MySql-如何查询删除数据表重复记录

Home

Database

Mysql Tutorial

MySql-如何查询删除数据表重复记录_MySQL

WBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWB

Jun 08, 2016 am 08:50 AM

demohttpmysql

查询

<code class="hljs oxygene">SELECT
    name, email, COUNT(*)
FROM
    users
GROUP BY
    name, email
HAVING 
    COUNT(*) > 1</code>

重点来了，查询容易，那应该如何删除重复记录呢？

演示数据 表结构：

<code class="hljs oxygene"><code class="hljs asciidoc">mysql> desc demo;
+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| id    | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| site  | varchar(100)     | NO   | MUL |         |                |
+-------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)</code></code>

<code class="hljs asciidoc">数据：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc">mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
|  4 | http://www.CodeBit.cn  |
|  5 | http://www.ShuoWen.org |
+----+------------------------+
5 rows in set (0.00 sec)</code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc">当没有创建表或创建索引权限的时候，可以用下面的方法：

<code class="hljs asciidoc"><code class="hljs asciidoc">如果你要删除较旧的重复记录，可以使用下面的语句：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml">mysql> delete from a 
    -> using demo as a, demo as b
    -> where (a.id > b.id)
    -> and (a.site = b.site);
Query OK, 2 rows affected (0.12 sec)</code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)</code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">如果你要删除较新的重复记录，可以使用下面的语句：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml">mysql> delete from a 
    -> using demo as a, demo as b
    -> where (a.id < b.id)
    -> and (a.site = b.site);
Query OK, 2 rows affected (0.12 sec)</code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  2 | http://YITU.org        |
|  4 | http://www.CodeBit.cn  |
|  5 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)</code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">你可以用下面的语句先确认将被删除的重复记录：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc">mysql> SELECT a.* 
    -> FROM demo a, demo b
    -> WHERE a.id > b.id
    -> AND (a.site = b.site);
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
2 rows in set (0.00 sec)</code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm">mysql> alter ignore table demo add unique index ukey (site);
Query OK, 5 rows affected (0.46 sec)
Records: 5  Duplicates: 2  Warnings: 0</code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc">mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)</code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc">重复记录被删除后，如果需要，可以删除索引：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm">mysql> alter table demo drop index ukey;
Query OK, 3 rows affected (0.37 sec)
Records: 3  Duplicates: 0  Warnings: 0</code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm">如果有创建表的权限，可以用下面的方法：

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm">创建一个新表，然后将原表中不重复的数据插入新表：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene">mysql> create table demo_new as select * from demo group by site;
Query OK, 3 rows affected (0.19 sec)
Records: 3  Duplicates: 0  Warnings: 0</code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc">mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| demo           |
| demo_new       |
+----------------+
2 rows in set (0.00 sec)</code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc">mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
|  4 | http://www.CodeBit.cn  |
|  5 | http://www.ShuoWen.org |
+----+------------------------+
5 rows in set (0.00 sec)</code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">mysql> select * from demo_new order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">然后将原表备份，将新表重命名为当前表：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">mysql> rename table demo to demo_old, demo_new to demo;
Query OK, 0 rows affected (0.04 sec)

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| demo           |
| demo_old       |
+----------------+
2 rows in set (0.00 sec)</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">注意：使用这种方式创建的表会丢失原表的索引信息！

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">mysql> desc demo;
+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| id    | int(11) unsigned | NO   |     | 0       |       |
| site  | varchar(100)     | NO   |     |         |       |
+-------+------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">如果要保持和原表信息一致，你可以使用 show create table demo; 来查看原表的创建语句，然后使用原表的创建语句创建新表，接着使用 insert … select 语句插入数据，再重命名表即可。

Statement

The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Explain the InnoDB Buffer Pool and its importance for performance.Apr 19, 2025 am 12:24 AM

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.

MySQL vs. Other Programming Languages: A ComparisonApr 19, 2025 am 12:22 AM

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.

Learning MySQL: A Step-by-Step Guide for New UsersApr 19, 2025 am 12:19 AM

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: Essential Skills for Beginners to MasterApr 18, 2025 am 12:24 AM

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: Structured Data and Relational DatabasesApr 18, 2025 am 12:22 AM

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: Key Features and Capabilities ExplainedApr 18, 2025 am 12:17 AM

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.

The Purpose of SQL: Interacting with MySQL DatabasesApr 18, 2025 am 12:12 AM

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.

MySQL for Beginners: Getting Started with Database ManagementApr 18, 2025 am 12:10 AM

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

See all articles