title: SQL query issue with achieving an encompasses all effect
body: <p>Below is the query I have been working on for this question.</p>
  
  <p>Find the names of the companies which have employees residing in every city where employees of Mutual of Omaha live.</p>
  
  <p>This means that if Mutual has employees in the cities Omaha, Lincoln, and Denver that the only company names it should return is a company that has employees in all 3 of those cities. This should also return Mutual. </p>
  
  <p>The below query returns the company which has an employee in any of those three cities. The lastname is there for me to manually check which employees it is counting.        </p>
  
  <pre><code>    SELECT COMPANY_NAME, e1.lastname
      FROM EMPLOYEE E1,WORKS W1 
      WHERE E1.CITY IN (SELECT CITY 
        FROM EMPLOYEE E2,WORKS W2 
        WHERE E2.firstname = W2.firstname
          AND E2.lastname = W2.lastname 
          AND W2.COMPANY_NAME= 'Mutual of Omaha')  
      AND E1.firstname = W1.firstname 
      AND E1.lastname = W1.lastname;
  </code></pre>
  
  <p>I realized I didn't put the tables down so here they are</p>
  
  <p>employee (Lastname, FirstName, MidInitial, gender, street, city)</p>
  
  <p>works (Lastname, FirstName, MidInitial, company_name, salary)</p>
  
  <p>manages(Lastname, FirstName, MidInitial, ManagerLastname, MFirstName, MMidInitial, start-date)</p>