MYSQL_03_ GROUP

GROUP / COUNT

OrderDate를 값별로 묶고, 해당 값들마다 COUNT를 해서 COUNT와 OrderDate 컬럼을 불러오기
SELECT
  COUNT(*), OrderDate
FROM Orders
GROUP BY OrderDate;

GROUP / SUM, MAX, MIN

SUM
SELECT
  ProductID,
  SUM(Quantity) AS QuantitySum
FROM OrderDetails
GROUP BY ProductID
ORDER BY QuantitySum DESC;

MAX, MIN (TRUNCATE 는 소수점 지우는거인듯)
SELECT
  CategoryID,
  MAX(Price) AS MaxPrice, 
  MIN(Price) AS MinPrice,
  TRUNCATE((MAX(Price) + MIN(Price)) / 2, 2) AS MedianPrice,
  TRUNCATE(AVG(Price), 2) AS AveragePrice
FROM Products
GROUP BY CategoryID;

예시

GROUP / WITH ROLLUP

전체 집계값 나옴