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

NULL can be entered only where NULL values are permitted for the column. Whether a column can accept NULL values is determined when the table is created. Assuming a column can accept NULL values, NULL is inserted:

Default If no data is entered, and the column has no other default setting.
Explicit entry You can explicitly insert the word NULL without quotation marks. If the word NULL is typed in a character column with quotation marks, it is treated as data, not as the NULL value.

For example, the DepartmentHeadID column of the Departments table allows NULL values. You can enter two rows for departments with no manager as follows:

INSERT INTO Departments (DepartmentID, DepartmentName)
   VALUES (201, 'Eastern Sales')
INSERT INTO Departments
   VALUES (202, 'Western Sales', NULL);

Returning NULL values

NULL values are returned to the client application for display, just as with other values. For example, the following example illustrates how NULL values are displayed in Interactive SQL:

SELECT *
FROM Departments;

DepartmentID	DepartmentName	DepartmentHeadID
100	R & D	501
200	Sales	904
300	Finance	1293
400	Marketing	1576
500	Shipping	703
201	Eastern Sales	(NULL)
202	Western Sales	(NULL)

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