Truncated incorrect DOUBLE value in update query

Databases
1

Hi guys, hope in your help.
I tried this select query and I’ve this output:

mysql> SELECT
	A,
	B,
	A * B AS output,
	C
FROM
	tbl_2
ORDER BY
	output DESC;
+-----+---+--------+------+
| A   | B | output | C    |
+-----+---+--------+------+
| 252 | 7 |   1764 | NULL |
| 134 | 7 |    938 | NULL |
| 116 | 6 |    696 | NULL |
| 560 | 1 |    560 | NULL |
| 220 | 2 |    440 | NULL |
| 161 | 2 |    322 | NULL |
| 261 | 1 |    261 | NULL |
| 192 | 1 |    192 | NULL |
| 183 | 1 |    183 | NULL |
| 164 | 1 |    164 | NULL |
| 150 | 1 |    150 | NULL |
| 126 | 1 |    126 | NULL |
|  98 | 1 |     98 | NULL |
|  79 | 1 |     79 | NULL |
|   0 |   |      0 | NULL |
|   0 |   |      0 | NULL |
| 149 | 0 |      0 | NULL |
|   0 |   |      0 | NULL |
+-----+---+--------+------+
18 rows in set

Now I need update the field C of tbl_2 with the result of the product (A * B):

mysql> UPDATE tbl_2
SET C = (
	A * B
);

1292 - Truncated incorrect DOUBLE value: ''

Why this error Truncated incorrect DOUBLE value ?
The field C in the tbl_2 is varchar 255.
Can you help me?
Thank you

2

three comments

first, you should rarely ever need to store the result of a row calculation – it can always be calculated in a SELECT and you run the risk of data inconsistencies

second, you shouldn’t be using DOUBLE here for what are obviously integers – DOUBLE is a floating point, i.e. approximate datatype

third, when you ask for help on something like this, you should always do a SHOW CREATE TABLE so that we can see what the table definition looks like

3

thank you for comments, solved with second comment.