SQL 从查询结果里查询[通俗易懂]

大家好，又见面了，我是全栈君。

有orders表：
订单表

我想要从从表中查出每天电动车和手机各自的销售总额。这个需求还是蛮简单的，仅仅须要依据createtime和product group by即可了。以下是我写的SQL语句：

SELECT DATE_FORMAT( createtime, "%Y-%m-%d" ) cr, product, SUM(price) total FROM orders GROUP BY DATE_FORMAT(createtime, "%Y-%m-%d"), product

查询结果例如以下：
这里写图片描写叙述

这个结果确实满足了我的需求，可是存在一个问题。就是当天数多了的时候，会看得我头晕眼花的。假设查询结果是以下这个样子，我想看起来会舒服得多：
这里写图片描写叙述

对于怎样直接从表中查询出这种结果，我是一点想法都没有，可是，我却知道怎样从上一个SQL语句的查询结果中查询从而得到想要的结果。以下是我写的SQL语句：

SELECT origintable.cr, SUM(CASE WHEN origintable.product = "电动车" THEN origintable.total ELSE 0 END) "电动车", SUM(CASE WHEN origintable.product = "手机" THEN origintable.total ELSE 0 END) "手机" FROM (SELECT DATE_FORMAT( createtime, "%Y-%m-%d" ) AS cr, product, SUM(price) AS total FROM orders GROUP BY DATE_FORMAT(createtime, "%Y-%m-%d"), product) AS origintable GROUP BY origintable.cr

假设大家想亲自试一试，能够用以下的SQL语句创建orders表和向表中插入数据：

create table orders( id int PRIMARY KEY auto_increment, product VARCHAR(64) null, price DOUBLE null, createtime DATETIME )

INSERT INTO orders (orders.product, orders.price, orders.createtime) VALUES ("电动车", 50, NOW());
INSERT INTO orders (orders.product, orders.price, orders.createtime) VALUES ("手机", 60, NOW());
INSERT INTO orders (orders.product, orders.price, orders.createtime) VALUES ("电动车", 50, NOW());
INSERT INTO orders (orders.product, orders.price, orders.createtime) VALUES ("手机", 60, NOW());
INSERT INTO orders (orders.product, orders.price, orders.createtime) VALUES ("电动车", 50, "2016-04-14 22:16:11");
INSERT INTO orders (orders.product, orders.price, orders.createtime) VALUES ("手机", 60, "2016-04-14 22:16:11");
INSERT INTO orders (orders.product, orders.price, orders.createtime) VALUES ("电动车", 50, "2016-04-14 22:16:11");
INSERT INTO orders (orders.product, orders.price, orders.createtime) VALUES ("手机", 60, "2016-04-14 22:16:11");