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A field with a NULL value is a field with no value
A field with a NULL value is a field with no value.
If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field. Then, the field will be saved with a NULL value.
Note: A NULL value is different from a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation!
It is not possible to test for NULL values with comparison operators, such as =, <, or <>.
We will have to use the IS NULL
and
IS NOT NULL
operators instead.
IS NULL Syntax
SELECT column_names
FROM table_name
WHERE column_name IS NULL;
IS NOT NULL Syntax
SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;
Below is a selection from the "Customers" table in the Northwind sample database:
CustomerID | CustomerName | ContactName | Address | City | PostalCode | Country |
---|---|---|---|---|---|---|
1 |
Alfreds Futterkiste | Maria Anders | Obere Str. 57 | Berlin | 12209 | Germany |
2 | Ana Trujillo Emparedados y helados | Ana Trujillo | Avda. de la Constitución 2222 | México D.F. | 05021 | Mexico |
3 | Antonio Moreno Taquería | Antonio Moreno | Mataderos 2312 | México D.F. | 05023 | Mexico |
4 |
Around the Horn | Thomas Hardy | 120 Hanover Sq. | London | WA1 1DP | UK |
5 | Berglunds snabbköp | Christina Berglund | Berguvsvägen 8 | Luleå | S-958 22 | Sweden |
The IS NULL
operator is used to test for empty values (NULL values).
The following SQL lists all customers with a NULL value in the "Address" field:
SELECT CustomerName, ContactName, Address
FROM Customers
WHERE Address
IS NULL;
Tip: Always use IS NULL to look for NULL values.
The IS NOT NULL
operator is used to test for non-empty values (NOT NULL
values).
The following SQL lists all customers with a value in the "Address" field:
SELECT CustomerName, ContactName, Address
FROM Customers
WHERE Address
IS NOT NULL;
Select all records from the Customers
where
the PostalCode
column is empty.
SELECT * FROM Customers WHERE ;