What does sql constraint mean?

SQL constraints are rules that limit data in tables to ensure data consistency, accuracy and integrity. They are divided into five categories: Primary key constraints: Ensure row unique identification. Foreign key constraints: Associate the table to ensure that the values ​​in the referenced table exist in the referenced table. Unique constraints: Ensure that a specific column or group column contains unique values. Non-empty constraint: prevents columns from being empty. Check constraints: Verify that the data complies with custom rules.

SQL Constraints

SQL constraints are rules and conditions used to restrict data in SQL tables. They ensure consistency, accuracy and completeness of data. Constraints can be divided into the following categories:

Primary key constraints

• Specifies the column in the table that uniquely identifies each row.
• Make sure no two rows contain the same primary key value.
• For example, in the employee table, employee_id column can be specified as the primary key, ensuring that each employee has a unique identifier.

Foreign key constraints

• Associate columns in one table with columns in another table.
• Make sure that the values ​​in the referenced table exist in the referenced table.
• For example, in order table, customer_id column can be used as a foreign key, referring to customer_id primary key in the customer table, ensuring that each order belongs to a valid customer.

Unique constraints

• Make sure that one column or group of columns in the table contains unique values.
• Similar to primary key constraints, but allows multiple rows in a table to have the same value as long as the other columns are different.
• For example, in the product table, product_name column can be specified as a unique constraint to prevent the creation of different products with the same name.

Non-empty constraints

• Make sure that specific columns cannot be empty.
• Prevent null values ​​in the table, thereby improving data integrity.
• For example, in the address table, street_address column can be specified as non-empty to force the user to provide a street address when inserting a new address.

Check constraints

• Perform specific conditions to verify the data in the table.
• Allows to specify custom rules to limit the data types and ranges of values ​​that can be inserted.
• For example, in the salary table, a check constraint can be defined to ensure that the salary is less than or equal to $100,000.

Benefits of constraints

• Improve data quality: Constraints prevent incorrect or incomplete data from being inserted.
• Maintain data integrity: Constraints ensure that relationships between tables are consistent.
• Simplified data management: Constraints automate data verification, thereby reducing management burden.
• Improve application performance: Constraints optimize query performance because it narrows the search and reduces unnecessary data retrieval.

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