今天说说数据库中的数据表数据约束问题,约束大概分为主键约束,外键约束,默认约束,唯一约束,非空约束这几种,关于讲解配置,我后面慢慢说 1、主键约束 其实是用来唯一标示表中的一个列或多个列,但是一个表中必须唯一,配置方法 (1)、初始化列级主键约
今天说说数据库中的数据表数据约束问题,约束大概分为主键约束,外键约束,默认值约束,唯一约束,非空约束这几种,关于讲解配置,我后面慢慢说
1、主键约束
其实是用来唯一标示表中的一个列或多个列,但是一个表中必须唯一,配置方法
(1)、初始化列级主键约束
create table custominfo ( id int primary key, name varchar(12), age int );
(2)、初始化表级主键约束
create table custominfo ( id int, name varchar(12), age int, primary key(id) );
(3)、修改主键约束
alter table customerinfo add constraint yueshu primary key(id);
(4)、修改添加联合主键约束
alter table customerinfo add constraint yueshu primary key(id,name);
(5)、删除主键约束
alter table customerinfo drop primary key;
2、外键约束
比如某个表B关联于表A,也就是表A中所有项目表B中都有,就可以设置表A主键关联,表B外键关联
(1)初始化添加
create table typeinfo ( typeid int primary key, type varchar(20) ); create table roominfo ( roomid int primary key, type varchar(20), price int constraint yueshu foreign key(roomid) references typeinfo(typeid) );
(2)、修改添加
alter table roominfo add constraint yueshu foreign key(roomid) references typeinfo(typeid);
(3)、删除外键约束
alter table roominfo drop foreign key yueshu;
3、默认值约束
将某一数据初始化为固定值
(1)、初始化设置
create table roominfo ( id int, name varchar(20), price int default 0 );
(2)、修改添加设置
alter table roominfo alter price set default 0;
(3)、删除默认约束
alter table roominfo alter price drop default;
4、非空约束
某项数据不得为空的约束
(1)、初始化设置
create table roominfo ( id int not null, name varchar(20) );
(2)、修改设置
alter table roominfo alter id int not null;
5、检查约束
就是固定行规定数据的范围
(1)、初始化列级设置
<pre name="code" class="sql"><pre name="code" class="sql" style="background-color: rgb(255, 255, 255); "><pre name="code" class="sql">create table roominfo ( id int, name varchar(20), price int check(price>=0) );
(2)、初始化表级设置
<pre name="code" class="sql">create table roominfo ( id int, name varchar(20), price int, check(price>=0) );
(3)、修改设置
alter table roominfo add constraint yueshu check(price>=0);
6、唯一约束
(1)、初始化列级唯一约束
create table roominfo ( id int unique, name varchar(20) );
(2)、初始化表级唯一约束
create table roominfo ( id int, name varchar(20), unique(id) );
可以设置多个独立唯一约束
(3)、初始化创建共同唯一约束
create table roominfo ( id int, name varchar(20), price int, unique(id,name) );
(4)、修改设置独立唯一约束
alter table roominfo add constraint yueshu unique(id);
(5)、修改设置共同唯一约束
alter table roominfo add constraint yueshu unqiue(id,name);
(6)、删除唯一约束
drop index yueshu on roominfo;

MySQL is an open source relational database management system, mainly used to store and retrieve data quickly and reliably. Its working principle includes client requests, query resolution, execution of queries and return results. Examples of usage include creating tables, inserting and querying data, and advanced features such as JOIN operations. Common errors involve SQL syntax, data types, and permissions, and optimization suggestions include the use of indexes, optimized queries, and partitioning of tables.

MySQL is an open source relational database management system suitable for data storage, management, query and security. 1. It supports a variety of operating systems and is widely used in Web applications and other fields. 2. Through the client-server architecture and different storage engines, MySQL processes data efficiently. 3. Basic usage includes creating databases and tables, inserting, querying and updating data. 4. Advanced usage involves complex queries and stored procedures. 5. Common errors can be debugged through the EXPLAIN statement. 6. Performance optimization includes the rational use of indexes and optimized query statements.

MySQL is chosen for its performance, reliability, ease of use, and community support. 1.MySQL provides efficient data storage and retrieval functions, supporting multiple data types and advanced query operations. 2. Adopt client-server architecture and multiple storage engines to support transaction and query optimization. 3. Easy to use, supports a variety of operating systems and programming languages. 4. Have strong community support and provide rich resources and solutions.

InnoDB's lock mechanisms include shared locks, exclusive locks, intention locks, record locks, gap locks and next key locks. 1. Shared lock allows transactions to read data without preventing other transactions from reading. 2. Exclusive lock prevents other transactions from reading and modifying data. 3. Intention lock optimizes lock efficiency. 4. Record lock lock index record. 5. Gap lock locks index recording gap. 6. The next key lock is a combination of record lock and gap lock to ensure data consistency.

The main reasons for poor MySQL query performance include not using indexes, wrong execution plan selection by the query optimizer, unreasonable table design, excessive data volume and lock competition. 1. No index causes slow querying, and adding indexes can significantly improve performance. 2. Use the EXPLAIN command to analyze the query plan and find out the optimizer error. 3. Reconstructing the table structure and optimizing JOIN conditions can improve table design problems. 4. When the data volume is large, partitioning and table division strategies are adopted. 5. In a high concurrency environment, optimizing transactions and locking strategies can reduce lock competition.

In database optimization, indexing strategies should be selected according to query requirements: 1. When the query involves multiple columns and the order of conditions is fixed, use composite indexes; 2. When the query involves multiple columns but the order of conditions is not fixed, use multiple single-column indexes. Composite indexes are suitable for optimizing multi-column queries, while single-column indexes are suitable for single-column queries.

To optimize MySQL slow query, slowquerylog and performance_schema need to be used: 1. Enable slowquerylog and set thresholds to record slow query; 2. Use performance_schema to analyze query execution details, find out performance bottlenecks and optimize.

MySQL and SQL are essential skills for developers. 1.MySQL is an open source relational database management system, and SQL is the standard language used to manage and operate databases. 2.MySQL supports multiple storage engines through efficient data storage and retrieval functions, and SQL completes complex data operations through simple statements. 3. Examples of usage include basic queries and advanced queries, such as filtering and sorting by condition. 4. Common errors include syntax errors and performance issues, which can be optimized by checking SQL statements and using EXPLAIN commands. 5. Performance optimization techniques include using indexes, avoiding full table scanning, optimizing JOIN operations and improving code readability.


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