The criteria for second normal form (2NF) are:
The table must be in 1NF.
None of the non-prime attributes of the table are functionally dependent on a part (proper subset) of a candidate key; in other words, all functional dependencies of non-prime attributes on candidate keys are full functional dependencies. For example, in an "Employees' Skills" table whose attributes are Employee ID, Employee Address, and Skill, the combination of Employee ID and Skill uniquely identifies records within the table. Given that Employee Address depends on only one of those attributes – namely, Employee ID – the table is not in 2NF.
Note that if none of a 1NF table's candidate keys are composite – i.e. every candidate key consists of just one attribute – then we can say immediately that the table is in 2NF.