MySQL8.0新特性--Group by

时间 2020-06-16

原文原文链接

Group by 语句用于结合聚合函数（如count，sum，avg，max，min），根据一个或多个列对结果集进行分组。html

（1）去掉重复值：根据group by后面的关键字只显示一行结果；mysql

（2）mysql5.7默认开启参数ONLY_FULL_GROUP_BY，表示彻底group by，即select后面跟的列group by后面也必须有，可是group by后面跟的列，select后面不必定须要出现； sql

mysql> select @@version;
+-----------+
| @@version |
+-----------+
| 8.0.13    |
+-----------+
1 row in set (0.00 sec)

mysql> show variables like '%sql_mode%';
+---------------+-----------------------------------------------------------------------------------------------------------------------+
| Variable_name | Value                                                                                                                 |
+---------------+-----------------------------------------------------------------------------------------------------------------------+
| sql_mode      | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION |
+---------------+-----------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

mysql> select * from t_group;
+--------+---------+------------+------------+
| emp_no | dept_no | from_date  | to_date    |
+--------+---------+------------+------------+
|  22744 | d006    | 1986-12-01 | 9999-01-01 |
|  24007 | d005    | 1986-12-01 | 9999-01-01 |
|  30970 | d005    | 1986-12-01 | 2017-03-29 |
|  31112 | d002    | 1986-12-01 | 1993-12-10 |
|  40983 | d005    | 1986-12-01 | 9999-01-01 |
|  46554 | d008    | 1986-12-01 | 1992-05-27 |
|  48317 | d008    | 1986-12-01 | 1989-01-11 |
|  49667 | d007    | 1986-12-01 | 9999-01-01 |
|  50449 | d005    | 1986-12-01 | 9999-01-01 |
|  10004 | d004    | 1986-12-01 | 9999-01-01 |
+--------+---------+------------+------------+
10 rows in set (0.00 sec)

mysql> select dept_no,count(*) from t_group group by dept_no;
+---------+----------+
| dept_no | count(*) |
+---------+----------+
| d006    |        1 |
| d005    |        4 |
| d002    |        1 |
| d008    |        2 |
| d007    |        1 |
| d004    |        1 |
+---------+----------+
6 rows in set (0.00 sec)

mysql> select dept_no,emp_no,count(*) from t_group group by dept_no;
ERROR 1055 (42000): Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'employees.t_group.emp_no' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

关闭ONLY_FULL_GROUP_BY参数后，不报错，可是结果是不彻底group by;
mysql> set session sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION';
Query OK, 0 rows affected (0.01 sec)

mysql> select dept_no,emp_no,count(*) from t_group group by dept_no;
+---------+--------+----------+
| dept_no | emp_no | count(*) |
+---------+--------+----------+
| d006    |  22744 |        1 |
| d005    |  24007 |        4 |
| d002    |  31112 |        1 |
| d008    |  46554 |        2 |
| d007    |  49667 |        1 |
| d004    |  10004 |        1 |
+---------+--------+----------+
6 rows in set (0.00 sec)

（3）mysql5.7group by 默认还有排序功能，8.0默认只分组不排序，须要加order by才排序，这点能够从执行结果是否有Using filesort来判断session

mysql> select @@version;
+-----------+
| @@version |
+-----------+
| 8.0.13    |
+-----------+
1 row in set (0.00 sec)

mysql> select dept_no,count(*) from t_group group by dept_no;
+---------+----------+
| dept_no | count(*) |
+---------+----------+
| d006    |        1 |
| d005    |        4 |
| d002    |        1 |
| d008    |        2 |
| d007    |        1 |
| d004    |        1 |
+---------+----------+
6 rows in set (0.00 sec)

mysql> desc select dept_no,count(*) from t_group group by dept_no;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | SIMPLE      | t_group | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   10 |   100.00 | Using temporary |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)


root@localhost [testdb]>select @@version;
+------------+
| @@version  |
+------------+
| 5.7.16-log |
+------------+
1 row in set (0.00 sec)

root@localhost [testdb]>select dept_no,count(*) from t_group group by dept_no;
+---------+----------+
| dept_no | count(*) |
+---------+----------+
| d002    |        1 |
| d004    |        1 |
| d005    |        4 |
| d006    |        1 |
| d007    |        1 |
| d008    |        2 |
+---------+----------+
6 rows in set (0.00 sec)

root@localhost [testdb]>desc select dept_no,count(*) from t_group group by dept_no;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                           |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
|  1 | SIMPLE      | t_group | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   10 |   100.00 | Using temporary; Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)

(4) group by是否能排序会直接影响分页查询结果app

8.0.13版本
mysql> select dept_no,count(*) from t_group group by dept_no limit 1;
+---------+----------+
| dept_no | count(*) |
+---------+----------+
| d006    |        1 |
+---------+----------+
1 row in set (0.01 sec)

5.7.16版本：
root@localhost [testdb]>select dept_no,count(*) from t_group group by dept_no limit 1;
+---------+----------+
| dept_no | count(*) |
+---------+----------+
| d002    |        1 |
+---------+----------+
1 row in set (0.00 sec)

参考连接ide

8.2.1.15 GROUP BY Optimization函数

MySQL 5.7有关group by说明的片断以下：this

In MySQL, GROUP BY is used for sorting, so the server may also apply ORDER BY optimizations to grouping. However, relying on implicit or explicit GROUP BY sorting is deprecated. See Section 8.2.1.14, “ORDER BY Optimization”.spa