MySQL Update CASE updates field with "0" - mysql

Question

I have a problem by updating my SQL-Table with the CASE statement.
SELECT number,amount,minimuminventory FROM artikel WHERE number=17;
+--------+--------+------------------+
| number | amount | minimuminventory |
+--------+--------+------------------+
| 17 | 10 | 0 |
+--------+--------+------------------+
I have an amount of 10 but when I update my table:
UPDATE artikel
SET amount = CASE WHEN amount - minimuminventory - 2 < 0
THEN amount=amount-2
ELSE amount=99
END
WHERE artnr=17;
Query OK, 1 rows affected (0,01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql updates my table an sets amount to 0
SELECT number,amount,minimuminventory FROM artikel WHERE number=17;
+--------+--------+------------------+
| number | amount | minimuminventory |
+--------+--------+------------------+
| 17 | 0 | 0 |
+--------+--------+------------------+
Do you you see the problem?

Answer 1

amount = CASE WHEN amount - minimuminventory - 2 < 0 THEN amount=amount-2 ELSE amount=99 END
The value from the CASE statement is the intended value for the amount column, but you're doing amount=x again inside the THEN and ELSE parts of it. Change it to this:
amount = CASE WHEN amount - minimuminventory - 2 < 0 THEN amount-2 ELSE 99 END

Answer 2

Do you you see the problem?
In your statement:
UPDATE artikel
SET amount = CASE WHEN amount - minimuminventory - 2 < 0
THEN amount=amount-2
ELSE amount=99
END
WHERE artnr=17;
The comparison is interpreted as boolean but not as assignment.
SET amount = (amount=amount-2) -- comparison as if amount is equal to (17=17-2)
SET amount = (amount=99) -- comparison as if amount is equal to (17=99)
SET amount = false -- i.e. 0, because 17 != 15 or 17 != 99
That resulted a value of 0 in amount column.
Change your update statement as below:
UPDATE artikel
SET amount = CASE WHEN amount - minimuminventory - 2 < 0
THEN amount-2
ELSE 99
END
WHERE artnr=17;