本文章讲了三款sql 查询重复记录与删除重复记录的sql语句,并且每个都利用了不同的方法来举例说明其它作用与可行性,以及它的效率。
本文章讲了三款sql 查询重复记录与删除重复记录的sql语句,并且每个都利用了不同的方法来举例说明其它作用与可行性,以及它的效率。
select *, count(distinct name) from table group by name
结果:
id name count(distinct name)
1 a 1
2 b 1
3 c 1
方法二
表 table1
id regname postionsn personsn
1 山东齐鲁制药 223 2
2 山东齐鲁制药 224 2
3 北京城建公司 225 2
4 科技公司 225 2
我想获得结果是
id regname postionsn personsn
1 山东齐鲁制药 223 2
3 北京城建公司 225 2
4 科技公司 225 2
select distinct regname,postionsn,personsn from table1
如果查询的是多列 distinct 用和不用一样
只能用group by
用group by regname
select * from table1 where id in (select min(id) from table1 group by regname) and personsn=2
实例三
个人成绩表:
学号 课号 成绩
01 01 80
01 02 79
01 03 88
02 01 87
02 02 77
02 03 68
用sql把上表转换为:
学号 课号01 课号02 课号03
01 80 79 88
02 87 77 68
---------建表----------
create table tab_score
(
bid int identity(0,1) primary key ,--流水号
sid varchar(20) not null,--学生号
cid varchar(20) not null,--课程号
score int--成绩
)
insert into tab_score select's01','c01','90' union all select 's01','c02','92' union all select 's01','c03','93'
union all select 's02','c01','81' union all select 's02','c02','82'
/*---固定列的写法,后面的写法将是根据有几个课程id来动态组装中间的sum语句,然后加上头尾就成了,
理解了这种'静态'写法,剩下的只是'动态'组装中间sum语句的工作----*/
select * from tab_score
select sid,sum(case cid when 'c01' then score else '0' end) as 'c01',
sum(case cid when 'c02' then score else '0' end) as 'c02',
sum(case cid when 'c03' then score else '0' end) as 'c03'
from tab_score group by sid
-----'动态'列的写法,定义一个变量来组装中间的sum语句,其中用到子查询表(原来不用这方法一直会出现重复列)-------
declare @s varchar(1000)
set @s=''
select @s=@s+', sum(case cid when '+''''+ a.cid+''''+' then score else ''0'' end) as '+''''+a.cid+''''
from ( select distinct cid from tab_score) a
print @*
**ec('select sid'+@s+'from tab_score group by sid')

InnoDBBufferPool reduces disk I/O by caching data and indexing pages, improving database performance. Its working principle includes: 1. Data reading: Read data from BufferPool; 2. Data writing: After modifying the data, write to BufferPool and refresh it to disk regularly; 3. Cache management: Use the LRU algorithm to manage cache pages; 4. Reading mechanism: Load adjacent data pages in advance. By sizing the BufferPool and using multiple instances, database performance can be optimized.

Compared with other programming languages, MySQL is mainly used to store and manage data, while other languages such as Python, Java, and C are used for logical processing and application development. MySQL is known for its high performance, scalability and cross-platform support, suitable for data management needs, while other languages have advantages in their respective fields such as data analytics, enterprise applications, and system programming.

MySQL is worth learning because it is a powerful open source database management system suitable for data storage, management and analysis. 1) MySQL is a relational database that uses SQL to operate data and is suitable for structured data management. 2) The SQL language is the key to interacting with MySQL and supports CRUD operations. 3) The working principle of MySQL includes client/server architecture, storage engine and query optimizer. 4) Basic usage includes creating databases and tables, and advanced usage involves joining tables using JOIN. 5) Common errors include syntax errors and permission issues, and debugging skills include checking syntax and using EXPLAIN commands. 6) Performance optimization involves the use of indexes, optimization of SQL statements and regular maintenance of databases.

MySQL is suitable for beginners to learn database skills. 1. Install MySQL server and client tools. 2. Understand basic SQL queries, such as SELECT. 3. Master data operations: create tables, insert, update, and delete data. 4. Learn advanced skills: subquery and window functions. 5. Debugging and optimization: Check syntax, use indexes, avoid SELECT*, and use LIMIT.

MySQL efficiently manages structured data through table structure and SQL query, and implements inter-table relationships through foreign keys. 1. Define the data format and type when creating a table. 2. Use foreign keys to establish relationships between tables. 3. Improve performance through indexing and query optimization. 4. Regularly backup and monitor databases to ensure data security and performance optimization.

MySQL is an open source relational database management system that is widely used in Web development. Its key features include: 1. Supports multiple storage engines, such as InnoDB and MyISAM, suitable for different scenarios; 2. Provides master-slave replication functions to facilitate load balancing and data backup; 3. Improve query efficiency through query optimization and index use.

SQL is used to interact with MySQL database to realize data addition, deletion, modification, inspection and database design. 1) SQL performs data operations through SELECT, INSERT, UPDATE, DELETE statements; 2) Use CREATE, ALTER, DROP statements for database design and management; 3) Complex queries and data analysis are implemented through SQL to improve business decision-making efficiency.

The basic operations of MySQL include creating databases, tables, and using SQL to perform CRUD operations on data. 1. Create a database: CREATEDATABASEmy_first_db; 2. Create a table: CREATETABLEbooks(idINTAUTO_INCREMENTPRIMARYKEY, titleVARCHAR(100)NOTNULL, authorVARCHAR(100)NOTNULL, published_yearINT); 3. Insert data: INSERTINTObooks(title, author, published_year)VA


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