Using Nulls in Expressions

Use is null or is not null in queries on columns defined to allow null values.

1 + column1

• Comparisons That Return TRUE
  In general, the result of comparing null values is UNKNOWN, since it is not possible to determine whether NULL is equal (or not equal) to a given value or to another NULL.
• Difference Between FALSE and UNKNOWN
  Although neither FALSE nor UNKNOWN returns values, there is an important logical difference between FALSE and UNKNOWN, because the opposite of false (“not false”) is true. For example, “1 = 2” evaluates to false and its opposite, “1 != 2”, evaluates to true. But “not unknown” is still unknown. If null values are included in a comparison, you cannot negate the expression to get the opposite set of rows or the opposite truth value.
• Using “NULL” as a Character String
  Only columns for which NULL was specified in the create table statement and into which you have explicitly entered NULL (no quotes), or into which no data has been entered, contain null values. Avoid entering the character string “NULL” (with quotes) as data for a character column. It can only lead to confusion. Use “N/A”, “none”, or a similar value instead. When you want to enter the value NULL explicitly, do not use single or double quotes.
• NULL Compared to the Empty String
  The empty string (“ ”or ‘ ’) is always stored as a single space in variables and column data.