mysql error 1215

1I'm guessing that Clients.Case_Number and/or Staff.Emp_ID are not exactly the same data type as Clients_has_Staff.Clients_Case_Number and Clients_has_Staff.Staff_Emp_ID.
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3Perhaps the columns in the parent tables are INT UNSIGNED?
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5They need to be exactly the same data type in both tables.

1When you are trying to reference a key on parent table which is not a candidate key (either a primary key or a unique key) you may get the error code: 1215 cannot add foreign key constraint. ... It does not necessarily to be a primary key. MySQL requires index on corresponding referenced key, so you need a unique key