Handle null values

A null value is not the same as zero or all blanks. A null value means unknown. Null values can be used as a condition in the WHERE and HAVING clauses. For example, a WHERE clause can specify a column that, for some rows, contains a null value. A basic comparison predicate using a column that contains null values does not select a row that has a null value for the column. This is because a null value is neither less than, equal to, nor greater than the value specified in the condition. The IS NULL predicate is used to check for null values. To select the values for all rows that contain a null value for the manager number, you can specify:

SELECT DEPTNO, DEPTNAME, ADMRDEPT
  FROM CORPDATA.DEPARTMENT
  WHERE MGRNO IS NULL

The result are:

To get the rows that do not have a null value for the manager number, you can change the WHERE clause like this:

WHERE MGRNO IS NOT NULL

Another predicate that is useful for comparing values that can contain the NULL value is the DISTINCT predicate. Comparing two columns using a normal equal comparison (COL1 = COL2) will be true if both columns contain an equal non-null value. If both columns are null, the result will be false since null is never equal to any other value, not even another null value. Using the DISTINCT predicate, null values are considered equal. So (COL1 is NOT DISTINCT from COL2) will be true if both columns contain an equal non-null value and also when both columns are the null value.

For example, suppose you want to select information from 2 tables that contain null values. The first table (T1) has a column (C1) with the following values:

The second table (T2) has a column (C2) with the following values:

Run the following SELECT statement:

SELECT * 
    FROM T1, T2
    WHERE C1 IS DISTINCT FROM C2

The results are:

For more information about the use of null values, see the SQL Reference topic collection.