Nested Queries ( with Correlations )

• No Correlation
  Select Name
  From emp
  sal = ( Select avg(sel) from emp )
  
• Compute and Reuse.
• Select Name
  from Emp X
  where salary > ( Select Salary avg(sal) from emp where empno = x.manager);
• frequent reevalution.
• Order access on correlation var value => no reevalution, use prev.
• Also : memoing schemes