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SQL Anywhere 11.0.1 » SQL Anywhere Server - SQL Usage » Querying and Modifying Data » Querying data » The WHERE clause: Specifying rows

 

Testing a column for NULL

You can use the IS NULL search conditions to compare column values to NULL, and to select them or perform a particular action based on the results of the comparison. Only columns that return a value of TRUE are selected or result in the specified action; those that return FALSE or UNKNOWN do not.

The following example selects only rows for which UnitPrice is less than $15 or is NULL:

SELECT Quantity, UnitPrice
   FROM Products
   WHERE UnitPrice < 15
   OR UnitPrice IS NULL;

The result of comparing any value to NULL is UNKNOWN, since it is not possible to determine whether NULL is equal (or not equal) to a given value or to another NULL.

There are some conditions that never return true, so that queries using these conditions do not return result sets. For example, the following comparison can never be determined to be true, since NULL means having an unknown value:

WHERE column1 > NULL

This logic also applies when you use two column names in a WHERE clause, that is, when you join two tables. A clause containing the condition WHERE column1 = column2 does not return rows where the columns contain NULL.

You can also find NULL or non-NULL with these patterns:

WHERE column_name IS NULL
WHERE column_name IS NOT NULL

For example:

WHERE advance < $5000
OR advance IS NULL

See NULL value.