How to fix ERROR 1216: Cannot add or update a child row: a foreign key constraint fails in MySQL

First, check that the parent record actually exists in the parent table. Query the parent table to confirm the referenced value is present:

-- Check if the parent record exists
SELECT * FROM parent_table WHERE id = 123;

If the query returns no results, the parent record does not exist and you must create it before inserting the child row.

Verify that the foreign key column data type in the child table matches the referenced column data type in the parent table. Use SHOW CREATE TABLE to inspect the schema:

-- Check parent table structure
SHOW CREATE TABLE parent_table\G

-- Check child table structure  
SHOW CREATE TABLE child_table\G

Look at the column definitions. The foreign key column (e.g., parent_id INT) must match the data type of the parent table's primary key. Common mismatches include INT vs BIGINT, VARCHAR length differences, or UNSIGNED vs signed integers.

Review the foreign key constraint to ensure it references the correct parent table and column:

-- View all foreign keys for a table
SELECT CONSTRAINT_NAME, TABLE_NAME, COLUMN_NAME, REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE TABLE_NAME = 'child_table' AND COLUMN_NAME = 'parent_id';

Confirm that the REFERENCED_TABLE_NAME and REFERENCED_COLUMN_NAME point to the correct parent table and column. If the constraint is malformed, drop and recreate it.

If loading data, ensure parent table records are inserted before child table records. For example:

-- Insert parent first
INSERT INTO parent_table (id, name) VALUES (123, 'Parent Name');

-- Then insert child
INSERT INTO child_table (id, parent_id, value) VALUES (1, 123, 'Child Value');

If you have a large dataset to import, consider inserting all parent data first, then all child data.

For large migrations or bulk data loads, you can temporarily disable foreign key checks. Use this approach with caution, as it bypasses referential integrity checks:

SET FOREIGN_KEY_CHECKS = 0;

-- Perform your INSERT, UPDATE, or LOAD DATA operations here
LOAD DATA INFILE '/path/to/data.csv'
INTO TABLE child_table
FIELDS TERMINATED BY ',';

SET FOREIGN_KEY_CHECKS = 1;

After re-enabling checks, MySQL will validate all foreign key constraints. If violations exist, the second SET command will fail and you'll need to fix the data.

On case-sensitive systems (Linux), ensure the parent table value case matches exactly:

-- Check actual values in parent table
SELECT DISTINCT column_name FROM parent_table;

-- Verify case matches in child insert
INSERT INTO child_table (parent_id) VALUES ('EXACT_CASE_VALUE');

If the parent table has 'Docker' but you're inserting 'docker', it will fail on case-sensitive systems.