Referencing column and referenced column are incompatible in MySQL
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This error occurs in MySQL when the column you are attempting to add a foreign key constraint on does not have a matching data type with the column you are linking to in the parent table.
Example
Consider the following table pupils that contains pupil names:
CREATE TABLE pupils ( id INT UNSIGNED AUTO_INCREMENT, name VARCHAR(30), PRIMARY KEY (id));
INSERT INTO pupils (name) VALUES ('axel'), ('bob'), ('cathy');
SELECT * FROM pupils;
+----+-------+| id | name |+----+-------+| 1 | axel || 2 | bob || 3 | cathy |+----+-------+
To create a new table product with a foreign key constraint on bought_by that refers to id from the pupil table:
CREATE TABLE product ( id INT UNSIGNED AUTO_INCREMENT, name VARCHAR(30), bought_by INT, PRIMARY KEY (id), FOREIGN KEY (bought_by) REFERENCES pupil (id));
ERROR 3780 (HY000): Referencing column 'bought_by' and referenced column 'id' in foreign key constraint 'product_ibfk_1' are incompatible.
Even though both columns are type INT, as the id column in pupils is UNSIGNED (i.e. cannot take negative values) and the bought_by column is SIGNED (i.e. can take negative values) these columns are considered incompatible. Updating one of the column types to match the other will allow you to create the table with the foreign key constraint.
Now specifying bought_by as INT UNSIGNED:
CREATE TABLE product ( id INT UNSIGNED AUTO_INCREMENT, name VARCHAR(30), bought_by INT UNSIGNED, PRIMARY KEY (id), FOREIGN KEY (bought_by) REFERENCES pupil (id));
Query OK, 0 rows affected (0.01 sec)
We are able to create the table successfully with the foreign key.
