List the full name and social security number of the account representative with average performance.
full name = firstname, lastname; social security number refers to ssn; account representative is a position title; average performance refers to performance = 'Average'
SELECT T1.firstname, T1.lastname, T1.ssn FROM employee AS T1  INNER JOIN position AS T2  ON T1.positionID = T2.positionID WHERE T1.performance = 'Average'