Example: Performing a Union Operation to Find Hours Worked - Teradata VantageCloud Lake

The following statement uses a union operation to return the number of man-hours charged by each employee who is working on a project and the names of employees who are not working on a project.

   SELECT Name, Proj_Id, Hours 
   FROM Employee, Charges 
   WHERE Employee.EmpNo = Charges.EmpNo
   UNION
   SELECT Name, Null (CHAR(8)), Null (DECIMAL(4,2)), 
   FROM Employee 
   WHERE EmpNo NOT IN 
   (SELECT EmpNo 
   FROM Charges
   )
   UNION
   SELECT Null (VARCHAR(12)), Proj_Id, Hours 
   FROM Charges 
   WHERE EmpNo NOT IN 
   (SELECT EmpNo 
   FROM Employee
   );

The first portion of the statement joins the Employee table with the Charges table on the EmpNo column. The second portion accounts for the employees who may be listed in the Employee table, but not the Charges table. The third portion of the statement accounts for the employees who may be listed in the Charges table and not in the Employee table. This makes sure that all the information asked for is included in the response.