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What are the constraints in mysql?

2024-04-27 09:27:211117browse

Constraints in MySQL are rules that limit the integrity and consistency of data in a table. Constraints include: Main constraints: primary key, foreign key, unique key Other constraints: non-null, default value, auto-increment, check Benefits: ensure data integrity, consistency, improve performance, enforce standardization Application method: when creating or Use the CREATE TABLE or ALTER TABLE statement when modifying the table

What are the constraints in mysql?

Constraint conditions in MySQL

Constraint conditions are used to limit the data in the database table rules to ensure data integrity and consistency. A wide range of constraint types are provided in MySQL to meet different data validation needs.

1. Main constraints

  • Primary key (PRIMARY KEY): The column or column combination that uniquely identifies each row in the table.
  • Foreign key (FOREIGN KEY): Force a column or column combination in one table to establish a relationship with the primary key column of another table.
  • Unique Key (UNIQUE): Ensures that values ​​for a specific column or combination of columns in a table are unique.

2. Other constraints

  • Non-null (NOT NULL): Requires that specific columns in the table cannot be NULL value.
  • Default value (DEFAULT): Specifies a default value for a specific column in the table, which is automatically populated when a new row is inserted.
  • Auto-increment (AUTO_INCREMENT): Creates an auto-incrementing sequence of integers for a specific column in a table, typically used for primary keys.
  • Check (CHECK): Verify that the value of a specific column or combination of columns in a table meets specified conditions.

3. Benefits of using constraints

Constraints provide the following benefits:

  • Ensure data integrity: prevent the insertion of invalid or inconsistent data .
  • Maintain data consistency: enforce relationships and referential integrity between tables.
  • Improve performance: Improve query efficiency by creating indexes and eliminating duplicate data.
  • Forced data standardization: Ensure that data follows predefined rules to facilitate data management and analysis.

4. Apply constraints

You can apply constraints in a MySQL table in the following ways:

  • CREATE TABLE statement : Specify constraints when creating a table.
  • ALTER TABLE Statement: Add or remove constraints to an existing table.

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