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sql-patterns

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Quick reference for common SQL patterns, CTEs, window functions, and indexing strategies. Triggers on: sql patterns, cte example, window functions, sql join, index strategy, pagination sql.

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SKILL.md

name sql-patterns
description Quick reference for common SQL patterns, CTEs, window functions, and indexing strategies. Triggers on: sql patterns, cte example, window functions, sql join, index strategy, pagination sql.
allowed-tools Read Write

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