SELECT sum(T2.hours) FROM assignedto AS T1 JOIN projects AS T2 ON T1.project  =  T2.code JOIN scientists AS T3 ON T1.scientist  =  T3.SSN WHERE T3.name  =  'Michael Rogers' OR T3.name  =  'Carol Smith'
What is the sum of hours for projects that scientists with the name Michael Rogers or Carol Smith are assigned to?
create table Scientists (
  SSN int,
  Name Char(30) not null,
  Primary Key (SSN)
);
Create table Projects (
  Code Char(4),
  Name Char(50) not null,
  Hours int,
  Primary Key (Code)
);
create table AssignedTo (
  Scientist int not null,
  Project char(4) not null,
  Primary Key (Scientist, Project),
  Foreign Key (Scientist) references Scientists (SSN),
  Foreign Key (Project) references Projects (Code)
);
INSERT INTO Scientists(SSN,Name) 
INSERT INTO Projects ( Code,Name,Hours)
INSERT INTO AssignedTo ( Scientist, Project)