Understanding SQL Fundamentals: A Beginner's Handbook:-

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What is a Database?

• A place to store and organize information so you can find it easily. 📂✅

• Example: Like a digital notebook to save and find information quickly. 📝💻

• Example: A phone contact list is a simple database. 📱

What is DBMS?

• DBMS means Database Management System.

• It’s a software that helps to store and manage data.

• Makes it easy to add, update, delete, and find data.

• Example**:** MySQL, Oracle, Microsoft SQL Server.

What is a Query?

• A query is a question or request sent to a database.

It asks the database to find, add, update, or delete data.

This query asks the database to show all friend names.

SELECT name FROM friends;

What is MySQL Workbench?

• It’s a tool to work with MySQL databases easily.

• You can write and run SQL commands.

• You can design tables and see data in a nice way.

What is a Primary Key in a database?

• It’s a special ID for each row in a table.

• No two rows have the same primary key.

• Example: Your student ID in a school.

What is a Secondary Key in a database?

• A key used to find or group data, but it’s not unique.

• Example:-Class name in a student list — many students share the same class.

• It’s like a helper key in the database.

• What is a Foreign Key in a database?

• A foreign key is like a bridge between two tables.

• It links one table’s data to another table’s unique ID (primary key).

• Helps keep data organized and connected.

• A foreign key is a link from one table to another .It helps tables talk to each other.

• Example:-In an orders list, Customer ID tells who placed the order by linking to the customers list.

---

-- Create a new database named startersql
     CREATE DATABASE startersql;

-- Use the startersql database
     USE startersql;

-- Delete the users table if it exists
     DROP TABLE IF EXISTS users;

-- (You can create tables or insert data here)
-- (You can create tables or insert data here)

-- Rename the table 'users' to 'customers'
     RENAME TABLE users TO customers;

-- Get all columns and rows from the 'users' table
    SELECT * FROM users;

-- It displays all the columns (fields) in the table named pre.
    SHOW COLUMNS FROM pre;


-- Delete the startersql database completely
     DROP DATABASE startersql;

---

Step 1: Create a Database:-

CREATE DATABASE startersql;
USE startersql;

Step 2: Create a Table:-

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,         
    -- Makes a user ID that goes up automatically (1, 2, 3...)

    name VARCHAR(100) NOT NULL,                
    -- Name of the user (must be filled in) Variable-length string, up to 100 characters.

    email VARCHAR(100) UNIQUE NOT NULL,        
    -- Email (must be filled in and cannot be the same as anyone else's)

    gender ENUM('Male', 'Female', 'Other'),    
    -- Gender (choose one from Male, Female, Other)

    date_of_birth DATE,                        
    -- Birth date (example: 2000-05-01)

    salary DECIMAL(10, 2),                     
    -- Salary with 2 decimal points (example: 50000.50)

    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP 
    -- Adds the date & time automatically when the user is added
);

Data Types Explained:-

• INT: Integer type, used for whole numbers.

• VARCHAR(100): Variable-length string, up to 100 characters.

• ENUM: A string object with a value chosen from a list of permitted values.

• DATE: Stores date values. eg -date_of_birth DATE.

• TIMESTAMP: Stores date and time, automatically set to the current timestamp when a row is created.

• BOOLEAN: Stores TRUE or FALSE values, often used for flags like is_active.

• DECIMAL(10, 2): Stores exact numeric data values, useful for financial data.

Constraints Explained:-

• AUTO_INCREMENT: Automatically generates a unique number for each row.

• PRIMARY KEY: Uniquely identifies each row in the table.

• NOT NULL: Ensures a column cannot have a NULL value.

• UNIQUE: Ensures all values in a column are different.

• DEFAULT: Sets a default value for a column if no value is provided. eg-created_at TIMESTAMP DEFAULT

• CURRENT_TIMESTAMP, is_active BOOLEAN DEFAULT TRUE

Step 3-Insert Multiple Rows at Once:-

INSERT INTO users (name, email, gender, date_of_birth, salary) VALUES
('Aarav', 'aarav@example.com', 'Male', '1995-05-14', 65000.00),
('Ananya', 'ananya@example.com', 'Female', '1990-11-23', 72000.00),
('Raj', 'raj@example.com', 'Male', '1988-02-17', 58000.00),
('Sneha', 'sneha@example.com', 'Female', '2000-08-09', 50000.00),
('Farhan', 'farhan@example.com', 'Male', '1993-12-30', 61000.00),
('Priyanka', 'priyanka@example.com', 'Female', '1985-07-12', 84000.00),
('Aisha', 'aisha@example.com', 'Female', '1997-03-25', 56000.00),
('Aditya', 'aditya@example.com', 'Male', '1992-06-17', 69000.00),
('Meera', 'meera@example.com', 'Female', '1989-09-05', 77000.00),
('Ishaan', 'ishaan@example.com', 'Male', '2001-10-02', 45000.00),
('Tanvi', 'tanvi@example.com', 'Female', '1994-04-18', 62000.00),
('Rohan', 'rohan@example.com', 'Male', '1986-12-01', 75000.00),

**Step 4-**Basic SQL Operations:-

-- Show all columns and rows from the users table
SELECT * FROM users;

-- Show only the name and email of each user
SELECT name, email FROM users;

-- Show all users where gender is Female
SELECT * FROM users WHERE gender = 'Female';

-- Show all users where gender is NOT Female
SELECT * FROM users WHERE gender <> 'Female';   -- (<> means Not Equal To)

-- Select only column1 and column2 from the table called table_name
SELECT column1, column2 FROM table_name;

-- Show users whose ID is greater than 10
SELECT * FROM users WHERE id > 10;

-- Show users who have a date of birth (not empty)
SELECT * FROM users WHERE date_of_birth IS NOT NULL;

-- Show users born between 1990 and 2000
SELECT * FROM users WHERE date_of_birth BETWEEN '1990-01-01' AND '2000-12-31';

-- Show users whose gender is either Male or Female
SELECT * FROM users WHERE gender IN ('Male', 'Female');

-- Show Male users who earn more than 70,000, sorted by birth date (oldest first)
SELECT * FROM users WHERE gender = 'Male' AND salary > '70000' ORDER BY date_of_birth ASC;

-- Show users whose name starts with the letter A
SELECT * FROM users WHERE name LIKE 'A%';  -- % means any characters after A

---

Altering a Table in SQL(Add a Column):-

--  Add a new column named 'phone' to the 'users' table to store phone numbers
   ALTER TABLE users
    ADD phone VARCHAR(15);

--  Remove the 'is_active' column from the 'users' table
   ALTER TABLE users DROP COLUMN is_active;

-- Change the size of the 'name' column to allow up to 150 characters
   ALTER TABLE users MODIFY COLUMN name VARCHAR(150);

Move a column to the first position:-

--ALTER TABLE table_name 
--MODIFY COLUMN column_name data_type 
--FIRST;


--ALTER TABLE table_name 
--MODIFY COLUMN column_name data_type 
--AFTER another_column;


-- Move the 'email' column to be the first column in the 'users' table
   ALTER TABLE users 
   MODIFY COLUMN email VARCHAR(100) 
   FIRST;




-- Move the 'gender' column to appear right after the 'name' column in the 'users' table
   ALTER TABLE users 
   MODIFY COLUMN gender ENUM('Male', 'Female', 'Other') 
   AFTER name;

LIKE (Pattern Matching):-

SELECT * FROM users WHERE name LIKE 'A%';  -- Starts with A
SELECT * FROM users WHERE name LIKE '%a';  -- Ends with a
SELECT * FROM users WHERE name LIKE '%li%'; -- Contains 'li'

ORDER BY:-

• ASC: Sorts from smallest to largest (e.g., A to Z, oldest to newest).

• DESC: Sorts from largest to smallest (e.g., Z to A, newest to oldest).

SELECT * FROM users ORDER BY date_of_birth ASC;
SELECT * FROM users ORDER BY name DESC;

LIMIT:-

SELECT * FROM users LIMIT 5;  -- Top 5 rows
SELECT * FROM users LIMIT 10 OFFSET 5;  -- Skip first 5 rows, then get next 10
SELECT * FROM users LIMIT 5, 10;  -- Get 10 rows starting from the 6th row (Same as above)
SELECT * FROM users ORDER BY created_at DESC LIMIT 10;

UPDATE - Modifying Existing Data:

CREATE DATABASE IF NOT EXISTS startersql;
USE startersql;

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    email VARCHAR(100) UNIQUE NOT NULL,
    gender ENUM('Male', 'Female', 'Other'),
    date_of_birth DATE,
    salary DECIMAL(10, 2),
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
INSERT INTO users (name, email, gender, date_of_birth, salary) VALUES
('Aarav', 'aarav@example.com', 'Male', '1995-05-14', 65000.00),
('Ananya', 'ananya@example.com', 'Female', '1990-11-23', 72000.00),
('Raj', 'raj@example.com', 'Male', '1988-02-17', 58000.00),
('Sneha', 'sneha@example.com', 'Female', '2000-08-09', 50000.00),
('Farhan', 'farhan@example.com', 'Male', '1993-12-30', 61000.00),
('Priyanka', 'priyanka@example.com', 'Female', '1985-07-12', 84000.00),
('Aisha', 'aisha@example.com', 'Female', '1997-03-25', 56000.00),
('Aditya', 'aditya@example.com', 'Male', '1992-06-17', 69000.00),
('Meera', 'meera@example.com', 'Female', '1989-09-05', 77000.00),
('Ishaan', 'ishaan@example.com', 'Male', '2001-10-02', 45000.00),
('Tanvi', 'tanvi@example.com', 'Female', '1994-04-18', 62000.00),
('Rohan', 'rohan@example.com', 'Male', '1986-12-01', 75000.00),
('Zoya', 'zoya@example.com', 'Female', '1998-01-15', 54000.00),
('Karan', 'karan@example.com', 'Male', '1990-08-22', 68000.00),
('Nikita', 'nikita@example.com', 'Female', '1987-03-10', 71000.00),
('Manav', 'manav@example.com', 'Male', '1996-11-29', 61000.00),
('Divya', 'divya@example.com', 'Female', '1991-02-28', 57000.00),
('Harshit', 'harshit@example.com', 'Male', '1993-09-09', 65000.00),
('Ritika', 'ritika@example.com', 'Female', '1999-05-05', 52000.00),
('Imran', 'imran@example.com', 'Male', '1995-07-30', 63000.00),
('Juhi', 'juhi@example.com', 'Female', '1992-10-14', 59000.00),
('Tushar', 'tushar@example.com', 'Male', '1990-01-08', 73000.00),
('Lata', 'lata@example.com', 'Female', '1984-11-11', 78000.00),
('Yash', 'yash@example.com', 'Male', '1997-06-06', 64000.00),
('Fatima', 'fatima@example.com', 'Female', '1993-03-03', 55000.00);

-- UPDATE users SET name = 'Ankit', email = 'Ankit@gmail.com' WHERE id = 1;
    DELETE FROM users WHERE id = 3;

SELECT * FROM users;

Increase salary by ₹10,000 for all users whose salary is less than ₹60,000.

UPDATE users
SET salary = salary + 10000
WHERE salary < 60000;

Set the gender of user Ommm to Other.

---

UPDATE users
SET gender = 'Other'
WHERE name = 'Ommm';

Reset salary of all users to ₹50,000 (Careful - affects all rows).

UPDATE users
SET salary = 50000;

-- Update the user with ID 1, set their name and email to new values
UPDATE users 
SET name = 'Ankit', email = 'Ankit@gmail.com' 
WHERE id = 1;

-- Delete the user who has ID 3
DELETE FROM users 
WHERE id = 3;

-- Check if the user with ID 3 still exists (should return nothing if deleted)
SELECT * FROM users 
WHERE id = 3;

-- Delete all users where salary is empty (NULL)
DELETE FROM users 
WHERE salary IS NULL;

-- Delete all users whose salary is less than 50000
DELETE FROM users 
WHERE salary < 50000;

MySQL Constraints:-

CREATE TABLE users (
    -- id will auto-increase for each new user
    id INT AUTO_INCREMENT PRIMARY KEY,  

    -- name cannot be empty (NOT NULL)
    name VARCHAR(100) NOT NULL,           

    -- email must be unique for each user
    email VARCHAR(100) UNIQUE,             

    -- user must be born after the year 2000 (real check)
    date_of_birth DATE,                    
    CONSTRAINT chk_dob CHECK (date_of_birth > '2000-01-01'),

    -- gender has default value 'Other' if not given
    gender ENUM('Male', 'Female', 'Other') DEFAULT 'Other',  

    -- user is active by default
    is_active BOOLEAN DEFAULT TRUE,         

    -- salary can be any number, but we will use this later in queries
    salary DECIMAL(10,2)                   
);

CREATE DATABASE IF NOT EXISTS db;
USE db;
DROP TABLE IF EXISTS users;
CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100),
    gender ENUM('Male', 'Female', 'Other'),
    email VARCHAR(100),
    salary DECIMAL(10, 2),
    date_of_birth DATE
);

INSERT INTO users (name, gender, email, salary, date_of_birth) VALUES 
('Ankit', 'Male', 'ankit@gmail.com', 60000, '1996-08-10'),
('Priya', 'Female', 'priya@example.com', 55000, '1998-04-22'),
('Ravi', 'Male', 'ravi@company.com', 72000, '1995-01-15'),
('Neha', 'Female', 'neha@work.com', 50000, '1997-12-05'),
('Rahul', 'Male', 'rahul@webmail.com', 65000, '1994-07-30'),
('Aditi', 'Female', 'aditi@domain.com', 48000, '1999-06-18'),
('Kiran', 'Other', 'kiran@misc.com', 53000, '2000-03-10');

-- --------------------------------------------
-- 5. Aggregate Functions
-- --------------------------------------------

SELECT COUNT(*) AS total_users FROM users; 
-- Output: 7 users total

SELECT COUNT(*) AS female_users FROM users WHERE gender = 'Female'; 
-- Output: 3 female users

SELECT MIN(salary) AS min_salary, MAX(salary) AS max_salary FROM users; 
-- Output: Min = 48000.00, Max = 72000.00

SELECT SUM(salary) AS total_payroll FROM users;
-- Output: 403000.00 (total of all salaries)

SELECT AVG(salary) AS avg_salary FROM users;
-- Output: 57642.86 (average salary)

SELECT gender, AVG(salary) AS avg_salary FROM users GROUP BY gender;
-- Output:
-- Male   → 65666.67
-- Female → 51000.00
-- Other  → 53000.00

-- --------------------------------------------
-- 6. String Functions
-- --------------------------------------------

SELECT name, LENGTH(name) AS name_length FROM users;
-- Output: Ankit → 5, Priya → 5, etc.

SELECT name, LOWER(name) AS lowercase_name FROM users;
-- Output: Ankit → ankit, Priya → priya, etc.

SELECT name, UPPER(name) AS uppercase_name FROM users;
-- Output: Ankit → ANKIT, etc.

SELECT CONCAT(name, ' <', email, '>') AS user_contact FROM users;
-- Output: Ankit <ankit@gmail.com>, etc.

SELECT name, LEFT(name, 3) AS first_3_letters FROM users;
-- Output: Ankit → Ank, Priya → Pri, etc.

SELECT email, RIGHT(email, 4) AS email_end FROM users;
-- Output: .com

SELECT name, SUBSTRING(name, 2, 3) AS mid_part FROM users;
-- Output: Ankit → nki, Priya → riy, etc.

SELECT email, REPLACE(email, '@', '[at]') AS safe_email FROM users;
-- Output: ankit[at]gmail.com, etc.

SELECT email, INSTR(email, '@') AS at_position FROM users;
-- Output: Position of '@' in email, e.g., 6

SELECT name, REVERSE(name) AS reversed_name FROM users;
-- Output: Ankit → tiknA, Priya → ayirP, etc.

-- --------------------------------------------
-- 7. Date Functions
-- --------------------------------------------

SELECT NOW() AS current_time;
-- Output: Current system date and time

SELECT name, YEAR(date_of_birth) AS birth_year FROM users;
-- Output: Ankit → 1996, Priya → 1998, etc.

SELECT name, DATEDIFF(CURDATE(), date_of_birth) AS days_lived FROM users;
-- Output: Ankit → approx 10500+ days, etc.

SELECT name, TIMESTAMPDIFF(YEAR, date_of_birth, CURDATE()) AS age FROM users;
-- Output: Ankit → 28 (as of 2025), etc.

-- --------------------------------------------
-- 8. Mathematical Functions
-- --------------------------------------------

SELECT salary,
       ROUND(salary) AS rounded,
       FLOOR(salary) AS floored,
       CEIL(salary) AS ceiled
FROM users;
-- Output: Rounded, floored, ceiled values of each salary (usually same here)

SELECT id, MOD(id, 2) AS remainder FROM users;
-- Output: 0 for even id, 1 for odd id

-- --------------------------------------------
-- 9. Conditional Functions
-- --------------------------------------------

SELECT name, gender,
       IF(gender = 'Female', 'Yes', 'No') AS is_female
FROM users;
-- Output: Priya → Yes, Ankit → No, etc.

---

---

MySQL Transactions and AutoCommit:-

• Transaction means running a group of SQL commands together, and we can save or cancel them using COMMIT or ROLLBACK.

• AutoCommit means MySQL automatically saves your changes after each SQL command.

-- Step 1: Create the 'users' table
CREATE TABLE users (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    salary DECIMAL(10,2)
);

-- Step 2: Insert sample data
INSERT INTO users (id, name, salary) VALUES
(1, 'Alice', 60000.00),
(2, 'Bob', 55000.00),
(3, 'Charlie', 70000.00),
(4, 'Diana', 75000.00),
(5, 'Evan', 50000.00);

-- Step 3: View initial data
SELECT * FROM users;

-- Output:
/*
+----+---------+----------+
| id | name    | salary   |
+----+---------+----------+
|  1 | Alice   | 60000.00 |
|  2 | Bob     | 55000.00 |
|  3 | Charlie | 70000.00 |
|  4 | Diana   | 75000.00 |
|  5 | Evan    | 50000.00 |
+----+---------+----------+
*/

-- Step 4: Turn off AutoCommit
SET autocommit = 0;

-- Step 5: Start a transaction (optional but recommended)
START TRANSACTION;

-- Step 6: Update Evan’s salary to 80000 (temporary change)
UPDATE users SET salary = 80000 WHERE id = 5;

-- Step 7: Check Evan’s salary before deciding
SELECT * FROM users WHERE id = 5;

-- Output:
/*
+----+-------+----------+
| id | name  | salary   |
+----+-------+----------+
|  5 | Evan  | 80000.00 |  <-- Updated but NOT saved yet
+----+-------+----------+
*/

-- Step 8: Something is wrong — rollback the change
ROLLBACK;

-- Step 9: Check Evan’s salary again after rollback
SELECT * FROM users WHERE id = 5;

-- Output:
/*
+----+-------+----------+
| id | name  | salary   |
+----+-------+----------+
|  5 | Evan  | 50000.00 |  <-- Back to original value
+----+-------+----------+
*/

-- Step 10: Turn AutoCommit back on (optional)
SET autocommit = 1;

-- Step 11: Final check – View all data
SELECT * FROM users;

-- Output:
/*
+----+---------+----------+
| id | name    | salary   |
+----+---------+----------+
|  1 | Alice   | 60000.00 |
|  2 | Bob     | 55000.00 |
|  3 | Charlie | 70000.00 |
|  4 | Diana   | 75000.00 |
|  5 | Evan    | 50000.00 |  <-- No change saved
+----+---------+----------+
*/

• SET autocommit = 0; → Turn off auto-save, you control when to save.

• UPDATE → Make a change in the data.

• COMMIT; → Save your changes permanently.

• ROLLBACK; → Undo your changes, don’t save.

• SET autocommit = 1; → Turn auto-save back on.

Can I Drop a PRIMARY KEY?

Yes, you can drop a primary key in MySQL using:

ALTER TABLE users DROP PRIMARY KEY;

Foreign key:-

• In Orders table, customer_id is the foreign key — it points to the customer in the Customers table.

-- Create Customers table (Parent table)
CREATE TABLE customers (
    customer_id INT PRIMARY KEY,
    name VARCHAR(50)
);

-- Create Orders table (Child table) with foreign key to Customers
CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    order_date DATE,
    customer_id INT,
    FOREIGN KEY (customer_id) REFERENCES customers(customer_id)
);

-- Insert data into Customers
INSERT INTO customers VALUES
(1, 'Alice'),
(2, 'Bob');

-- Insert data into Orders linked to Customers
INSERT INTO orders VALUES
(101, '2025-07-27', 1),  -- Alice's order
(102, '2025-07-28', 2);  -- Bob's order

-- Check Customers table
SELECT * FROM customers;
-- Output:
-- +-------------+-------+
-- | customer_id | name  |
-- +-------------+-------+
-- |           1 | Alice |
-- |           2 | Bob   |
-- +-------------+-------+

-- Check Orders table
SELECT * FROM orders;
-- Output:
-- +----------+------------+-------------+
-- | order_id | order_date | customer_id |
-- +----------+------------+-------------+
-- |      101 | 2025-07-27 |           1 |
-- |      102 | 2025-07-28 |           2 |
-- +----------+------------+-------------+

-- Trying to insert an order with a customer_id that doesn't exist will fail
-- INSERT INTO orders VALUES (103, '2025-07-29', 3);
-- Error: Cannot add or update a child row: a foreign key constraint fails

SQL JOINs in MySQL:-

• JOIN = connect two tables using a common column to get related data together.

-- ----------------------------------------
-- 1. Create Sample Tables
-- ----------------------------------------

-- Drop tables if they already exist
DROP TABLE IF EXISTS Employees;
DROP TABLE IF EXISTS Departments;

-- Employees Table
CREATE TABLE Employees (
    EmpID INT PRIMARY KEY,
    EmpName VARCHAR(100),
    DeptID INT
);

-- Departments Table
CREATE TABLE Departments (
    DeptID INT PRIMARY KEY,
    DeptName VARCHAR(100)
);

-- ----------------------------------------
-- 2. Insert Sample Data
-- ----------------------------------------

-- Insert Employees
INSERT INTO Employees (EmpID, EmpName, DeptID) VALUES
(1, 'Alice', 10),
(2, 'Bob', 20),
(3, 'Charlie', 30),
(4, 'Diana', NULL),
(5, 'Eve', 40);  -- No matching department

-- Insert Departments
INSERT INTO Departments (DeptID, DeptName) VALUES
(10, 'HR'),
(20, 'Engineering'),
(30, 'Marketing'),
(50, 'Sales'); -- No matching employee

-- ----------------------------------------
-- 3. INNER JOIN: Only matching rows
-- ----------------------------------------

SELECT 
    E.EmpID, E.EmpName, D.DeptName
FROM 
    Employees E
INNER JOIN 
    Departments D ON E.DeptID = D.DeptID;

-- ✅ Shows only employees that belong to existing departments

-- ----------------------------------------
-- 4. LEFT JOIN: All employees, matched or not
-- ----------------------------------------

SELECT 
    E.EmpID, E.EmpName, D.DeptName
FROM 
    Employees E
LEFT JOIN 
    Departments D ON E.DeptID = D.DeptID;

-- ✅ Shows all employees, including those without a department (NULLs for Dept)

-- ----------------------------------------
-- 5. RIGHT JOIN: All departments, matched or not
-- ----------------------------------------

SELECT 
    E.EmpName, D.DeptName
FROM 
    Employees E
RIGHT JOIN 
    Departments D ON E.DeptID = D.DeptID;

-- ✅ Shows all departments, including those with no employees

-- ----------------------------------------
-- 6. FULL OUTER JOIN: All rows from both sides
-- ----------------------------------------
-- Note: Not all SQL databases (like MySQL) support FULL OUTER JOIN natively.
-- You can simulate it using UNION

SELECT 
    E.EmpName, D.DeptName
FROM 
    Employees E
LEFT JOIN 
    Departments D ON E.DeptID = D.DeptID

UNION

SELECT 
    E.EmpName, D.DeptName
FROM 
    Employees E
RIGHT JOIN 
    Departments D ON E.DeptID = D.DeptID;

-- ✅ Shows all combinations: matched and unmatched from both tables

-- ----------------------------------------
-- 7. CROSS JOIN: Cartesian product (every row with every other)
-- ----------------------------------------

SELECT 
    E.EmpName, D.DeptName
FROM 
    Employees E
CROSS JOIN 
    Departments D;

Employees Table

Departments Table

---

INNER JOIN:-

SELECT E.EmpID, E.EmpName, D.DeptName
FROM Employees E
INNER JOIN Departments D ON E.DeptID = D.DeptID;

LEFT JOIN:-

SELECT E.EmpID, E.EmpName, D.DeptName
FROM Employees E
LEFT JOIN Departments D ON E.DeptID = D.DeptID;

RIGHT JOIN:-

SELECT E.EmpName, D.DeptName
FROM Employees E
RIGHT JOIN Departments D ON E.DeptID = D.DeptID;

FULL OUTER JOIN (Simulated with UNION):-

SELECT E.EmpName, D.DeptName
FROM Employees E
LEFT JOIN Departments D ON E.DeptID = D.DeptID
UNION
SELECT E.EmpName, D.DeptName
FROM Employees E
RIGHT JOIN Departments D ON E.DeptID = D.DeptID;

CROSS JOIN:-

SELECT E.EmpName, D.DeptName
FROM Employees E
CROSS JOIN Departments D;

---

Summary Table

UNION ALL:-

SELECT E.EmpName, D.DeptName
FROM Employees E
LEFT JOIN Departments D ON E.DeptID = D.DeptID

UNION ALL

SELECT E.EmpName, D.DeptName
FROM Employees E
RIGHT JOIN Departments D ON E.DeptID = D.DeptID
WHERE E.DeptID IS NULL; -- Avoid double-counting matches

• UNION: Removes duplicates between the two queries.

• UNION ALL: Includes all rows, including duplicates.

Self JOIN:-

-- Create the table
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    manager_id INT
);

-- Insert sample data
INSERT INTO employees (id, name, manager_id) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 2);

-- Self join: get employees and their manager's name
SELECT
    e.id AS employee_id,
    e.name AS employee_name,
    m.name AS manager_name
FROM
    employees e
LEFT JOIN
    employees m ON e.manager_id = m.id;

Project1:-

SELECT DISTINCT name_hos       -- Get different hospital names  
FROM pre                       -- From the table called 'pre'  
WHERE name_hos IS NOT NULL     -- Only names that are not empty  
  AND TRIM(name_hos) != ''     -- And not just spaces  
ORDER BY name_hos;             -- Put the names in order  
-----------------------------------------------------------------------------------------------------
--SELECT p51.basic_diagnosis, COUNT(DISTINCT pre.pat_id) AS patient_count
    --FROM pre
    --JOIN demohealth.p51 AS p51 ON pre.preid = p51.preid
    --WHERE pre.name_hos = ?
      --AND (
       -- DATE(pre.entrydate) BETWEEN ? AND ?
        --OR DATE(pre.apptdate) BETWEEN ? AND ?
     -- )
    --GROUP BY p51.basic_diagnosis
    --ORDER BY patient_count DESC
----------------------------------------
SELECT p51.basic_diagnosis,               -- Get the diagnosis name  
       COUNT(DISTINCT pre.pat_id) AS patient_count  -- Count unique patients for each diagnosis  
FROM pre                                  -- From the table named 'pre'  
JOIN demohealth.p51 AS p51 ON pre.preid = p51.preid  -- Join with 'p51' table where preid matches  
WHERE pre.name_hos = 'VBCR019'            -- Only for hospital named 'VBCR019'  
  AND (                                  
    DATE(pre.entrydate) BETWEEN '2013-01-01' AND '2024-12-31'  -- Dates between 2013 and 2024 for entry date  
    OR DATE(pre.apptdate) BETWEEN '2013-01-01' AND '2024-12-31'  -- Or dates between 2013 and 2024 for appointment date  
  )  
GROUP BY p51.basic_diagnosis              -- Group results by diagnosis  
ORDER BY patient_count DESC;              -- Show diagnoses with most patients first  
-----------------------------------------------------------------------------------------------------
--SELECT pre.pat_id, p51.basic_diagnosis, p51.value, MIN(pre.preid) AS preid
    --FROM pre
   -- JOIN demohealth.p51 AS p51 ON pre.preid = p51.preid
   -- WHERE pre.name_hos = ?
     -- AND p51.basic_diagnosis = ?
     -- AND (
       -- DATE(pre.entrydate) BETWEEN ? AND ?
       -- OR DATE(pre.apptdate) BETWEEN ? AND ?
      --)
   -- GROUP BY pre.pat_id, p51.basic_diagnosis, p51.value
   -- ORDER BY MIN(pre.entrydate) DESC
----------------------------------------
SELECT pre.pat_id,                       -- Patient ID  
       p51.basic_diagnosis,              -- Diagnosis name  
       p51.value,                        -- Related value for diagnosis  
       MIN(pre.preid) AS preid           -- Smallest preid for this patient and diagnosis  
FROM pre  
JOIN demohealth.p51 AS p51 ON pre.preid = p51.preid  -- Join on preid  
WHERE pre.name_hos = 'VBCR019'      -- Hospital name to filter  
  AND p51.basic_diagnosis = 'DM'        -- Diagnosis to filter  
  AND (                                  
    DATE(pre.entrydate) BETWEEN '2020-01-01' AND '2024-12-31'  -- Entry dates between Jan 1, 2020 and Dec 31, 2024  
    OR DATE(pre.apptdate) BETWEEN '2020-01-01' AND '2024-12-31' -- Or appointment dates in the same range  
  )  
GROUP BY pre.pat_id, p51.basic_diagnosis, p51.value  -- Group by patient, diagnosis, value  
ORDER BY MIN(pre.entrydate) DESC;        -- Sort by earliest entry date, newest first  
-----------------------------------------------------------------------------------------------------

Project2:-

SELECT DISTINCT name_hos              -- Get only different (unique) hospital names  
FROM healthcaredb.pre                 -- From the 'pre' table in the 'healthcaredb' database  
ORDER BY name_hos;                    -- Sort the hospital names in alphabetical order  
-----------------------------------------------------------------------------------------------------------------------
SELECT DISTINCT basic_diagnosis              -- Get only unique diagnosis names  
FROM demohealth.p51                          -- From the 'p51' table in the 'demohealth' database  
ORDER BY basic_diagnosis;                    -- Sort the diagnosis names alphabetically  
----------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------------

 --SELECT DISTINCT pr.preid
   -- FROM healthcaredb.pre pr
    --JOIN demohealth.p51 p ON pr.preid = p.preid
    --WHERE TRIM(UPPER(p.basic_diagnosis)) = TRIM(UPPER(?))
      --AND TRIM(UPPER(pr.name_hos)) = TRIM(UPPER(?))
      --AND pr.entrydate BETWEEN ? AND ?
-----------------------------------------------
SELECT DISTINCT pr.preid                            -- Get unique preid values
FROM healthcaredb.pre pr                            -- From the 'pre' table
JOIN demohealth.p51 p ON pr.preid = p.preid         -- Join with 'p51' table on preid
WHERE TRIM(UPPER(p.basic_diagnosis)) = TRIM(UPPER('DM'))  -- Filter diagnosis (e.g., 'Diabetes')
  AND TRIM(UPPER(pr.name_hos)) = TRIM(UPPER('VBCR019'))   -- Filter hospital name (e.g., 'City Hospital')
  AND pr.entrydate BETWEEN '2016-03-15' AND '2024-11-30';       -- Random date range between 2013 and 2024
----------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------------

--SELECT DISTINCT basic_diagnosis AS disease
    --FROM demohealth.p51
    --WHERE preid = ?

SELECT DISTINCT basic_diagnosis AS disease
FROM demohealth.p51
WHERE preid = 1713347150718;
-----------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------------

--SELECT DISTINCT pr.preid
    --FROM healthcaredb.pre pr
    --JOIN demohealth.p51 p ON pr.preid = p.preid
    --WHERE TRIM(UPPER(p.basic_diagnosis)) = TRIM(UPPER(?))
     -- AND TRIM(UPPER(pr.name_hos)) = TRIM(UPPER(?))
     -- AND pr.entrydate BETWEEN ? AND ?
----------------------------------------------------

SELECT DISTINCT pr.preid                     -- Select unique preid values (no duplicates)
FROM healthcaredb.pre pr                     -- From the 'pre' table in 'healthcaredb' (alias pr)
JOIN demohealth.p51 p ON pr.preid = p.preid  -- Join with 'p51' table on matching preid
WHERE TRIM(UPPER(p.basic_diagnosis)) = TRIM(UPPER('CMD'))  -- Filter by diagnosis 'Diabetes' (case-insensitive, trimmed)
  AND TRIM(UPPER(pr.name_hos)) = TRIM(UPPER('VBCR019'))   -- Filter by hospital name 'City Hospital' (case-insensitive, trimmed)
  AND pr.entrydate BETWEEN '2015-01-01' AND '2024-12-31';        -- Filter records with entrydate between Jan 1, 2015 and Dec 31, 2023
----------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------------
--SELECT basic_diagnosis AS disease, COUNT(DISTINCT preid) AS count
      --FROM demohealth.p51
      --WHERE preid IN (${placeholders})
        --AND TRIM(UPPER(basic_diagnosis)) != TRIM(UPPER(?))
        --AND TRIM(UPPER(basic_diagnosis)) != 'OTHERS'
      --GROUP BY basic_diagnosis
      --ORDER BY count DESC
      --LIMIT ?-
----------------------------------------
SELECT basic_diagnosis AS disease,                -- Rename 'basic_diagnosis' column as 'disease'
       COUNT(DISTINCT preid) AS count             -- Count how many unique 'preid' values for each diagnosis
FROM demohealth.p51                               -- Use the 'p51' table from 'demohealth' database
WHERE preid IN (1001, 1002, 1003, 1004)           -- Filter rows where 'preid' is in this list (replace with real IDs)
  AND TRIM(UPPER(basic_diagnosis)) != TRIM(UPPER('Flu'))  -- Exclude the diagnosis 'Flu' (case-insensitive)
  AND TRIM(UPPER(basic_diagnosis)) != 'OTHERS'           -- Also exclude the diagnosis labeled as 'OTHERS'
GROUP BY basic_diagnosis                          -- Group results by diagnosis
ORDER BY count DESC                               -- Show diagnoses with the most patients first
LIMIT 5;                                          -- Show only the top 5 results
----------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------------