SQL Patterns
Quick reference for common SQL patterns.
CTE (Common Table Expressions)
WITH active_users AS (
SELECT id, name, email
FROM users
WHERE status = 'active'
)
SELECT * FROM active_users WHERE created_at > '2024-01-01';
Chained CTEs
WITH
active_users AS (
SELECT id, name FROM users WHERE status = 'active'
),
user_orders AS (
SELECT user_id, COUNT(*) as order_count
FROM orders GROUP BY user_id
)
SELECT u.name, COALESCE(o.order_count, 0) as orders
FROM active_users u
LEFT JOIN user_orders o ON u.id = o.user_id;
Window Functions (Quick Reference)
| Function | Use |
|---|
ROW_NUMBER() | Unique sequential numbering |
RANK() | Rank with gaps (1, 2, 2, 4) |
DENSE_RANK() | Rank without gaps (1, 2, 2, 3) |
LAG(col, n) | Previous row value |
LEAD(col, n) | Next row value |
SUM() OVER | Running total |
AVG() OVER | Moving average |
SELECT
date,
revenue,
LAG(revenue, 1) OVER (ORDER BY date) as prev_day,
SUM(revenue) OVER (ORDER BY date) as running_total
FROM daily_sales;
JOIN Reference
| Type | Returns |
|---|
INNER JOIN | Only matching rows |
LEFT JOIN | All left + matching right |
RIGHT JOIN | All right + matching left |
FULL JOIN | All rows, NULL where no match |
Pagination
-- OFFSET/LIMIT (simple, slow for large offsets)
SELECT * FROM products ORDER BY id LIMIT 20 OFFSET 40;
-- Keyset (fast, scalable)
SELECT * FROM products WHERE id > 42 ORDER BY id LIMIT 20;
Index Quick Reference
| Index Type | Best For |
|---|
| B-tree | Range queries, ORDER BY |
| Hash | Exact equality only |
| GIN | Arrays, JSONB, full-text |
| Covering | Avoid table lookup |
Anti-Patterns
| Mistake | Fix |
|---|
SELECT * | List columns explicitly |
WHERE YEAR(date) = 2024 | WHERE date >= '2024-01-01' |
NOT IN with NULLs | Use NOT EXISTS |
| N+1 queries | Use JOIN or batch |
Additional Resources
For detailed patterns, load:
./references/window-functions.md - Complete window function patterns./references/indexing-strategies.md - Index types, covering indexes, optimization
