分组数据

创建分组

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

GROUP BY 子句必须出现在WHERE子句之后，OEDER BY子句之前。

过滤分组

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

select子句顺序

SELECT

FROM

WHERE

GROUP BY

HAVING

ORDER BY

子查询

SELECT cust_id
FROM Orders
WHERE order_num IN （SELECT order_num
                     FROM OrderItems
                     WHERE prod_id ='RGAN01'）;