Error Code: 1822 when data types are matching, with composite key

Question

Getting an

Error Code: 1822. Failed to add the foreign key constraint. Missing index for constraint \'subject_ibfk_1\' in the referenced table \'enro

Answer 1

The problem is due to the fact that the foreign key, subj_code, is part of a multi-column primary key (PK) in the referenced table enrolment:

primary key (stud_id, subj_code, semester, year)

where this column (subj_code) is not the leftmost one.

Table student does not have this problem because its foreign key column stud_id is the leftmost column of the PK in the referenced table.

To resolve this you can create a new index for the referened column:

ALTER TABLE enrolment ADD INDEX subj_code_idx (subj_code);

Note: You have to do the same for referenced table grade in the other foreign key.

Demo here