SQL Cheat Sheet
Common SQL queries, joins, clauses, and database operations.
2 PagesBeginnerMay 15, 2026
SELECT Queries
Retrieve data from tables.
sql
SELECT * FROM users;SELECT name, email FROM users WHERE age > 18;SELECT * FROM users ORDER BY name ASC LIMIT 10;SELECT COUNT(*) FROM orders WHERE status = 'paid';
JOINs
Combine rows from multiple tables.
sql
-- INNER JOINSELECT u.name, o.totalFROM users uINNER JOIN orders o ON u.id = o.user_id;-- LEFT JOIN (all users even without orders)SELECT u.name, o.totalFROM users uLEFT JOIN orders o ON u.id = o.user_id;
CRUD Operations
Create, read, update, and delete.
sql
INSERT INTO users (name, email) VALUES ('Alice', '[email protected]');UPDATE users SET email = '[email protected]' WHERE id = 1;DELETE FROM users WHERE id = 1;
GROUP BY & Aggregates
Summarize rows with aggregate functions.
sql
SELECT department, COUNT(*) AS headcount, AVG(salary) AS avg_salary, MAX(salary) AS top_salaryFROM employeesWHERE active = 1GROUP BY departmentHAVING COUNT(*) > 5ORDER BY avg_salary DESC;
Subqueries & CTEs
Nested queries and readable common table expressions.
sql
-- subquerySELECT name FROM employeesWHERE salary > (SELECT AVG(salary) FROM employees);-- CTEWITH dept_avg AS ( SELECT department, AVG(salary) AS avg_sal FROM employees GROUP BY department)SELECT e.name FROM employees eJOIN dept_avg d ON e.department = d.departmentWHERE e.salary > d.avg_sal;
Window Functions
Rankings and running totals without collapsing rows.
sql
SELECT name, department, salary, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rank_in_dept, SUM(salary) OVER (PARTITION BY department) AS dept_total, LAG(salary) OVER (ORDER BY hire_date) AS prev_hire_salaryFROM employees;
Indexes & Constraints
Schema features for integrity and performance.
- PRIMARY KEY- uniquely identifies each row; implies NOT NULL + unique index
- FOREIGN KEY- enforces a reference to a key in another table
- UNIQUE- prevents duplicate values in a column or set of columns
- CREATE INDEX- speeds up lookups/joins on a column at write-time cost
- CHECK- validates a boolean condition on inserted/updated rows
Pro Tip
Always use parameterized queries or prepared statements to prevent SQL injection attacks.
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