# NULL(data type)

**NULL** indicates unknown data. **NULL** and **0** cannot be compared because they are not equivalent to each other.

When creating a table, you can specify wtypehether **NULL** can be stored in a column. For details, see [NOT NULL](https://docs.opengauss.org/en/docs/latest/docs/BriefTutorial/constraints.html#section74155314398). This section describes the IS NULL and IS NOT NULL operators.

Create the **customer\_t1** table. The data in the table is as follows:

```plaintext
""openGauss=# SELECT * FROM customer_t1;
 c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
          3869 | hello         | Grace        |             |   1000
          3869 |               | Grace        |             |
          3869 | hello         |              |             |
          6985 | maps          | Joes         |             |   2200
          9976 | world         | James        |             |   5000
          4421 | Admin         | Local        |             |   3000
(6 rows)
```

## **IS NOT NULL**

In the **WHERE** clause, locate the column whose value is **NULL**.

For example, list the column whose **c\_customer\_id** is not set to a null value in the **customer\_t1** table.

```plaintext
""openGauss=# SELECT * FROM customer_t1 WHERE c_customer_id IS NOT NULL;
 c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
          3869 | hello         | Grace        |             |   1000
          3869 | hello         |              |             |
          6985 | maps          | Joes         |             |   2200
          9976 | world         | James        |             |   5000
          4421 | Admin         | Local        |             |   3000
(5 rows)
```

## **IS NULL**

In the **WHERE** clause, locate the column whose value is **NULL**.

For example, list the column whose **c\_customer\_id** is set to a null value in the **customer\_t1** table.

```plaintext
""openGauss=# SELECT * FROM customer_t1 WHERE c_customer_id IS NULL;
 c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
          3869 |               | Grace        |             |
(1 row)
```