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;
Transaction Isolation Levels
| Level |
Dirty Read |
Non-Repeatable Read |
Phantom Read |
Use |
READ UNCOMMITTED |
Possible |
Possible |
Possible |
Rarely (PostgreSQL treats as READ COMMITTED) |
READ COMMITTED |
No |
Possible |
Possible |
Default in most databases |
REPEATABLE READ |
No |
No |
Possible* |
Consistent multi-statement reads |
SERIALIZABLE |
No |
No |
No |
Critical invariants (retry on serialization failure) |
*PostgreSQL's REPEATABLE READ also prevents phantoms via snapshot isolation.
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- ... critical operations ...
COMMIT; -- be prepared to retry on serialization failure
Keep the default READ COMMITTED globally; raise the level per-transaction only where the logic requires it.
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
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