Why Am I Getting MySQL Error 1215: Cannot Add Foreign Key Constraint?

Unable to Add Foreign Key: MySQL Error 1215

In an attempt to engineer a new database schema, a user encountered the MySQL error "Error 1215: Cannot add foreign key constraint." They have already set the database engine to InnoDB and verified that the keys designated as foreign keys are primary keys in their respective tables.

The issue may stem from a difference in data types between the foreign key columns in Clients_has_Staff and the corresponding primary key columns in the parent tables (Clients and Staff).

Possible Solution:

The user should check if the following columns have the same data type in all three tables:

• Clients_has_Staff.Clients_Case_Number
• Clients.Case_Number
• Clients_has_Staff.Staff_Emp_ID
• Staff.Emp_ID

For example, if the parent table columns are INT UNSIGNED, the foreign key columns in Clients_has_Staff must also be INT UNSIGNED. Failure to match data types will prevent the foreign key constraint from being added successfully.

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