A NULL in a column means no entry has been made in that column. A data value for the column is “unknown” or “not available.”
NULL is not synonymous with “zero” or “blank.” 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.
In a column where null values are permitted:
If you do not enter any data, Adaptive Server automatically enters “NULL”.
Users can explicitly enter the word “NULL” or “null” without quotation marks.
If you type the word ‘‘NULL’’ in a character column and include quotation marks, it is treated as data, rather than a null value.
Query results display the word NULL. For example, the advance column of the titles table allows null values. By inspecting the data in that column, you can tell whether a book had no advance payment by agreement (the row MC2222 has zero in the advance column) or whether the advance amount was not known when the data was entered (the row MC3026 has NULL in the advance column).
select title_id, type, advance from titles where pub_id = "0877"
title_id type advance -------- ---------- --------- MC2222 mod_cook 0.00 MC3021 mod_cook 15,000.00 MC3026 UNDECIDED NULL PS1372 psychology 7,000.00 TC3218 trad_cook 7,000.00 TC4203 trad_cook 4,000.00 TC7777 trad_cook 8,000.00 (7 rows affected)
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