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Why am I violating an integrity constraint? (ORA-02291)

When I try to alter one of my tables to set one of it’s rows as a foreign key, I get the following error: “ORA-02291: integrity constraint (SYSTEM.M_FRAN_FK) violated – parent key not found” This error only occurs when I insert a row into the database with a none-null value. I’m …

I want to join two tables, removing duplicate values

I have to say I’m an amateur on MySQL, plus english is not my mother’s language. Here is my first table “Teacher”: And here my second table “Course”: I would like to know which country has more teachers giving courses, but I don’t want that it count the same teacher t…