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SQL for beginners

Checking for NULL values #

NULL represents a missing or unknown value. It isn't equal to anything, not even to another NULL, so = NULL never matches. To check for it, SQL gives us the dedicated IS NULL (and IS NOT NULL) operators.

Syntax #

SELECT
    column_name
FROM
    table_name
WHERE
    column_name IS NULL;

SELECT
    column_name
FROM
    table_name
WHERE
    column_name IS NOT NULL;

Example #

In the orders table, shipped_date is NULL whenever an order hasn't shipped yet, that is, when its status is 'Pending' or 'Cancelled'. We can find those orders directly:

SELECT
    *
FROM
    orders
WHERE
    shipped_date IS NULL;

Result:

+----+-------------+------------+-----------+--------------+
| id | customer_id | order_date |  status   | shipped_date |
+----+-------------+------------+-----------+--------------+
| 1  |      3      | 2025-...   | Cancelled |     None     |
| 4  |      1      | 2025-...   | Cancelled |     None     |
| 17 |      9      | 2025-...   |  Pending  |     None     |
| .. |     ...     |    ...     |    ...    |     ...      |

If we wanted the opposite, the orders that have shipped, we would use WHERE shipped_date IS NOT NULL instead.

Exercise #

Write a query that retrieves all columns from the orders table, keeping only the rows where shipped_date is NULL.

  • The result has a shipped_date column.
  • The result contains exactly the rows where shipped_date is NULL, and no others.
Solution 
SELECT
    *
FROM
    orders
WHERE
    shipped_date IS NULL;
Output will be displayed here