EXISTS Predicate Versus NOT IN and Nulls

Teradata Vantageā„¢ SQL Functions, Expressions, and Predicates

brand
Software
Teradata Vantage
prodname
Teradata Database
Teradata Vantage NewSQL Engine
vrm_release
16.20
category
Programming Reference
featnum
B035-1145-162K

Use the NOT EXISTS predicate instead of NOT IN if the following conditions are true:

  • Some column of the NOT IN condition is defined as nullable.
  • Any rows from the main query with a null in any column of the NOT IN condition should always be returned.
  • Any nulls returned in the select list of the subquery should not prevent any rows from the main query from being returned.

For example, if all of the previous conditions are true for the following query, use NOT EXISTS instead of NOT IN:

   SELECT dept, DeptName 
   FROM Department 
   WHERE Dept NOT IN 
    (SELECT Dept 
     FROM Course);

The NOT EXISTS version looks like this:

   SELECT dept, DeptName
   FROM Department 
   WHERE NOT EXISTS
    (SELECT Dept 
     FROM Course 
     WHERE Course.Dept=Department.Dept);

That is, either Course.Dept or Department.Dept is nullable and a row from Department with a null for Dept should be returned and a null in Course.Dept should not prevent rows from Department from being returned.