Structured Query Language (SQL) is the standard language used for managing and manipulating databases.
This SQL Cheatsheet is your complete quick reference for creating, updating, and managing data — whether you use MySQL, SQL Server, PostgreSQL, or Oracle.
Table of Contents:
SQL Basics for Beginners
| Command | Purpose | Example |
|---|---|---|
CREATE DATABASE | Creates a new database | CREATE DATABASE company; |
USE | Selects a database | USE company; |
CREATE TABLE | Creates a new table | CREATE TABLE employees (id INT, name VARCHAR(50)); |
DROP TABLE | Deletes a table | DROP TABLE employees; |
ALTER TABLE | Modifies a table structure | ALTER TABLE employees ADD salary INT; |
SQL Data Types
| Category | Data Types | Example |
|---|---|---|
| Numeric | INT, FLOAT, DECIMAL | salary DECIMAL(10,2) |
| String | CHAR, VARCHAR, TEXT | name VARCHAR(100) |
| Date/Time | DATE, TIME, DATETIME, TIMESTAMP | created_at DATETIME |
| Boolean | BOOLEAN, BIT | is_active BOOLEAN |
Inserting and Updating Data in SQL
Insert Data into a Table
INSERT INTO employees (id, name, salary)
VALUES (1, 'John Doe', 60000);
Update Existing Records
UPDATE employees
SET salary = 70000
WHERE id = 1;
Delete a Record
DELETE FROM employees
WHERE id = 1;
SQL SELECT Statement Explained
The SELECT statement is used to retrieve data from one or more tables.
SELECT name, salary FROM employees;
Aliases Example:
SELECT name AS EmployeeName, salary AS MonthlySalary FROM employees;
SQL WHERE Clause
Used to filter records based on a condition.
SELECT * FROM employees WHERE salary > 50000;
Common Operators:=, !=, >, <, >=, <=, BETWEEN, IN, LIKE, IS NULL
Example:
SELECT * FROM employees WHERE name LIKE 'J%';
SQL ORDER BY Clause
Sorts the result in ascending (ASC) or descending (DESC) order.
SELECT * FROM employees ORDER BY salary DESC;
SQL GROUP BY and HAVING Clauses
GROUP BY groups similar data.
HAVING filters aggregated data.
SELECT department, COUNT(*) AS total_employees
FROM employees
GROUP BY department
HAVING COUNT(*) > 5;
SQL Joins — Combining Data from Multiple Tables
| Type | Description | Example |
|---|---|---|
| INNER JOIN | Returns matching records from both tables | SELECT * FROM employees e INNER JOIN departments d ON e.dept_id = d.id; |
| LEFT JOIN | All records from left + matched from right | SELECT * FROM employees e LEFT JOIN departments d ON e.dept_id = d.id; |
| RIGHT JOIN | All records from right + matched from left | SELECT * FROM employees e RIGHT JOIN departments d ON e.dept_id = d.id; |
| FULL JOIN | All records when there is a match | SELECT * FROM employees e FULL JOIN departments d ON e.dept_id = d.id; |
SQL Aggregate Functions
| Function | Description | Example |
|---|---|---|
COUNT() | Returns number of rows | SELECT COUNT(*) FROM employees; |
SUM() | Returns total sum | SELECT SUM(salary) FROM employees; |
AVG() | Returns average value | SELECT AVG(salary) FROM employees; |
MIN() | Returns minimum value | SELECT MIN(salary) FROM employees; |
MAX() | Returns maximum value | SELECT MAX(salary) FROM employees; |
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SQL Constraints
| Constraint | Purpose | Example |
|---|---|---|
PRIMARY KEY | Uniquely identifies each record | id INT PRIMARY KEY |
FOREIGN KEY | Links to another table | FOREIGN KEY (dept_id) REFERENCES departments(id) |
NOT NULL | Ensures column cannot be null | name VARCHAR(50) NOT NULL |
UNIQUE | Prevents duplicate values | email VARCHAR(100) UNIQUE |
CHECK | Validates data | CHECK (salary > 0) |
DEFAULT | Sets default value | status VARCHAR(10) DEFAULT 'Active' |
SQL Subqueries
A subquery is a query inside another query.
SELECT name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);
SQL Views
A view is a virtual table.
CREATE VIEW high_salary AS
SELECT name, salary FROM employees WHERE salary > 70000;
SQL Indexes
Indexes improve query performance.
CREATE INDEX idx_name ON employees(name);
SQL Transactions
Used for safe execution of multiple statements.
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;
Use ROLLBACK; to undo changes.
SQL Interview Questions
Q1. What is the difference between WHERE and HAVING?
➡️ WHERE filters rows before grouping, HAVING filters after grouping.
Q2. What is normalization?
➡️ The process of organizing data to reduce redundancy.
Q3. What is a foreign key?
➡️ It’s a constraint that creates a relationship between two tables.
Q4. What is the difference between INNER JOIN and OUTER JOIN?
➡️ INNER JOIN returns matched rows only, while OUTER JOIN returns all rows including unmatched ones.
FAQ — SQL Cheatsheet
Q1: What is SQL used for?
SQL is used to store, retrieve, and manipulate data in relational databases.
Q2: Which databases use SQL?
MySQL, PostgreSQL, SQL Server, Oracle, and SQLite all use SQL syntax.
Q3: Is SQL case-sensitive?
SQL keywords are not case-sensitive, but string comparisons may be.
Q4: What are DDL, DML, and DCL commands?
- DDL: Data Definition Language (
CREATE,ALTER,DROP) - DML: Data Manipulation Language (
INSERT,UPDATE,DELETE) - DCL: Data Control Language (
GRANT,REVOKE)
