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. 

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