Group by 語句用于結合聚合函數（如count，sum，avg，max，min），根據一個或多個列對結果集進行分組。
（1）去掉重復值：根據group by后面的關鍵字只顯示一行結果；
（2）mysql5.7默認開啟參數ONLY_FULL_GROUP_BY，表示完全group by，即select后面跟的列group by后面也必須有，但是group by后面跟的列，select后面不一定需要出現；
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 關閉ONLY_FULL_GROUP_BY參數后，不報錯，但是結果是不完全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 默認還有排序功能，8.0默認只分組不排序，需要加order by才排序，這點可以從執行結果是否有Using filesort來判斷
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是否能排序會直接影響分頁查詢結果
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)
參考鏈接
8.2.1.15 GROUP BY Optimization
MySQL 5.7有關group by說明的片段如下：
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”.
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