#### 優化group by 默認情況下， mysql對所有的group by col1,col2進行排序。這與在查詢中指定order by col1,col2類型，如果查詢中包括group by 但用戶想要避免排序結果的消耗，則可以使用order by null禁止排序。 根據class_id進行分組 ~~~ select * from 表 group by class_id; ~~~ 結果: mysql自動對結果進行了排序. ~~~ +----+------+------+----------+ | id | name | age | class_id | +----+------+------+----------+ | 4 | jack | 30 | 1 | | 2 | jack1| 24 | 3 | | 1 | jack2| 20 | 4 | | 3 | jack3| 14 | 5 | +----+------+------+----------+ ~~~ 通過執行計劃可以看到使用了排序 ~~~ id: 1 select_type: SIMPLE table: user partitions: NULL type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 4 filtered: 100.00 Extra: Using temporary; Using filesort //使用了排序 ~~~ 優化: ~~~ select * from user group by class_id order by null; ~~~ 結果: ~~~ id: 1 select_type: SIMPLE table: user partitions: NULL type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 4 filtered: 100.00 Extra: Using temporary //這里沒有了排序 ~~~