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Compare - NULL

 Definition: When a variable has no value, it considered to be null. Having a null value is different than having a value of 0, since 0 is an actual value. However, when used in a boolean  test, both null and zero result in a FALSE value. Programmers often use boolean tests to determine whether a variable has been given a value or not.


Display a NULL Value
Database
Syntax
 OracleSELECT ProductName
     , UnitPrice * (UnitsInStock + NVL(UnitsOnOrder,0))
  FROM Products;


 SQL Server
SELECT ProductName
     , UnitPrice * (UnitsInStock + ISNULL(UnitsOnOrder,0))
  FROM Products


Check if NULL (Filter in Where Clause)
Database
Syntax
OracleSELECT ProductName
     , UnitPrice * (UnitsInStock + UnitsOnOrder)
  FROM Products
 WHERE UnitsOnOrder IS NOT NULL;
SQL Server
SELECT ProductName
     , UnitPrice * (UnitsInStock + UnitsOnOrder)
  FROM Products
 WHERE UnitsOnOrder IS NOT NULL