What are the MySQL database constraint types

MySQL What are the types of database constraints

This article mainly introduces MySQL What are the relevant knowledge of database constraint types , The content is detailed and easy to understand , The operation is simple and fast , It has certain reference value , I believe that after reading this article MySQL What articles on database constraint types will be fruitful , Let's have a look .

Constraints in the database , As the name suggests, it is to limit the data inserted into the database , The purpose of this is to ensure the validity and integrity of the data . This greatly improves the quality of data in the database , It saves the space of the database and the time of calling data .

1.NOT NULL： Non empty constraint , Specifies that a column cannot be empty , Can only be used as a column level constraint , Only column level constraint syntax can be used to define .

SQL Medium null value ,null Case insensitive , Has the following characteristics ： The values of all data types can be null, Include int、float、boolean etc. . Empty string not equal to null,0 Also is not equal to null. Set a non empty constraint for the specified column when creating a table , Just add after the column definition not null that will do , for example ：

create table t_test
(
   id int not null,
   name varchar(255) default 'xyz' not null,
   gender varchar(2) null
);

Use alter table Add or delete non empty constraints when modifying tables , for example ：

# Add non NULL constraint alter table t_test modify gender varchar(2) not null;# Remove non empty constraints alter table t_test modify gender varchar(2) null;# Remove non empty constraints , And specify the default value alter table t_test modify name varchar(255) default 'abc' null;

2.UNIQUE: Unique constraint , The specified column or combination of columns cannot be repeated .

Although columns with unique constraints cannot have duplicate values , But there can be multiple null value , Because in the database null It's not equal to null. Multiple unique constraints can be created in the same table , Unique constraints can also be composed of multiple columns . When creating a unique constraint for a column ,MySQL A unique index will be created for this column , If you don't name a unique constraint , This unique constraint defaults to the same column name . Unique constraints can be created using column level syntax , You can also use table level syntax to create . If you are creating a combined constraint for multiple columns , Or you need to specify a constraint name for the constraint , Only table level syntax can be used .

Use column level syntax to create unique constraints , Just add... After the column definition unique Key words can be used :

# Create unique constraints when creating tables , Use column level syntax to create create table unique_test
(
   # Create a non NULL constraint , signify id Not for null   id int not null,
   # Establish unique constraints , This means that the column values of multiple rows of data cannot be equal    name varchar(255) unique
);

Table level syntax creates a unique constraint format ：[constraint Constraint name ] unique ( Name [, Name ,…]), The above table level constraint syntax format can be placed in create table Statement juxtaposed with the column definition , It can also be placed in alter table Use in statement add Keyword addition ：

# Create table time , Create unique constraints using table level syntax create table uniques_test2
(
   # by id Create a non NULL constraint , signify id Not for null   id int not null,
   name varchar(255),
   pass varchar(255),
   # The table level syntax used is name Establish unique constraints    unique (name),
   # The table level syntax used is pass Establish unique constraints , And specify the constraint name as test2_uk   constraint test2_uk unique (pass)
);# Create table time , Use table level syntax to create unique constraints on combined columns create table unique_test3
(
   id int not null,
   name varchar(255),
   pass varchar(255),
   # Appoint name and pass Two column combinations cannot be repeated    constraint test3_uk unique (name, pass)
);# When modifying a table , Use add Keyword to add unique constraints alter table unique_test3 add unique (id, name);# When modifying a table , Use modify keyword , To set unique constraints for a single column alter table unique_test3 modify name varchar(100) unique;

MySQL Delete unique constraint in ：

alter table tableName drop index  Constraint name ;
    example sentence ：# Delete unique_test3 In the table test3_uk Unique constraint alter table unique_test3 drop index test3_uk;

3.PRIMARY KEY: Primary key constraint , Specify the value of the column to uniquely identify the record .

4.FOREIGN KEY: Foreign key constraints , Specifies that the row record is subordinate to a record in the main table , It is mainly used to ensure referential integrity .

5.CHECK(MySQL I won't support it ): Check constraint , Specify a Boolean expression , The value used to specify the corresponding column must satisfy the expression .

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