select department,group_concat(name) from employee group by department; 结果如下: GROUP BY +聚合函数 for example: 将职员表按照部门分组 查询每个部门职员的薪水 和薪水总数 select department,group_concat(salary),sum(salary) from employee group by department; 查询每个部门的名称 以及 每个部门的人数 sel...
在MySQL 中,MySQL Query Optimizer 首先会选择尝试通过松散索引扫描来实现 GROUP BY 操作,当发现某些情况无法满足松散索引扫描实现 GROUP BY 的要求之后,才会尝试通过紧凑索引扫描来实现。 当GROUP BY 条件字段并不连续或者不是索引前缀部分的时候,MySQL Query Optimizer 无法使用松散索引扫描,设置无法直接通过索引完成 GR...
1 sky @localhost : example 08 : 49 : 45 > create index idx_gid_uid_gc 2 3 -> on group_message(group_id, user_id ,gmt_create); 4 5 Query OK, rows affected ( 0.03 sec) 6 7 Records: 96 Duplicates: 0 Warnings: 0 8 9 sky @localhost : example 09 : 07 : 30 > drop index ...
In my previous blog post, we learned that indexes or other means of finding data might not be the most expensive part of query execution. For example, MySQL GROUP BY could potentially be responsible for 90% or more of the query execution time. 当MySQL执行GROUP BY的时候,最复杂的操作就是聚...
key_len:4ref: example.m.idrows:11Extra: 看看上面的这个Query语句,明明有ORDER BY user_id,为什么在执行计划中却没有排序操作呢?其实这里正是因为MySQL Query Optimizer选择了一个有序的索引来进行访问表中的数据(idx_group_message_gid_uid),这样,我们通过group_id的条件得到的数据已经是按照group_id和user_...
GROUP BY B.deptno; 但为什么mysql就能支持呢? SQL:1999 and later permits such nonaggregates per optional feature T301 if they are functionally dependent on GROUP BY columns: If such a relationship exists between name and custid, the query is legal. This would be the case, for example, were...
但为什么mysql就能支持呢? SQL:1999 and later permits such nonaggregates per optional feature T301 if they are functionally dependent on GROUP BY columns: If such a relationship exists between name and custid, the query is legal. This would be the case, for example, were custid a primary key...
在我的上一篇文章中,我们知道了通过索引或者其他的方式获取数据可能不是语句执行最耗时的操作。比如,MySQL的GROUP BY可能会占据语句执行时间的90%. In my previous blog post, we learned that indexes or other means of finding data might not be the most expensive part of query execution. For example, My...
MySQL also permits a nonaggregate column not named in aGROUP BYclause when SQLONLY_FULL_GROUP_BYmode is enabled, provided that this column is limited to a single value, as shown in the following example: mysql>CREATETABLEmytable(->idINTUNSIGNEDNOTNULLPRIMARYKEY,->aVARCHAR(10),->bINT->)...
Here is an example of the data and what I am getting with my query, but also what I need to get. -- use a test database use testdb ; drop table if exists testdb.testtable ; -- create the test table create table testdb.testtable(IDColumn int default null, NameColumn varchar(50...