EXTRACT, DATE_TRUNC, AGE

SELECT EXTRACT(YEAR  FROM created_at) AS yr,
       EXTRACT(MONTH FROM created_at) AS mo,
       EXTRACT(DAY   FROM created_at) AS d,
       EXTRACT(DOW   FROM created_at) AS day_of_week
FROM orders;

SELECT EXTRACT(EPOCH FROM (NOW() - created_at)) AS seconds_ago
FROM events;

SELECT date_trunc('day',   NOW());   -- 2024-03-15 00:00:00
SELECT date_trunc('week',  NOW());
SELECT date_trunc('month', NOW());
SELECT date_trunc('hour',  NOW());

SELECT date_trunc('month', created_at) AS month,
       SUM(total) AS revenue
FROM orders
GROUP BY 1
ORDER BY 1;

-- This month so far
SELECT SUM(total) FROM orders
WHERE created_at >= date_trunc('month', NOW())
  AND created_at <  date_trunc('month', NOW()) + INTERVAL '1 month';

SELECT AGE('2024-03-15', '2020-12-01');
-- → '3 years 3 months 14 days'

SELECT EXTRACT(YEAR FROM AGE('2024-03-15', '2020-12-01'));  -- 3

SELECT AGE(birth_date) FROM users LIMIT 5;

SELECT * FROM orders
WHERE EXTRACT(ISODOW FROM created_at) IN (6, 7);  -- weekend orders

SELECT EXTRACT(HOUR FROM created_at AT TIME ZONE 'UTC') FROM events;
SELECT EXTRACT(HOUR FROM created_at AT TIME ZONE 'Europe/Berlin') FROM events;

SELECT to_timestamp(
  floor(EXTRACT(EPOCH FROM created_at) / 300) * 300
) AS bucket_5min,
COUNT(*)
FROM events
GROUP BY 1
ORDER BY 1;

CREATE INDEX events_day_idx ON events ((date_trunc('day', created_at)));