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The foreign key for table child5 is an error because even though the parent key column has a unique index, the index uses a different collating sequence.
Tables child6 and child7 are incorrect because while both have UNIQUE indices on their parent keys, the keys are not an exact match to the columns of a single UNIQUE index.
Because the error occurs during a schema change, this is a DDL error.
Foreign key DDL errors are reported regardless of whether or not foreign key constraints are enabled when the table is created.
In the above paragraph, the term "equal" means equal when values are compared using the rules specified here.
If they are not the primary key, then the parent key columns must be collectively subject to a UNIQUE constraint or have a UNIQUE index.