mysql 多列distinct 基础概念 DISTINCT 是SQL 中的一个关键字,用于返回唯一不同的值。当你在 SELECT 语句中使用 DISTINCT 关键字时,MySQL 会从查询结果中删除重复的行,只返回唯一的行。 多列DISTINCT 在MySQL 中,你可以使用 DISTINCT 关键字对多个列进行去重。这意味着查询结果中的每一行在指定的多个列上都必须是...
DISTINCT实际上和GROUP BY操作非常相似,只不过是在GROUP BY之后的每组中只取出一条记录而已 所以,DISTINCT的实现方式和GROUP BY基本相同,同样可以通过索引扫描来实现,当然...,在仅使用索引无法完成DISTINCT时,MySQL只能通过临时表来完成。...也就是说,当只进行DISTINCT操作的Query仅利用索引无法完成操作时,MySQL会利用...
$sql='select DISTINCT n.nid,tn.tid,n.title,n.created,ni.thumbpath from {term_node} tn INNER JOIN {node} n ON n.nid=tn.nid INNER JOIN {node_images} ni ON ni.nid=n.nid where tn.tid IN('.implode(',', $tids).') ORDER BY n.nid DESC'; $res=db_query($sql); $t_data=ar...
Bug #98249 Distinct in sub-query causes infinite run time with large table Submitted: 16 Jan 2020 10:31Modified: 16 Jul 2020 14:11 Reporter: Andras Janurik Email Updates: Status: Duplicate Impact on me: None Category: MySQL ServerSeverity: S5 (Performance) Version: 8.0.18OS: Windows (...
MySQL distinct 结果大于1 mysql in_use 大于0,容易踩雷造成的全表扫描尽量避免null值判断,会导致数据库引擎放弃索引进行全表扫描SELECT*FROMuserWHEREageISNULL优化方式:可以给字段添加默认值0,对0值进行判断。如下:SELECT*FROMuserWHEREage=0避免where条件中等号在左
5 rows in set (0.03 sec) 区别示例 两者的语法区别在于,group by可以进行单列去重,group by的原理是先对结果进行分组排序,然后返回每组中的第一条数据。且是根据group by的后接字段进行去重的。例如: mysql> select sex,age from student group by sex; ...
When combining LIMIT row_count with DISTINCT, MySQL stops as soon as it finds row_count unique rows. If you do not use columns from all tables named in a query, MySQL stops scanning any unused tables as soon as it finds the first match. In the following case, assuming that t1 is us...
Bug #117702 Incorrect Query Results When Using DISTINCT and Logical AND in Subquery Submitted: 14 Mar 8:48Modified: 14 Mar 10:21 Reporter: zhiqiang cheng Email Updates: Status: Verified Impact on me: None Category: MySQL Server: OptimizerSeverity: S2 (Serious) Version: 9.1.0 , 8.0.41,...
When I use DISTINCT in an INNER JOIN query that returns more than 1 column in the SELECT a full table scan is performed rather than using an index. When 1 column is returned in the SELECT the index is used. (1) Create the following tables: ...
MySQL Query Error: SELECT COUNT(*) FROM feedback GROUP BY songid ORDER BY new_time DESC Error Info:Unknown column 'new_time' in 'order clause' 本文转自 chengxuyonghu 51CTO博客,原文链接:http://blog.51cto.com/6226001001/1665581,如需转载请自行联系原作者...