Key Points
The special value NULL
means no data, a statement of the fact that the value is unknown. By default, columns, and variables of any type can take this value unless they have a NOT NULL
constraint. Also, the DBMS
automatically adds a NOT NULL
constraint to columns included in the table's primary key.
The main feature of NULL
is that it is not equal to anything, not even another NULL
. You cannot compare any value with it using any operators: =
, <
, >
, like ... Even the expression NULL != NULL
will not be true because one cannot uniquely compare one unknown with another. By the way, this expression will not be false either because when calculating the conditions, Oracle is not limited to the TRUE
and FALSE
states. Due to the presence of an element of uncertainty in the form of NULL
, there is one more state — UNKNOWN
.