mysql group by 之 rollup
在group by之后使用 rollup 修饰符其实就是为分组（group by）以后的每个小组再进行聚合操作。
CREATE TABLE sales
(
year INT,
country VARCHAR(20),
product VARCHAR(32),
profit INT
);
1、单个分组
》普通分组-group by：
mysql> SELECT year, SUM(profit) AS profit
FROM sales
GROUP BY year;
+------+--------+
| year | profit |
+------+--------+
| 2000 | 4525 |
| 2001 | 3010 |
+------+--------+
》group by … with roll up：
这里介绍两个聚合函数 SUM() 和 AVG()。
mysql> SELECT year, SUM(profit) AS profit
FROM sales
GROUP BY year WITH ROLLUP;
+------+--------+
| year | profit |
+------+--------+
| 2000 | 4525 |
| 2001 | 3010 |
| NULL | 7535 |
+------+--------+
mysql> SELECT year, AVG(profit) AS profit
FROM sales
GROUP BY year WITH ROLLUP;
+------+--------+
| year | profit |
+------+--------+
| 2000 | 4520 |
| 2001 | 3010 |
| NULL | 3765 |
+------+--------+
2、多个分组
》普通分组-group by：
mysql> SELECT year, country, product, SUM(profit) AS profit
FROM sales
GROUP BY year, country, product;
+------+---------+------------+--------+
| year | country | product | profit |
+------+---------+------------+--------+
| 2000 | Finland | Computer | 1500 |
| 2000 | Finland | Phone | 100 |
| 2000 | India | Calculator | 150 |
| 2000 | India | Computer | 1200 |
| 2000 | USA | Calculator | 75 |
| 2000 | USA | Computer | 1500 |
| 2001 | Finland | Phone | 10 |
| 2001 | USA | Calculator | 50 |
| 2001 | USA | Computer | 2700 |
| 2001 | USA | TV | 250 |
+------+---------+------------+--------+
》group by … with roll up：
mysql> SELECT year, country, product, SUM(profit) AS profit
FROM sales
GROUP BY year, country, product WITH ROLLUP;
+------+---------+------------+--------+
| year | country | product | profit |
+------+---------+------------+--------+
| 2000 | Finland | Computer | 1500 |
| 2000 | Finland | Phone | 100 |
| 2000 | Finland | NULL | 1600 |
| 2000 | India | Calculator | 150 |
| 2000 | India | Computer | 1200 |
| 2000 | India | NULL | 1350 |
| 2000 | USA | Calculator | 75 |
| 2000 | USA | Computer | 1500 |
| 2000 | USA | NULL | 1575 |
| 2000 | NULL | NULL | 4525 |
| 2001 | Finland | Phone | 10 |
| 2001 | Finland | NULL | 10 |
| 2001 | USA | Calculator | 50 |
| 2001 | USA | Computer | 2700 |
| 2001 | USA | TV | 250 |
| 2001 | USA | NULL | 3000 |
| 2001 | NULL | NULL | 3010 |
| NULL | NULL | NULL | 7535 |
+------+---------+------------+--------+
3、rollup + limit
rollup 可以和 limit 一起使用，limit 位于 with rollup 之后。
mysql> SELECT year, country, product, SUM(profit) AS profit
FROM sales
GROUP BY year, country, product WITH ROLLUP
LIMIT 5;
+------+---------+------------+--------+
| year | country | product | profit |
+------+---------+------------+--------+
| 2000 | Finland | Computer | 1500 |
| 2000 | Finland | Phone | 100 |
| 2000 | Finland | NULL | 1600 |
| 2000 | India | Calculator | 150 |
| 2000 | India | Computer | 1200 |
+------+---------+------------+--------+
4、注意
rollup 和 order by 是互斥的，不能同时应用在同一个子句中。但可以结合嵌套使用。
mysql> SELECT * FROM
(SELECT year, SUM(profit) AS profit
FROM sales GROUP BY year WITH ROLLUP) AS dt
ORDER BY year DESC;
+------+--------+
| year | profit |
+------+--------+
| 2001 | 3010 |
| 2000 | 4525 |
| NULL | 7535 |
+------+--------+