MySQL8.0新特性--Groupby
Group by 語(yǔ)句用于結(jié)合聚合函數(shù)（如count，sum，avg，max，min），根據(jù)一個(gè)或多個(gè)列對(duì)結(jié)果集進(jìn)行分組。
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（1）去掉重復(fù)值：根據(jù)group by后面的關(guān)鍵字只顯示一行結(jié)果；
（2）MySQL5.7默認(rèn)開(kāi)啟參數(shù)ONLY_FULL_GROUP_BY，表示完全group by，即select后面跟的列g(shù)roup 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(xiàn)；
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(guān)閉ONLY_FULL_GROUP_BY參數(shù)后，不報(bào)錯(cuò)，但是結(jié)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(rèn)還有排序功能，8.0默認(rèn)只分組不排序，需要加order by才排序，這點(diǎn)可以從執(zhí)行結(jié)果是否有Using filesort來(lái)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(huì)直接影響分頁(yè)查詢結(jié)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(guān)group by說(shuō)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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