Order by two columnsPosted by: Gaby Lopez Date: October 30, 2014 11:21AM Hello I'd like to order my query by two colums. I'm making a comments section and I'd like to show the comments and (if it has) their replies. I have one column with the ID called id_comment and ...
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...
In the next two queries, whether the index is used is similar to the same queries without DESC shown previously: SELECT * FROM t1 ORDER BY key_part1 DESC, key_part2 DESC; SELECT * FROM t1 WHERE key_part1 = constant ORDER BY key_part2 DESC; Two columns in an ORDER BY can sort ...
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
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...
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InnoDB存储引擎以B+树作为索引的底层实现,B+树的叶子节点存储着...
Below we take a look at several examples of how to use the MySQL LIMIT clause to retrieve specific results, including date created, category IDs, and running queries on multiple columns. Using MySQL LIMIT 10 to find result sets by ‘date_created’ and ‘category_id” For example, if I do...
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. ...
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...