What Is Null Actually?

Null is a special value that represents an unknown value. Any arithmatic operation (+,-,*, /) that has at least one Null operand results in Null. The same thing applies with comparisons: All comparisons with Null would result in Null. Even Null = Null returns Null. Therefore,

• and operator:
  • true and Null results in Null
  • false and Null results in false
  • Null and Null results in Null
• or operator:
  • true or Null results in true
  • false or Null results in Null
  • Null or Null results in Null
• not operator:
  • not Null results in Null
• To test if a value is Null, use IS NULL or IS NOT NULL
• In where clause, recods that evaluates to either false or Null are excluded
• Aggregation functions such as sum and average discard Null records
• count(*) returns the number of records in a table. But count(field) returns the number of not Null records in the table. This means if the field is empty (Null values only), count(field) will return zero.