Database Reference
In-Depth Information
Input
SELECT cust_id
FROM customers
WHERE cust_email IS NULL;
Output
+---------+
| cust_id |
+---------+
| 10002 |
| 10005 |
+---------+
Caution
NULL and Nonmatches You might expect that when you filter to select all rows that
do not have a particular value, rows with a NULL will be returned. But they will not.
Because of the special meaning of unknown , the database does not know whether they
match, and so they are not returned when filtering for matches or when filtering for non-
matches.
When filtering data, make sure to verify that the rows with a NULL in the filtered column
are really present in the returned data.
Summary
In this chapter, you learned how to filter returned data using the SELECT state-
ment's WHERE clause. You learned how to test for equality, nonequality, greater
than and less than, value ranges, and NULL values.
 
 
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