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 |

```sql
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

sql-ops

SQL Patterns

CTE (Common Table Expressions)

Chained CTEs

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