Given Clients table <pre><code>+------+-----------

Given Clients table

+------+--------------+----------+
| ID   |  Name        |  Summa   |
+------+--------------+----------+
| 1    |  Sasha       |  4000    |
| 2    |  Marina      |  2000    |
| 3    |  Stepan      |   0      |
| 4    |  Klara       |  NULL    |
+------+--------------+----------+

After it is updated using:

UPDATE Clients SET Summa = Summa + 700

What will be the result of the following query execution?

SELECT AVG(Summa) FROM Clients;

Explanation

If binary arithmetic operator (+ in our case) accepts NULL as one of its arguments, the result will always be NULL.
Aggregate functions (like AVG() in our case) ignore NULL values.