claude-mods/skills/sql-patterns/SKILL.md

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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, CTEs, window functions, and indexing strategies.

CTE (Common Table Expressions)

Basic CTE

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;

Recursive CTE (Hierarchies)

WITH RECURSIVE org_tree AS (
    -- Base case: top-level managers
    SELECT id, name, manager_id, 1 as level
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    -- Recursive case: employees under managers
    SELECT e.id, e.name, e.manager_id, t.level + 1
    FROM employees e
    JOIN org_tree t ON e.manager_id = t.id
)
SELECT * FROM org_tree ORDER BY level, name;

Window Functions

ROW_NUMBER (Unique sequential)

SELECT
    name,
    department,
    salary,
    ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) as rank
FROM employees;

RANK / DENSE_RANK (Ties allowed)

-- RANK: 1, 2, 2, 4 (skips after ties)
-- DENSE_RANK: 1, 2, 2, 3 (no skip)
SELECT
    name,
    score,
    RANK() OVER (ORDER BY score DESC) as rank,
    DENSE_RANK() OVER (ORDER BY score DESC) as dense_rank
FROM contestants;

LAG / LEAD (Previous/Next row)

SELECT
    date,
    revenue,
    LAG(revenue, 1) OVER (ORDER BY date) as prev_day,
    revenue - LAG(revenue, 1) OVER (ORDER BY date) as change
FROM daily_sales;

Running Total

SELECT
    date,
    amount,
    SUM(amount) OVER (ORDER BY date) as running_total
FROM transactions;

Moving Average

SELECT
    date,
    value,
    AVG(value) OVER (ORDER BY date ROWS BETWEEN 6 PRECEDING AND CURRENT ROW) as moving_avg_7day
FROM metrics;

JOIN Reference

Type	Returns
INNER JOIN	Only matching rows from both
LEFT JOIN	All from left + matching from right
RIGHT JOIN	All from right + matching from left
FULL JOIN	All from both, NULL where no match
CROSS JOIN	Cartesian product (all combinations)

Self Join (Same table)

SELECT e.name as employee, m.name as manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.id;

Pagination Patterns

OFFSET/LIMIT (Simple, slow for large offsets)

SELECT * FROM products
ORDER BY id
LIMIT 20 OFFSET 40;  -- Page 3, 20 per page

Keyset Pagination (Fast, scalable)

-- First page
SELECT * FROM products ORDER BY id LIMIT 20;

-- Next page (where last id was 42)
SELECT * FROM products WHERE id > 42 ORDER BY id LIMIT 20;

Index Strategies

Index Type	Best For
B-tree	Default, range queries, ORDER BY
Hash	Exact equality only
GIN	Arrays, JSONB, full-text
GiST	Geometric, full-text
Covering	Include columns to avoid table lookup

Covering Index

-- Query needs name but filters on email
CREATE INDEX idx_users_email_name ON users(email) INCLUDE (name);

-- Now this is index-only:
SELECT name FROM users WHERE email = 'x@y.com';

Composite Index Order

-- Leftmost prefix rule: (a, b, c) supports:
-- WHERE a = ?
-- WHERE a = ? AND b = ?
-- WHERE a = ? AND b = ? AND c = ?
-- NOT: WHERE b = ? (a must be present)
CREATE INDEX idx_orders ON orders(user_id, status, created_at);

EXISTS vs IN

-- EXISTS: Often faster for large outer, small inner
SELECT * FROM orders o
WHERE EXISTS (SELECT 1 FROM users u WHERE u.id = o.user_id AND u.status = 'active');

-- IN: Often faster for small list, can be optimized
SELECT * FROM orders
WHERE user_id IN (SELECT id FROM users WHERE status = 'active');

Anti-Patterns to Avoid

Anti-Pattern	Problem	Fix
SELECT *	Over-fetches, breaks on schema change	List columns explicitly
Function on indexed column	WHERE YEAR(date) = 2024 prevents index	WHERE date >= '2024-01-01'
OR in WHERE	May prevent index usage	Use UNION or rewrite
N+1 queries	Loop with query per item	Single JOIN or batch
DISTINCT to fix duplicates	Masks JOIN issues	Fix the JOIN logic
NOT IN with NULLs	Returns wrong results	Use NOT EXISTS instead

NULL Handling

-- NULL comparisons
WHERE column IS NULL        -- Correct
WHERE column IS NOT NULL    -- Correct
WHERE column = NULL         -- WRONG (always false)

-- COALESCE for defaults
SELECT COALESCE(nickname, name, 'Anonymous') as display_name FROM users;

-- NULLIF to create NULLs
SELECT amount / NULLIF(count, 0) as average FROM stats;  -- Avoids divide by zero

Batch Operations

-- Insert multiple rows
INSERT INTO users (name, email) VALUES
    ('Alice', 'a@x.com'),
    ('Bob', 'b@x.com'),
    ('Carol', 'c@x.com');

-- Update with limit (process in batches)
UPDATE orders SET status = 'archived'
WHERE status = 'completed' AND updated_at < '2023-01-01'
LIMIT 1000;