jan.kneschke.de/projects/mysql/groupwise-max C. Pivot Tables A second frequently asked question involves the transposition of rows to columns, often involving the aggregation of data, so-called 'Pivot Table' queries. Although my preferred solution is to handle the transposition at the application...
Two columns in anORDER BYcan sort in the same direction (bothASC, or bothDESC) or in opposite directions (oneASC, oneDESC). A condition for index use is that the index must have the same homogeneity, but need not have the same actual direction. ...
虽然索引能提高查询效率,但在一条SQL里,对于一张表的查询 一次只能使用一个索引(注:排除发生index merge的可能性),也就是说当WHERE子句与ORDER BY子句要使用的索引不一致时,MySQL只能使用其中一个索引(B+树)。 也就是说,一个既有WHERE又有ORDER BY的SQL中,使用索引有三个可能的场景:只用于WHERE子句 筛选出满...
Two columns in anORDER BYcan sort in the same direction (bothASC, or bothDESC) or in opposite directions (oneASC, oneDESC). A condition for index use is that the index must have the same homogeneity, but need not have the same actual direction. ...
1.回表读取两次数据(two-pass):两次传输排序 2.回表读取一次数据(single-pass):单次传输排序 两次传输排序 两次传输排序会进行两次回表操作:第一次回表用于在WHERE子句中筛选出满足条件的rowid以及rowid对应的ORDER BY的列值;第二次回表发生在ORDER BY子句对指定列进行排序之后,通过rowid回表查出SELECT子句需要的...
MySQL has two filesort algorithms for sorting and retrieving results. The original method uses only the ORDER BY columns. The modified method uses not just the ORDER BY columns, but all the columns used in the query. The optimizer selects which filesort algorithm to use. Prior to MySQL 4.1...
虽然索引能提高查询效率,但在一条SQL里,对于一张表的查询 一次只能使用一个索引(注:排除发生index merge的可能性),也就是说当WHERE子句与ORDER BY子句要使用的索引不一致时,MySQL只能使用其中一个索引(B+树)。 也就是说,一个既有WHERE又有ORDER BY的SQL中,使用索引有三个可能的场景: 只用于WHERE子句 筛选出...
1 概述 MySQL有两种方式可以实现ORDER BY: 1.通过索引扫描生成有序的结果 2.使用文件排序(filesort) 围绕着这两种排序方式,我们试着理解一下ORDER BY的执行过程以及回答一些常见的问题(下文仅讨论InnoDB存储引擎)。 2 索引扫描排序和文件排序(filesort)简介 我们知道Inn
You can do that by declaring those columns as the leading columns of the PRIMARY KEY of your table. You can create a UNIQUE INDEX on the id column. 1 2 3 4 5 6 7 8 9 10 CREATE TABLE `tableMain` ( `id` INT(11) NOT NULL AUTO_INCREMENT , `value1` VARCHAR(45) NOT NULL , ...
Written By Posted order by max of two columns Tobias Schützer September 06, 2004 08:46AM Re: order by max of two columns Ramalingam Chelliah September 07, 2004 06:53AM Sorry, you can't reply to this topic. It has been closed. ...