When Should You Use 'ON UPDATE CASCADE' in SQL?

Understand the applicable scenarios of ON UPDATE CASCADE

Although ON DELETE CASCADE is often used, the application scenarios of ON UPDATE CASCADE may not be intuitive. This article explores scenarios where ON UPDATE CASCADE can be beneficial and answers some frequently asked questions about its functionality.

Scenario Analysis

Consider the following example:

<code class="language-sql">CREATE TABLE parent (
    id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (id)
);

CREATE TABLE child (
    id INT NOT NULL AUTO_INCREMENT,
    parent_id INT,
    INDEX par_ind (parent_id),
    FOREIGN KEY (parent_id)
        REFERENCES parent(id)
        ON DELETE CASCADE
);</code>

Question 1: Impact of update operations

Indeed, when the primary key of the "parent" table is modified, ON UPDATE CASCADE the record in the "child" table will be updated. However, this feature does not apply if the primary key is not updatable (for example, auto-incremented or set by a timestamp).

Question 2: Other scenarios

In addition to updating non-updatable primary keys, ON UPDATE CASCADE is also suitable for scenarios where the primary keys can be updated. For example, if the primary key is a UPC barcode and needs to be modified from 10 digits to 13 digits, ON UPDATE CASCADE make sure foreign key references in other tables are modified accordingly.

Problem 3: Invalid update

If a foreign key value is intentionally modified to an invalid value, a foreign key constraint error will usually be triggered, thus maintaining the integrity of the database.

Problem 4: Database vendor dependency

The behavior of

ON UPDATE CASCADE is generally independent of the database vendor. It is a standard feature supported by most relational database management systems.

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