子查询

下面的查询会先执行内层，再执行外层。

SELECT
    product_type,
    cnt_product
FROM
    (
        SELECT
            product_type,
            COUNT(*) AS cnt_product
        FROM
            Product
        GROUP BY
            product_type
    ) AS ProductSum;

内层是

SELECT product_type, COUNT(*) AS cnt_product FROM Product
GROUP BY product_type;

外层是

SELECT product_type, cnt_product FROM ProductSum;

可认为，每个子查询都会形成一张新的表，每个外层查询都是从子查询，即内层查询生成的新的表中去操作数据。