The difference between functional dependencies in the database, primary code, candidate code, etc.

   From the above definition, it can be obtained that there must be a code in a relational table, and in addition:

    Strictly speaking: 
    primary attribute : refers to the primary key column, that is, the primary key consists of one column. 
    Primary key definition: an attribute or attribute set that can uniquely identify a tuple , that is, it can be composed of multiple columns. 

    In teaching, most of the instances are the primary key consisting of one column, so it can be simply said that the primary attribute is no different from the primary key.