Group By

GROUP BY 子句用于与 SELECT 语句一起使用，来对相同的数据进行分组。

在 SELECT 语句中，GROUP BY 子句放在 WHERE 子句之后，放在 ORDER BY 子句之前。

-- GROUP BY 子句必须放在 WHERE 子句中的条件之后，必须放在 ORDER BY 子句之前。
-- 可以在 GROUP BY 子句中使用多个列。确保使用的分组列在列清单中。
SELECT column-list
FROM table_name
WHERE [ conditions ]
GROUP BY column1, column2....columnN
ORDER BY column1, column2....columnN

-- 数据准备
sqlite> .headers on
sqlite> .mode column
sqlite> select * from COMPANY ;
ID          NAME        AGE         ADDRESS     SALARY
----------  ----------  ----------  ----------  ----------
1           Paul        32          California  20000.0
2           Allen       25          Texas       15000.0
3           Teddy       23          Norway      20000.0
4           Mark        25          Rich-Mond   65000.0
5           David       27          Texas       85000.0
6           Kim         22          South-Hall  45000.0
7           James       24          Houston     10000.0

实例

-- 获取每个客户的工资总额
sqlite> SELECT NAME, SUM(SALARY) FROM COMPANY GROUP BY NAME;
NAME        SUM(SALARY)
----------  -----------
Allen       15000.0
David       85000.0
James       10000.0
Kim         45000.0
Mark        65000.0
Paul        20000.0
Teddy       20000.0

-- 插入有重复NAME的记录
INSERT INTO COMPANY VALUES (8, 'Paul', 24, 'Houston', 20000.00 );
INSERT INTO COMPANY VALUES (9, 'James', 44, 'Norway', 5000.00 );
INSERT INTO COMPANY VALUES (10, 'James', 45, 'Texas', 5000.00 );

sqlite> select * from COMPANY ;
ID          NAME        AGE         ADDRESS     SALARY
----------  ----------  ----------  ----------  ----------
1           Paul        32          California  20000.0
2           Allen       25          Texas       15000.0
3           Teddy       23          Norway      20000.0
4           Mark        25          Rich-Mond   65000.0
5           David       27          Texas       85000.0
6           Kim         22          South-Hall  45000.0
7           James       24          Houston     10000.0
8           Paul        24          Houston     20000.0
9           James       44          Norway      5000.0
10          James       45          Texas       5000.0

-- 对所有记录按 NAME 列进行分组
sqlite> SELECT NAME, SUM(SALARY) FROM COMPANY GROUP BY NAME ORDER BY NAME;
NAME        SUM(SALARY)
----------  -----------
Allen       15000.0
David       85000.0
James       20000.0
Kim         45000.0
Mark        65000.0
Paul        40000.0
Teddy       20000.0

-- 把 ORDER BY 子句与 GROUP BY 子句一起使用
sqlite> SELECT NAME, SUM(SALARY) FROM COMPANY GROUP BY NAME ORDER BY NAME DESC;
NAME        SUM(SALARY)
----------  -----------
Teddy       20000.0
Paul        40000.0
Mark        65000.0
Kim         45000.0
James       20000.0
David       85000.0
Allen       15000.0