AND / OR truth table in SQL

Question

why null is introduced in the AND and OR truth table?when will a statement/part of a statement return a null value??
|AND               |True          |False             |NULL??        |
|True               |True          |False             |Null             |
|False              |False          |False             |False           |
|Null                |Null             |Null               |Null             |is it introduced in AND/OR truth table for the null containing row(s)??
select employee_id,last_name,job_id,salary
from employees
wheere salary >= 1000
And job_id like '%man%';
how the statement after the where cluz can return null value??Edited by: sayantan chakraborty on Aug 14, 2009 11:36 AM

Answer 1

sayantan chakraborty wrote:
how either of X and Y will return a null value?can you put any example?is there any syntax/ command which will print the return value of a statement as null?Say you had a table with columns X and Y. As long as NOT NULL constraints WERE NOT defined these values could contain NULLs. Then if you apply the WHERE condition of X = Y you would have the case of NULL = NULL which results in UNKNOWN.
Simple Example:
SQL > WITH test_data AS
  2  (
  3          SELECT 1 AS X, 1 AS Y FROM DUAL UNION ALL
  4          SELECT 2 AS X, 2 AS Y FROM DUAL UNION ALL
  5          SELECT 3 AS X, 5 AS Y FROM DUAL UNION ALL
  6          SELECT 4 AS X, 6 AS Y FROM DUAL UNION ALL
  7          SELECT 1 AS X, NULL AS Y FROM DUAL
  8  )
  9  SELECT  *
10  FROM    test_data
11  WHERE   X = Y
12  /
         X          Y
         1          1
         2          2