SQL Development Skill | Agent Skills

SQL Development

Write efficient SQL queries and design schemas.

When to Use

Writing complex queries
Query optimization
Schema design
Index strategy
Migration planning

Query Patterns

Window Functions

-- Running totals
SELECT
    date,
    amount,
    SUM(amount) OVER (ORDER BY date) as running_total,
    AVG(amount) OVER (ORDER BY date ROWS BETWEEN 6 PRECEDING AND CURRENT ROW) as moving_avg_7d
FROM transactions;

-- Ranking
SELECT
    name,
    score,
    RANK() OVER (ORDER BY score DESC) as rank,
    DENSE_RANK() OVER (ORDER BY score DESC) as dense_rank,
    ROW_NUMBER() OVER (ORDER BY score DESC) as row_num
FROM players;

-- Partition by category
SELECT
    category,
    product,
    sales,
    sales * 100.0 / SUM(sales) OVER (PARTITION BY category) as pct_of_category
FROM products;

CTEs (Common Table Expressions)

WITH
monthly_sales AS (
    SELECT
        DATE_TRUNC('month', order_date) as month,
        SUM(amount) as total
    FROM orders
    GROUP BY 1
),
growth AS (
    SELECT
        month,
        total,
        LAG(total) OVER (ORDER BY month) as prev_month,
        (total - LAG(total) OVER (ORDER BY month)) / NULLIF(LAG(total) OVER (ORDER BY month), 0) * 100 as growth_pct
    FROM monthly_sales
)
SELECT * FROM growth WHERE growth_pct < 0;

Recursive CTEs

-- Hierarchical data (org chart, categories)
WITH RECURSIVE subordinates AS (
    SELECT id, name, manager_id, 1 as level
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    SELECT e.id, e.name, e.manager_id, s.level + 1
    FROM employees e
    JOIN subordinates s ON e.manager_id = s.id
)
SELECT * FROM subordinates ORDER BY level, name;

Query Optimization

Index Strategy

-- Composite index for common queries
CREATE INDEX idx_orders_user_date ON orders(user_id, order_date DESC);

-- Partial index for filtered queries
CREATE INDEX idx_active_users ON users(email) WHERE status = 'active';

-- Check query plan
EXPLAIN ANALYZE SELECT * FROM orders WHERE user_id = 123;

Common Issues

| Problem | Symptom | Solution | | --------------- | --------------- | -------------------------- | | Missing index | Seq Scan | Add appropriate index | | N+1 queries | Many small hits | Use JOIN or batch | | SELECT * | Slow + memory | Select only needed columns | | No LIMIT | Large result | Add pagination | | Function on col | Index not used | Rewrite condition |

Schema Design

-- Normalized schema
CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    email VARCHAR(255) UNIQUE NOT NULL,
    created_at TIMESTAMP DEFAULT NOW()
);

CREATE TABLE orders (
    id SERIAL PRIMARY KEY,
    user_id INTEGER REFERENCES users(id),
    total DECIMAL(10,2) NOT NULL,
    status VARCHAR(20) DEFAULT 'pending',
    created_at TIMESTAMP DEFAULT NOW()
);

CREATE INDEX idx_orders_user ON orders(user_id);
CREATE INDEX idx_orders_status ON orders(status) WHERE status != 'completed';

Examples

Input: "Optimize this slow query" Action: Run EXPLAIN, identify bottlenecks, add indexes or rewrite query

Input: "Get top 10 customers by revenue" Action: Write aggregation with proper joins, ordering, and limit

Agent Skills: SQL Development

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