



Unknown values: NULL

A NULL in a column means that the user or application has made no entry in that column. That is, a data value for the column is unknown or not available.

NULL does not mean the same as zero (numerical values) or blank (character values). Rather, NULL values allow you to distinguish between a deliberate entry of zero for numeric columns or blank for character columns and a non-entry, which is NULL for both numeric and character columns.

 Entering NULL
 Returning NULL values