DEFAULT;
  

• Insert a user with specific values into the "users" table:

  INSERT INTO users VALUES(4, 'john1', 'john1@gmail.com');
  

• Rename the "demo" column to "country" in the "users" table:

  ALTER TABLE users RENAME COLUMN demo TO country;
  ALTER TABLE users ALTER COLUMN country SET NOT NULL;
  

• Drop the "NOT NULL" constraint from the "country" column:

  ALTER TABLE users ALTER COLUMN country DROP NOT NULL;
  

• Add a unique constraint to the "email" column in the "users" table:

  ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE(email);
  

• Delete the unique constraint from the "email" column in the "users" table:

  ALTER TABLE users DROP CONSTRAINT unique_email;
  

• Retrieve all data from the "users" table:

  SELECT * FROM users;
  
  

• Create a table named "Employees" to store employee information.:

  CREATE TABLE Employees (
      empID SERIAL PRIMARY KEY,
      empName VARCHAR(50) NOT NULL,
      departmentID INT,
      CONSTRAINT fk_constraint_dept FOREIGN KEY (departmentID) REFERENCES Department(deptID)
  );
  

Explanation: Create a table to store employee details with a foreign key constraint.

• Retrieve Specific table column:

  SELECT empid, name, email, joining_date, salary, from employees;
  
  SELECT *
  FROM employees
  WHERE
      name <> 'Eve'
      AND salary > 9000
      OR name = 'Greta'
  

• Update Course Data:

  UPDATE courses
  SET
      course_name = 'laravel',
      description = 'super'
      WHERE course_id = 1;
  

Explanation: Update course data where course_id is 1.

• Update Multiple Course Data:

  UPDATE courses
  SET
      course_name = 'laravel',
      description = 'super'
      WHERE course_id > 1;
  

Explanation: Update course data for courses with course_id greater than 1.

• Delete Course Data:

  DELETE FROM courses WHERE course_id = 1;
  

Explanation: Delete course data where course_id is 1.

• Create Employees Table (Conditional):

  CREATE TABLE IF NOT EXISTS employees (
    empID SERIAL PRIMARY KEY,
    name text NOT NULL,
    email TEXT NOT NULL,
    salary INT not NULL,
    joining_data DATE NOT NULL,
    deptID INT NOT NULL,
    CONSTRAINT fk_deptID FOREIGN KEY (deptID) REFERENCES department(deptID));
  

Explanation: Create employees table if it doesn't exist.

• Select Specific Columns from Employees Table:

  SELECT empid, name, email, joining_date, salary FROM employees;
  

Explanation: Retrieve specific columns from the employees table.

• Filtering Rows with Conditions:

  SELECT * FROM employees WHERE name LIKE '_r%';
  
  SELECT * FROM employees WHERE name LIKE '__r__';
  
  SELECT * FROM employees WHERE name LIKE 'G%a';
  
  SELECT * FROM employees WHERE deptid IS NULL;
  

Explanation: Retrieve rows based on specified conditions.

• Sorting and Limiting Results:

  SELECT * FROM employees ORDER BY name ASC LIMIT 10 OFFSET 2;
  

Explanation: Retrieve sorted and limited rows from employees table.

• Retrieving Maximum Salary:

  SELECT * FROM employees ORDER BY salary DESC LIMIT 1;
  

Explanation: Retrieve employee with the highest salary.

• IN, NOT IN, BETWEEN, and LIKE Operators:

  SELECT * FROM employees WHERE empid NOT IN (2, 3, 5);
  SELECT * FROM employees WHERE salary BETWEEN 10000 AND 15000;
  SELECT * FROM employees WHERE name LIKE 'A%';
  

Explanation: Retrieve rows based on various filtering conditions.

• Inner Join with Explanation:

  SELECT *
  FROM employees
    INNER JOIN department ON employees.deptid = department.deptid;
  

Explanation: Retrieve all columns from both tables where the department ID matches in both tables.

• Left Join:

  SELECT *
  FROM employees
  LEFT JOIN department ON department.department_id = employees.department_id;
  

Explanation: Retrieve all rows from the left table and matching rows from the right table.

• Right Join:

  SELECT *
  FROM employees
  RIGHT JOIN department ON department.department_id = employees.department_id;
  

Explanation: Retrieve all rows from the right table and matching rows from the left table.

• Full Outer Join:

  SELECT *
  FROM employees
  FULL JOIN department ON department.department_id = employees.department_id;
  

Explanation: Retrieve all rows when there is a match in either the left or right table.

• Cross Join:

  SELECT * FROM employees CROSS JOIN department;
  

Explanation: Generate all possible combinations of rows between two tables.

Perform calculations on groups of rows and summarize data

• Aggregate Functions with Explanation:

  SELECT
    d.name,
    AVG(e.salary),
    SUM(e.salary),
    MAX(e.salary)
  FROM employees e
    FULL JOIN department d ON e.dept = d.dept
  GROUP BY d.name
  HAVING AVG(e.salary) > 60000;
  

Explanation: Calculate average, sum, and maximum salary for each department, filtering results using the HAVING clause.

• Aggregate Functions with GROUP BY:

  SELECT
    d.name,
    SUM(salary),
    AVG(salary),
    MIN(salary),
    COUNT(*)
  FROM department d
    FULL JOIN employees e ON e.dept = d.deptid
  GROUP BY d.deptid;
  

Explanation: Group and aggregate salary data by department, calculating sum, average, minimum, and count.

Use subqueries to perform complex queries within other queries

• Aggregate Functions with GROUP BY:

  SELECT
    film_id,
    title,
    rental_rate
  FROM film
  WHERE rental_rate > 2.98;
  

Explanation: Retrieve film details where the rental rate is greater than 2.98.