SQL

SQL Skill

When to activate

• Writing complex queries (joins, aggregations, subqueries, window functions)
• Optimising a slow query — reading and improving EXPLAIN plans
• Designing or reviewing a database schema
• Writing database migrations (PostgreSQL, MySQL, SQLite)
• Generating reports or analytics queries from raw data

When NOT to use

• Simple CRUD — your ORM handles these better
• Schema design for NoSQL (MongoDB, DynamoDB) — different paradigm
• Data warehouse–specific syntax (Snowflake, BigQuery, Redshift) — use the relevant skill

Instructions

Query patterns

CTEs for readability (prefer over nested subqueries):

WITH monthly_revenue AS (
    SELECT
        DATE_TRUNC('month', created_at) AS month,
        SUM(amount) AS revenue
    FROM orders
    WHERE status = 'completed'
    GROUP BY 1
),
revenue_growth AS (
    SELECT
        month,
        revenue,
        LAG(revenue) OVER (ORDER BY month) AS prev_revenue,
        ROUND(
            (revenue - LAG(revenue) OVER (ORDER BY month))
            / NULLIF(LAG(revenue) OVER (ORDER BY month), 0) * 100,
            2
        ) AS growth_pct
    FROM monthly_revenue
)
SELECT * FROM revenue_growth ORDER BY month DESC;

Window functions for rankings and running totals:

SELECT
    user_id,
    order_id,
    amount,
    -- Running total per user
    SUM(amount) OVER (PARTITION BY user_id ORDER BY created_at) AS running_total,
    -- Rank within user (dense = no gaps in rank numbers)
    DENSE_RANK() OVER (PARTITION BY user_id ORDER BY amount DESC) AS order_rank,
    -- Percentile within all orders
    PERCENT_RANK() OVER (ORDER BY amount) AS percentile
FROM orders;

Self-joins for hierarchical data:

-- Manager → employee hierarchy
SELECT
    e.name AS employee,
    m.name AS manager,
    e.department
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.id
ORDER BY m.name, e.name;

Lateral join for top-N per group:

-- Top 3 orders per customer
SELECT c.name, o.amount, o.created_at
FROM customers c
CROSS JOIN LATERAL (
    SELECT amount, created_at
    FROM orders
    WHERE user_id = c.id
    ORDER BY amount DESC
    LIMIT 3
) o;

Upsert (INSERT ... ON CONFLICT):

-- PostgreSQL
INSERT INTO user_stats (user_id, login_count, last_login)
VALUES ($1, 1, NOW())
ON CONFLICT (user_id) DO UPDATE SET
    login_count = user_stats.login_count + 1,
    last_login = NOW();

Query optimisation

1. Read the EXPLAIN PLAN first:

EXPLAIN ANALYZE
SELECT * FROM orders o
JOIN users u ON o.user_id = u.id
WHERE o.status = 'pending' AND o.created_at > NOW() - INTERVAL '7 days';

Signs of a slow query:

• Seq Scan on a large table — needs an index
• Hash Join with large row estimates — consider reducing the result set earlier
• Sort on a non-indexed column — add an index or use a covering index
• High rows removed by filter — the index isn't selective enough

2. Index patterns:

-- Compound index for common filter + sort pattern
CREATE INDEX idx_orders_status_created ON orders (status, created_at DESC);

-- Partial index for a frequent subset
CREATE INDEX idx_orders_pending ON orders (created_at) WHERE status = 'pending';

-- Covering index (includes all selected columns, avoids table lookup)
CREATE INDEX idx_orders_covering ON orders (user_id, status) INCLUDE (amount, created_at);

-- Expression index for computed values
CREATE INDEX idx_users_email_lower ON users (LOWER(email));

3. Avoid N+1 in raw SQL:

-- Bad: one query per user to get their orders
-- (this happens in application code)

-- Good: single query with JOIN
SELECT u.id, u.email, COUNT(o.id) AS order_count, SUM(o.amount) AS total_spent
FROM users u
LEFT JOIN orders o ON o.user_id = u.id AND o.status = 'completed'
GROUP BY u.id, u.email;

Schema design rules

-- Always:
CREATE TABLE orders (
    id          BIGSERIAL PRIMARY KEY,           -- surrogate key
    user_id     BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    status      TEXT NOT NULL DEFAULT 'pending'
                CHECK (status IN ('pending', 'completed', 'cancelled')),
    amount      NUMERIC(10, 2) NOT NULL CHECK (amount >= 0),
    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
);

-- Indexes for foreign keys and common filter columns
CREATE INDEX ON orders (user_id);
CREATE INDEX ON orders (status) WHERE status != 'completed';  -- partial
CREATE INDEX ON orders (created_at DESC);

Naming conventions:

• Tables: snake_case, plural (orders, user_sessions)
• Columns: snake_case (created_at, user_id)
• Indexes: idx_{table}_{columns} (idx_orders_user_id_status)
• Foreign keys: fk_{table}_{referenced_table} (fk_orders_users)

Migrations

-- Always wrap in a transaction
BEGIN;

-- Add nullable column first, then backfill, then add NOT NULL constraint
ALTER TABLE orders ADD COLUMN shipping_address TEXT;

UPDATE orders SET shipping_address = 'Unknown' WHERE shipping_address IS NULL;

ALTER TABLE orders ALTER COLUMN shipping_address SET NOT NULL;

-- Add index CONCURRENTLY to avoid locking (PostgreSQL)
CREATE INDEX CONCURRENTLY idx_orders_shipping ON orders (shipping_address);

COMMIT;

Rules for zero-downtime migrations:

1. Never drop a column that code still reads
2. Add columns as nullable first, backfill, then add NOT NULL
3. Use CREATE INDEX CONCURRENTLY in PostgreSQL
4. Deploy code that handles both old and new schema before migrating

Analytics query patterns

Cohort retention:

WITH cohorts AS (
    SELECT user_id, DATE_TRUNC('month', MIN(created_at)) AS cohort_month
    FROM orders GROUP BY user_id
),
activity AS (
    SELECT DISTINCT o.user_id,
        DATE_TRUNC('month', o.created_at) AS active_month
    FROM orders o
)
SELECT
    c.cohort_month,
    a.active_month,
    DATE_DIFF('month', c.cohort_month, a.active_month) AS month_number,
    COUNT(DISTINCT a.user_id) AS users
FROM cohorts c JOIN activity a USING (user_id)
GROUP BY 1, 2, 3 ORDER BY 1, 3;

Funnel analysis:

WITH funnel AS (
    SELECT user_id,
        MAX(CASE WHEN event = 'signup'   THEN 1 END) AS did_signup,
        MAX(CASE WHEN event = 'onboard'  THEN 1 END) AS did_onboard,
        MAX(CASE WHEN event = 'purchase' THEN 1 END) AS did_purchase
    FROM events GROUP BY user_id
)
SELECT
    COUNT(*) FILTER (WHERE did_signup  = 1) AS signups,
    COUNT(*) FILTER (WHERE did_onboard = 1) AS onboarded,
    COUNT(*) FILTER (WHERE did_purchase= 1) AS purchased
FROM funnel;

Example

Request: "Show me the top 10 customers by revenue in the last 90 days, with their order count, average order value, and whether they've placed an order in the last 7 days."

Expected query:

SELECT
    u.id,
    u.email,
    COUNT(o.id)                         AS order_count,
    SUM(o.amount)                       AS total_revenue,
    ROUND(AVG(o.amount), 2)             AS avg_order_value,
    MAX(o.created_at)                   AS last_order_at,
    MAX(o.created_at) > NOW() - INTERVAL '7 days' AS active_recently
FROM users u
JOIN orders o ON o.user_id = u.id
WHERE o.status = 'completed'
  AND o.created_at > NOW() - INTERVAL '90 days'
GROUP BY u.id, u.email
ORDER BY total_revenue DESC
LIMIT 10;