Get the first and last names of all employee who entered the project at the same time as at least one other employee
You are missing conditions that say that the employee is working on the project in w1, and that the project in w2 is the same as the project in w1 but that the employee in w2 is not the same as the employee in w1.

The reason you get all rows is because the where condition always evaluates to true, for all employees. That is because the sub query simply asks for at least one row that has the same date in both w1 and w2. Consider that WorksOn_m has only one row:


  WorksOn_m
  -------------- EnterDate 12/03/2014


Is there a record in both w1 and w2 with the date 12/03/2014? Yes, because they're the same table. Are you applying any other conditions in the WHERE clause? No. So you'll get all employees, because you're basically saying "Give me all employees WHERE 1=1".