sql group by 用法

GROUP BY 语句
GROUP BY 语句用于结合合计函数，根据一个或多个列对结果集进行分组。

SQL GROUP BY 语法
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name

mysql教程> CREATE TABLE Employee(
    ->     id            int,
    ->     first_name    VARCHAR(15),
    ->     last_name     VARCHAR(15),
    ->     start_date    DATE,
    ->     end_date      DATE,
    ->     salary        FLOAT(8,2),
    ->     city          VARCHAR(10),
    ->     description   VARCHAR(15)
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql>
mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)
-> values (1,'Jason', 'Martin', '19960725', '20060725', 1234.56, 'Toronto', 'Programmer');
Query OK, 1 row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)
-> values(2,'Alison', 'Mathews', '19760321', '19860221', 6661.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.02 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)
-> values(3,'James', 'Smith', '19781212', '19900315', 6544.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)
-> values(4,'Celia', 'Rice', '19821024', '19990421', 2344.78, 'Vancouver','Manager');
Query OK, 1 row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)
-> values(5,'Robert', 'Black', '19840115', '19980808', 2334.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.01 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)
-> values(6,'Linda', 'Green', '19870730', '19960104', 4322.78,'New York', 'Tester');
Query OK, 1 row affected (0.02 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)
-> values(7,'David', 'Larry', '19901231', '19980212', 7897.78,'New York', 'Manager');
Query OK, 1 row affected (0.02 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)
-> values(8,'James', 'Cat', '19960917', '20020415', 1232.78,NULL, 'Tester');
Query OK, 1 row affected (0.00 sec)

mysql>
mysql> select * from Employee;
+------+------------+-----------+------------+------------+---------+-----------+-------------+
| id   | first_name | last_name | start_date | end_date   | salary | city      | description |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
|    1 | Jason      | Martin    | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto   | Programmer |
|    2 | Alison     | Mathews   | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver | Tester      |
|    3 | James      | Smith     | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver | Tester      |
|    4 | Celia      | Rice      | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver | Manager     |
|    5 | Robert     | Black     | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver | Tester      |
|    6 | Linda      | Green     | 1987-07-30 | 1996-01-04 | 4322.78 | New York | Tester      |
|    7 | David      | Larry     | 1990-12-31 | 1998-02-12 | 7897.78 | New York | Manager     |
|    8 | James      | Cat       | 1996-09-17 | 2002-04-15 | 1232.78 | NULL      | Tester      |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
8 rows in set (0.00 sec)

mysql>
mysql>
mysql> SELECT city, COUNT(*) FROM employee GROUP BY city;
+-----------+----------+
| city      | COUNT(*) |
+-----------+----------+
| NULL      |        1 |
| New York |        2 |
| Toronto   |        1 |
| Vancouver |        4 |
+-----------+----------+
4 rows in set (0.00 sec)

mysql>
mysql>
mysql>
mysql>
mysql> drop table Employee;
Query OK, 0 rows affected (0.00 sec)

与count()同用

mysql> SELECT city, COUNT(*) FROM employee
    -> WHERE description = 'Tester' OR description = 'Programmer'
    -> GROUP BY city;
+-----------+----------+
| city      | COUNT(*) |
+-----------+----------+
| New York |        1 |
| Toronto   |        1 |
| Vancouver |        4 |
+-----------+----------+
3 rows in set (0.00 sec)

mysql>
mysql> drop table Employee;
Query OK, 0 rows affected (0.00 sec)

带上is not null

mysql> SELECT city, COUNT(*) FROM employee
    -> WHERE description IS NOT NULL
    -> GROUP BY city;
+-----------+----------+
| city      | COUNT(*) |
+-----------+----------+
| New York |        2 |
| Toronto   |        1 |
| Vancouver |        4 |
+-----------+----------+
3 rows in set (0.02 sec)

最基本的

mysql> SELECT * FROM employee
    -> GROUP BY city;
+------+------------+-----------+------------+------------+---------+-----------+-------------+
| id   | first_name | last_name | start_date | end_date   | salary | city      | description |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
|    6 | Linda      | Green     | 1987-07-30 | 1996-01-04 | 4322.78 | New York | Tester      |
|    1 | Jason      | Martin    | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto   | Programmer |
|    2 | Alison     | Mathews   | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver | Tester      |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
3 rows in set (0.05 sec)

注:合计函数 (比如 SUM) 常常需要添加 GROUP BY 语句。
where 子句的作用是在对查询结果进行分组前，将不符合where条件的行去掉，即在分组之前过滤数据，条件中不能包含聚组函数，使用where条件显示特定的行。
having 子句的作用是筛选满足条件的组，即在分组之后过滤数据，条件中经常包含聚组函数，使用having 条件显示特定的组，也可以使用多个分组标准进行分组。

补充：数据库,mysql教程