Group By

1、每一列都有效（不能是聚集函式），Group by 後接相同表示式（不可是別名）

2、列中不能是長度可變的資料型別（如文字或備註型）

3、NULL單獨成組

10.1 資料分組

我們找出ID號為DLL01的產品個數

-- 我們找出ID號為DLL01的產品個數

SELECT COUNT(*) AS num_prods
FROM Products
WHERE vend_id = 'DLL01'

10.2 建立分組

但怎麼找到各個ID號不同的產品個數呢？

SELECT vend_id, COUNT(*) AS num_prods
FROM Products
GROUP BY
 
 vend_id;

10.3 過濾分組

哪些貨品超過2單？

SELECT cust_id, COUNT(*) AS orders
FROM Orders
GROUP BY cust_id
HAVING COUNT(*) >= 2;

價格超過4，同時數量超過2的？

SELECT vend_id, COUNT(*) AS num_prods
FROM Products
WHERE prod_price >= 4
GROUP BY vend_id
HAVING COUNT(*) >= 2;

10.4 分組和排序

SELECT order_num, COUNT
 
(*) AS items
FROM OrderItems
GROUP BY order_num
HAVING COUNT(*) >= 3
ORDER BY items, order_num;

10.5 SELECT 子句順序