Cannot add or update a child row: a foreign key constraint fails - How to solve MySQL error: child row has foreign key constraint

How to solve the MySQL error: The sub-row has foreign key constraints and requires specific code examples

When using the MySQL database, we may encounter "Cannot add or update a child row: a foreign key constraint fails" error. This error usually indicates a foreign key constraint problem when inserting or updating data. This article explains how to solve this problem and provides specific code examples.

First, let’s understand foreign key constraints. In a database, a foreign key is a relationship that is used to establish a connection between tables. It points to the primary key of another table and is used to maintain data consistency and integrity. Foreign key constraints restrict when inserting or updating data, specified conditions must be met, otherwise an error will be thrown.

When we encounter the "Cannot add or update a child row: a foreign key constraint fails" error, it is usually due to one of the following reasons:

1. There is a foreign key constraint, But the inserted or updated data does not satisfy the constraints.
2. There is a foreign key constraint, but the corresponding primary key does not exist in the main table.

The following are several common solutions:

1. Check whether the data satisfies the foreign key constraints: Before inserting or updating the data, ensure that the data satisfies the foreign key constraints. condition. For example, if the foreign key is related to the primary key of another table, then the data being inserted or updated on the foreign key field must exist in the primary table.
2. Check whether the corresponding primary key exists in the main table: If the corresponding primary key does not exist in the main table, a foreign key constraint error will be triggered when inserting or updating data in the subtable. In this case, you need to insert the corresponding primary key data into the main table first, and then insert or update the sub-table.

The following is a specific code example to illustrate how to solve this problem:

Suppose we have two tables: users table (users) and orders table (orders). The primary key of the user table is user ID (user_id), and the foreign key of the order table is user ID, which is used to associate the user table.

First, we create the user table:

CREATE TABLE users(
user_id INT PRIMARY KEY AUTO_INCREMENT,
username VARCHAR(50) NOT NULL
);

Then, we create the order table:

CREATE TABLE orders(
order_id INT PRIMARY KEY AUTO_INCREMENT,
user_id INT,
order_date DATE,
FOREIGN KEY (user_id) REFERENCES users(user_id)
);

Now, we want to insert a piece of data into the order table, but when the user ID does not exist in the user table, a foreign key constraint error will be triggered. In order to solve this problem, we can first insert a corresponding user data in the user table, and then insert the order data:

-- Insert user data first
INSERT INTO users(username) VALUES ('John ');

-- Then insert order data
INSERT INTO orders(user_id, order_date) VALUES (1, '2022-01-01');

Through the above example code , we can solve the error "Cannot add or update a child row: a foreign key constraint fails".

To summarize, when solving this error, we need to ensure that the data satisfies the conditions of the foreign key constraints and the corresponding primary key exists in the main table. By carefully checking the data and table structure, and inserting or updating data in the correct order, we can successfully solve this problem.

I hope the content of this article will be helpful in solving the "Cannot add or update a child row: a foreign key constraint fails" error. If you encounter similar problems, please take appropriate solutions based on the specific situation and refer to the above code samples. Good luck with your MySQL database operations!

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