[MySQL] 테이블 내의 데이터 개수 구하기 (+ DISTINCT)

SELECT COUNT(*) FROM Product 와 같이 COUNT()를 SELECT 하면 된다.

SELECT COUNT(DISTINCT player_id) FROM Activity 와 같이 DISTINCT를 COUNT() 내에 사용할 수도 있다.

COUNT() 내에 DISTINCT를 써야하는 때에 주의해야 한다!!

관련 문제: https://leetcode.com/problems/customers-who-bought-all-products/

SELECT customer_id
FROM Customer
GROUP BY customer_id
HAVING COUNT(DISTINCT product_key) = (SELECT COUNT(*) FROM Product);

관련 문제: https://leetcode.com/problems/game-play-analysis-iv/

SELECT ROUND(COUNT(player_id) / (SELECT COUNT(DISTINCT player_id) FROM Activity), 2) AS fraction
FROM Activity
WHERE (player_id, event_date) IN (
    SELECT player_id, DATE_ADD(MIN(event_date), INTERVAL 1 DAY)
    FROM Activity
    GROUP BY player_id
);