SQL

SELECT
    u.id,
    u.name,
    u.email,
    COUNT(o.id)     AS order_count,
    SUM(o.total)   AS total_spent
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
WHERE
    u.created_at >= '2024-01-01'
    AND u.status = 'active'
GROUP BY u.id, u.name, u.email
HAVING COUNT(o.id) > 0
ORDER BY total_spent DESC
LIMIT 25 OFFSET 0;

-- Single row
INSERT INTO products (name, price, category_id, stock)
VALUES ('Mechanical Keyboard', 89.99, 4, 150);

-- Multiple rows
INSERT INTO tags (name, color)
VALUES
    ('backend', '#0084ff'),
    ('frontend', '#00d4aa'),
    ('database', '#ffa500');

-- Insert from SELECT
INSERT INTO archive_orders
SELECT * FROM orders
WHERE created_at < '2023-01-01';

UPDATE products
SET
    price = price * 0.9,          -- 10% discount
    updated_at = NOW()
WHERE
    category_id = 4
    AND stock > 0
    AND sale_end IS NULL;

-- Delete with condition
DELETE FROM sessions
WHERE expires_at < NOW();

-- Delete with subquery
DELETE FROM users
WHERE id IN (
    SELECT user_id FROM banned_users
    WHERE reason = 'spam'
);

CREATE TABLE orders (
    id           BIGSERIAL          PRIMARY KEY,
    user_id      BIGINT             NOT NULL
                                       REFERENCES users(id),
    status       VARCHAR(20)        NOT NULL
                                       DEFAULT 'pending',
    total        DECIMAL(12,2)     NOT NULL
                                       CHECK (total >= 0),
    metadata     JSONB,
    created_at   TIMESTAMP          NOT NULL
                                       DEFAULT NOW(),
    updated_at   TIMESTAMP          NOT NULL
                                       DEFAULT NOW()
);

CREATE INDEX idx_orders_user_status
    ON orders (user_id, status);

CREATE INDEX idx_orders_created
    ON orders (created_at DESC);

ALTER TABLE users
    ADD COLUMN phone       VARCHAR(20),
    ADD COLUMN verified_at TIMESTAMP,
    ALTER COLUMN email     SET NOT NULL,
    DROP COLUMN legacy_id;

ALTER TABLE users
    RENAME COLUMN username TO handle;

CREATE VIEW active_user_summary AS
SELECT
    u.id,
    u.name,
    COUNT(o.id)   AS orders,
    MAX(o.created_at) AS last_order
FROM users u
JOIN orders o ON o.user_id = u.id
WHERE u.status = 'active'
GROUP BY u.id, u.name;

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

EXPLAIN ANALYZE
SELECT u.name, COUNT(o.id)
FROM users u
JOIN orders o ON o.user_id = u.id
GROUP BY u.id;

-- Seq Scan vs Index Scan vs Hash Join
-- Look for: rows estimate vs actual, loops, buffers hit
-- Add index if seq scan on large table

BEGIN;                              -- Start transaction

UPDATE accounts SET balance = balance - 500
WHERE id = 1;

UPDATE accounts SET balance = balance + 500
WHERE id = 2;

SAVEPOINT transfer_done;

-- If something goes wrong:
ROLLBACK TO transfer_done;

-- On success:
COMMIT;