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🗄️ SQL Cheatsheet

SQL command reference for SELECT queries, filtering, joins, aggregation, schema changes, and performance tips.

SQL query patterns that power dashboards, reporting, and migrations. Copy the essentials for filtering, joining, aggregating, and managing schema.

SELECT Essentials

Build reliable projections and calculations.

Query	Purpose	Notes
`SELECT * FROM customers LIMIT 50;`	Sample data	Use LIMIT for fast previews; replace * with named columns in production.
`SELECT name, email FROM users ORDER BY created_at DESC;`	Sorted projection	ORDER BY supports ASC/DESC and multi-column priority.
`SELECT DISTINCT country FROM orders;`	Unique values	DISTINCT applies to the full set of selected columns.
`SELECT NOW() AS server_time;`	Built-in functions	Popular helpers: NOW(), CURRENT_DATE, LOWER(), UPPER(), LENGTH().

Target precise records with WHERE clauses.

Query	Purpose	Notes
`SELECT * FROM orders WHERE status = 'paid';`	Equality filter	Use parameterized queries to avoid SQL injection.
`SELECT * FROM orders WHERE total BETWEEN 50 AND 200;`	Range filter	BETWEEN is inclusive; combine with decimals for price bands.
`SELECT * FROM users WHERE email LIKE '%@toolhop.app';`	Pattern search	Escape % or _ when matching literal characters.
`SELECT * FROM events WHERE occurred_at >= NOW() - INTERVAL '7 days';`	Relative time	Intervals vary by database (INTERVAL, DATEADD, DATE_SUB).

Combine tables and summarize metrics.

Query	Purpose	Notes
`SELECT * FROM orders o JOIN customers c ON c.id = o.customer_id;`	Inner join	Return rows where both tables match.
`SELECT * FROM posts p LEFT JOIN comments c ON c.post_id = p.id;`	Left join	Return all posts even when comments are missing.
`SELECT city, COUNT(*) FROM users GROUP BY city;`	Aggregation	Columns in SELECT must be aggregated or appear in GROUP BY.
`SELECT city, COUNT() FROM users GROUP BY city HAVING COUNT() > 100;`	Aggregation filter	HAVING filters aggregated results after GROUP BY.

DDL commands you reach for during migrations.

Define columns, constraints, and default values.

CREATE TABLE accounts (
  id SERIAL PRIMARY KEY,
  email TEXT UNIQUE NOT NULL,
  created_at TIMESTAMPTZ DEFAULT NOW()
);

Add/remove columns or constraints with ALTER.

ALTER TABLE accounts ADD COLUMN last_login TIMESTAMPTZ;

Add indexes to columns used in joins or WHERE filters.

CREATE INDEX idx_orders_customer_id ON orders (customer_id);

UPSERT prevents duplicate keys and keeps counts fresh.

INSERT INTO inventory (sku, qty) VALUES ($1, $2)
ON CONFLICT (sku) DO UPDATE SET qty = excluded.qty;

Keep queries fast and resource-friendly.

Explain plans
Use EXPLAIN (ANALYZE, BUFFERS) SELECT ... to inspect query cost.
Limit result size
Paginate with LIMIT/OFFSET or keyset pagination for deep scrolls.
Avoid SELECT *
Select only necessary columns to reduce network and cache pressure.
Batch writes
INSERT ... VALUES (),(),() avoids chattiness.
Use transactions
BEGIN; ... COMMIT; ensures integrity across multiple statements.

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