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Editor to share with you a brief introduction to the constraints in MySQL, I believe most people do not know much about it, so share this article for your reference, I hope you can learn a lot after reading this article, let's go to know it!

1) Primary key constraint

Primary key constraints are the most frequently used constraints. When designing a data table, it is generally required to set a primary key in the table.

A primary key is a special field of a table that uniquely identifies each piece of information in the table. For example, the student number in the student information table is unique.

PS: null values are not allowed to set primary key constraints.

2) Foreign key constraint

Foreign key constraints are often used in conjunction with primary key constraints to ensure data consistency.

For example, a fruit stand, only apples, peaches, plums, watermelons four kinds of fruit, then you come to the fruit stall to buy fruit can only choose apples, peaches, plums and watermelons, can not buy other fruits.

3) unique constraint

Unique constraints guarantee that the data in a field or set of fields is unique compared to the data in other rows in the table. The only similarity between a constraint and a primary key constraint is that they both ensure the uniqueness of the column. Unlike primary key constraints, unique constraints can have more than one in a table, and columns that set unique constraints are allowed to have null values, although only one null value is allowed.

4) check constraints

A check constraint is a rule that confirms acceptable field values for data in a record in a table. Checking constraints helps enforce domain integrity. Domain integrity defines valid values for fields in a database table. Check constraints can verify the domain integrity of a single field or some fields. You can have multiple checks for integrity on a single field. If the inserted or updated data violates a check constraint, the database engine will not allow the insert or update to occur.

5) non-empty constraint

A non-null constraint means that the contents of a field in the table are not allowed to be empty, and if you want to use a non-null constraint, you only need to declare it with "NOT NULL" after each column.

6) default value constraint

The default value constraint is used to specify the default value for a column. For example, if there are more male students in the class, then when setting up the "gender" column of the class table, you can set it to "male" by default.

The above is all the content of the article "introduction to constraints in MySQL". Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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