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CREATE TABLE child10(x,y,z, FOREIGN KEY(x,y,z) REFERENCES parent2); -- Error!By contrast, if foreign key errors can be recognized simply by looking at the definition of the child table and without having to consult the parent table definition, then the CREATE TABLE statement for the child table fails.

The foreign key declared as part of table child4 is an error because even though the parent key column is indexed, the index is not UNIQUE.The English language error message for foreign key DML errors is usually "foreign key mismatch" but can also be "no such table" if the parent table does not exist.Foreign key DML errors are reported if: CREATE TABLE parent2(a, b, PRIMARY KEY(a,b)); CREATE TABLE child8(x, y, FOREIGN KEY(x,y) REFERENCES parent2); -- Ok CREATE TABLE child9(x REFERENCES parent2); -- Error!Attempting to do so does not return an error; it simply has no effect.Usually, the parent key of a foreign key constraint is the primary key of the parent table.

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