用distinct关键字只能过滤查询字段中所有记录相同的(记录集相同),而如果要指定一个字段却没有效果,另外distinct关键字会排序
用distinct关键字只能过滤查询字段中所有记录相同的(记录集相同),而如果要指定一个字段却没有效果,另外distinct关键字会排序,效率很低 。
select distinct name from t1 能消除重复记录,但只能取一个字段,现在要同时取id,name这2个字段的值。
select distinct id,name from t1 可以取多个字段,但只能消除这2个字段值全部相同的记录
所以用distinct达不到想要的效果,用group by 可以解决这个问题。
例如要显示的字段为A、B、C三个,而A字段的内容不能重复可以用下面的语句:
select A, min(B),min(C),count(*) from [table] where [条件] group by A
having [条件] order by A desc
为了显示标题头好看点可以把select A, min(B),min(C),count(*) 换称select A as A, min(B) as B,min(C) as C,count(*) as 重复次数
显示出来的字段和排序字段都要包括在group by 中
但显示出来的字段包有min,max,count,avg,sum等聚合函数时可以不在group by 中
如上句的min(B),min(C),count(*)
一般条件写在where 后面
有聚合函数的条件写在having 后面
如果在上句中having加 count(*)>1 就可以查出记录A的重复次数大于1的记录
如果在上句中having加 count(*)>2 就可以查出记录A的重复次数大于2的记录
如果在上句中having加 count(*)>=1 就可以查出所有的记录,,但重复的只显示一条,并且后面有显示重复的次数----这就是所需要的结果,而且语句可以通过hibernate
下面语句可以查询出那些数据是重复的:
select 字段1,字段2,count(*) from 表名 group by 字段1,字段2 having count(*) > 1
将上面的>号改为=号就可以查询出没有重复的数据了。
例如 select count(*) from (select gcmc,gkrq,count(*) from gczbxx_zhao t group by gcmc,gkrq having
count(*)>=1 order by GKRQ)
select * from gczbxx_zhao where viewid in ( select max(viewid) from gczbxx_zhao group by
gcmc ) order by gkrq desc ---还是这个可行。
有一面试题说:distinct去重复的效率很底下,我在网上看到这遍文章的方法好像说是用 group by having 效率很高了?
我在了一个测试,有一商品表,26万条记录,只有商品编号建了索引,对品牌名称字段做 distinct
select brand,count(*) from tab_commbaseinfo group by brand having count(*) =1
平均时间是:0.453
select distinct brand from tab_commbaseinfo
平均时间是:0.39
搞不懂是否还有其它方法。

MySQL processes data replication through three modes: asynchronous, semi-synchronous and group replication. 1) Asynchronous replication performance is high but data may be lost. 2) Semi-synchronous replication improves data security but increases latency. 3) Group replication supports multi-master replication and failover, suitable for high availability requirements.

The EXPLAIN statement can be used to analyze and improve SQL query performance. 1. Execute the EXPLAIN statement to view the query plan. 2. Analyze the output results, pay attention to access type, index usage and JOIN order. 3. Create or adjust indexes based on the analysis results, optimize JOIN operations, and avoid full table scanning to improve query efficiency.

Using mysqldump for logical backup and MySQLEnterpriseBackup for hot backup are effective ways to back up MySQL databases. 1. Use mysqldump to back up the database: mysqldump-uroot-pmydatabase>mydatabase_backup.sql. 2. Use MySQLEnterpriseBackup for hot backup: mysqlbackup--user=root-password=password--backup-dir=/path/to/backupbackup. When recovering, use the corresponding life

The main reasons for slow MySQL query include missing or improper use of indexes, query complexity, excessive data volume and insufficient hardware resources. Optimization suggestions include: 1. Create appropriate indexes; 2. Optimize query statements; 3. Use table partitioning technology; 4. Appropriately upgrade hardware.

MySQL view is a virtual table based on SQL query results and does not store data. 1) Views simplify complex queries, 2) Enhance data security, and 3) Maintain data consistency. Views are stored queries in databases that can be used like tables, but data is generated dynamically.

MySQLdiffersfromotherSQLdialectsinsyntaxforLIMIT,auto-increment,stringcomparison,subqueries,andperformanceanalysis.1)MySQLusesLIMIT,whileSQLServerusesTOPandOracleusesROWNUM.2)MySQL'sAUTO_INCREMENTcontrastswithPostgreSQL'sSERIALandOracle'ssequenceandt

MySQL partitioning improves performance and simplifies maintenance. 1) Divide large tables into small pieces by specific criteria (such as date ranges), 2) physically divide data into independent files, 3) MySQL can focus on related partitions when querying, 4) Query optimizer can skip unrelated partitions, 5) Choosing the right partition strategy and maintaining it regularly is key.

How to grant and revoke permissions in MySQL? 1. Use the GRANT statement to grant permissions, such as GRANTALLPRIVILEGESONdatabase_name.TO'username'@'host'; 2. Use the REVOKE statement to revoke permissions, such as REVOKEALLPRIVILEGESONdatabase_name.FROM'username'@'host' to ensure timely communication of permission changes.


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

PhpStorm Mac version
The latest (2018.2.1) professional PHP integrated development tool

ZendStudio 13.5.1 Mac
Powerful PHP integrated development environment

DVWA
Damn Vulnerable Web App (DVWA) is a PHP/MySQL web application that is very vulnerable. Its main goals are to be an aid for security professionals to test their skills and tools in a legal environment, to help web developers better understand the process of securing web applications, and to help teachers/students teach/learn in a classroom environment Web application security. The goal of DVWA is to practice some of the most common web vulnerabilities through a simple and straightforward interface, with varying degrees of difficulty. Please note that this software

Atom editor mac version download
The most popular open source editor

Notepad++7.3.1
Easy-to-use and free code editor
