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 ...
Whether the optimizer actually does so depends on whether reading the index is more efficient than a table scan if columns not in the index must also be read. 官方文档告诉我们optimizer会帮我们选择一种高效的ORDER BY方式。 但也不能完全依赖optimizer的判断,这时合理建立索引、引导它使用指定索引可能是...
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子句需要的字段信...
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子句需要的字段信...
Whether the optimizer actually does so depends on whether reading the index is more efficient than a table scan if columns not in the index must also be read.官方文档告诉我们optimizer会帮我们选择一种高效的ORDER BY方式。但也不能完全依赖optimizer的判断,这时合理建立索引、引导它使用指定索引可能是更...
(class) -- 索引 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4; -- 查看表结构 DESCRIBE students; -- 或 SHOW COLUMNS FROM students; -- 修改表结构 ALTER TABLE students ADD COLUMN email VARCHAR(100); ALTER TABLE students MODIFY COLUMN name VARCHAR(100); ALTER TABLE students DROP COLUMN email; --...
The storage engine can’t optimize accesses with any columns to the right of the first range condition. For example, if your query is WHERE last_name="Smith" AND first_name LIKE 'J%' AND dob='1976-12-23', the index access will use only the first two columns in the index, because ...
The query joins many tables, and the columns in the ORDER BY are not all from the first nonconstant table that is used to retrieve rows. (This is the first table in the EXPLAIN output that does not have a const join type.) So for my case, given that the two columns I'm ordering...
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. ...