What does sql foreign key mean

SQL foreign keys are defined as relationship constraints between tables, ensuring data consistency by associating foreign keys and primary keys. The functions include: maintaining data consistency, preventing deletion of dependent data, and mandatory reference integrity. The syntax is CREATE TABLE child_table (child_id INT PRIMARY KEY, parent_id INT, FOREIGN KEY (parent_id) REFERENCES parent_table (parent_id)). The advantages are guaranteed data integrity, manage deletion dependencies and simplify queries, and the disadvantages are that they may reduce performance and increase inter-table relationship complexity.

SQL Foreign Keys

definition:
SQL foreign keys are a database constraint that establish relationships between two tables to ensure consistency and integrity of the data. A foreign key associates a column in one table with a column in the primary key in another table.

effect:
Foreign keys have the following functions:

• Maintain data consistency: Prevent data inconsistencies by ensuring that data in related tables match.
• Prevent deletion of dependent data: When records in the primary key table are deleted, foreign keys prevent relevant records from being deleted from the slave table.
• Forced reference integrity: Foreign keys ensure that values ​​in slave tables refer to valid values ​​present in primary key tables.

grammar:
In SQL, foreign key constraints are created using FOREIGN KEY clause:

 <code class="sql">CREATE TABLE child_table ( child_id INT PRIMARY KEY, parent_id INT, FOREIGN KEY (parent_id) REFERENCES parent_table (parent_id) );</code>

In this example, parent_id column in child_table is a foreign key that references parent_id primary key in parent_table .

advantage:

• Data integrity is guaranteed
• Delete dependencies to be managed
• Complex queries are simplified

shortcoming:

• May degrade insertion and update performance
• The relationship between tables becomes more complex

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