520
1. SQL Syntax Overview
SQL (Structured Query Language) is used to manage and manipulate databases.
2. Database and Table Management
1. Create a Database
CREATE DATABASE my_database;
2. Delete a Database
DROP DATABASE my_database;
3. Use a Database
USE my_database;
4. Create a Table
CREATE TABLE employees (
id INT PRIMARY KEY,
name VARCHAR(50),
position VARCHAR(50),
salary DECIMAL(10, 2)
);
5. Delete a Table
DROP TABLE employees;
6. Modify a Table
Add a Column:
ALTER TABLE employees ADD age INT;
Delete a Column:
ALTER TABLE employees DROP COLUMN age;
Rename a Column:
ALTER TABLE employees RENAME COLUMN name TO full_name;
3. Inserting Data
1. Insert Single Row
INSERT INTO employees (id, name, position, salary) VALUES (1, 'Alice', 'Manager', 75000);
2. Insert Multiple Rows
INSERT INTO employees (id, name, position, salary) VALUES (2, 'Bob', 'Developer', 60000), (3, 'Carol', 'Analyst', 55000);
4. Querying Data (SELECT)
1. Select All Columns
SELECT * FROM employees;
2. Select Specific Columns
SELECT name, salary FROM employees;
3. Rename Columns (Alias)
SELECT name AS employee_name, salary AS earnings FROM employees;
4. Distinct Values
SELECT DISTINCT position FROM employees;
5. Filtering Data (WHERE Clause)
1. Basic WHERE Clause
SELECT * FROM employees WHERE salary > 60000;
2. Multiple Conditions (AND/OR)
SELECT * FROM employees WHERE salary > 50000 AND position = 'Developer';
3. Range Filter (BETWEEN)
SELECT * FROM employees WHERE salary BETWEEN 50000 AND 70000;
4. Set Membership (IN)
SELECT * FROM employees
WHERE position IN ('Manager', 'Analyst');
5. Pattern Matching (LIKE)
SELECT * FROM employees WHERE name LIKE 'A%'; -- Names starting with A
- % – Matches any number of characters
- _ – Matches a single character
6. Sorting Data (ORDER BY)
1. Sort by Single Column
SELECT * FROM employees ORDER BY salary DESC;
2. Sort by Multiple Columns
SELECT * FROM employees ORDER BY position ASC, salary DESC;
7. Limiting and Paginating Results
1. Limit the Number of Results
SELECT * FROM employees LIMIT 5;
2. Paginate Results (OFFSET)
SELECT * FROM employees LIMIT 5 OFFSET 10;
8. Aggregation and Grouping
1. Aggregate Functions
SELECT COUNT(*) FROM employees; -- Count SELECT AVG(salary) FROM employees; -- Average SELECT MAX(salary) FROM employees; -- Maximum SELECT MIN(salary) FROM employees; -- Minimum SELECT SUM(salary) FROM employees; -- Sum
2. Grouping Data (GROUP BY)
SELECT position, AVG(salary) FROM employees GROUP BY position;
3. Filtering Groups (HAVING)
SELECT position, COUNT(*) FROM employees GROUP BY position HAVING COUNT(*) > 1;
9. Updating Data
1. Update a Single Row
UPDATE employees SET salary = 80000 WHERE id = 1;
2. Update Multiple Rows
UPDATE employees SET position = 'Senior Developer' WHERE position = 'Developer';
10. Deleting Data
1. Delete Specific Rows
DELETE FROM employees WHERE id = 3;
2. Delete All Rows
DELETE FROM employees;
11. Table Joins
1. Inner Join
SELECT employees.name, departments.name FROM employees INNER JOIN departments ON employees.department_id = departments.id;
2. Left Join
SELECT employees.name, departments.name FROM employees LEFT JOIN departments ON employees.department_id = departments.id;
3. Right Join
SELECT employees.name, departments.name FROM employees RIGHT JOIN departments ON employees.department_id = departments.id;
4. Full Outer Join
SELECT employees.name, departments.name FROM employees FULL OUTER JOIN departments ON employees.department_id = departments.id;
12. Subqueries
1. Subquery in WHERE Clause
SELECT name, salary FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);
2. Subquery in SELECT
SELECT name,
(SELECT AVG(salary) FROM employees) AS average_salary
FROM employees;
13. Indexing
1. Create an Index
CREATE INDEX idx_salary ON employees(salary);
2. Drop an Index
DROP INDEX idx_salary;
14. Backup and Restore
1. Backup a Database
mysqldump -u username -p my_database > backup.sql
2. Restore a Database
mysql -u username -p my_database < backup.sql
15. Common SQL Functions
String Functions
SELECT UPPER(name) FROM employees; -- Uppercase SELECT LOWER(name) FROM employees; -- Lowercase SELECT LENGTH(name) FROM employees; -- String Length
Date Functions
SELECT NOW(); -- Current date and time SELECT YEAR(hire_date); -- Extract Year SELECT DATEDIFF(NOW(), hire_date) FROM employees; -- Date difference
Tips for SQL
- Always backup your database before performing large operations.
- Use LIMIT to avoid selecting large datasets unintentionally.
- Test queries in a sandbox environment before applying to production.
- Use JOINs to efficiently combine data from multiple tables.