Update MySQL record twice using 'CASE' - mysql

Question

I am working on an accountant software , some times the invoice contain the same item id twice (every one with its unique serial number) .. so after selling the two items I need its quantity equals quantity-1
I am using this statement for updating some records
UPDATE `table1` SET `quantity` = CASE
WHEN id = 1 THEN quantity-1
WHEN id = 1 THEN quantity-1
END
WHERE id in (1)
after updating using this statement ,the value of quantity ignores the second statement
How can I solve this ?
EDIT
The answer
Thank You ... I found the trick by myself
UPDATE `table1` SET `quantity` = CASE
WHEN id = (1*(1/1)) THEN quantity-1
WHEN id = (1*(2/2)) THEN quantity-1
WHEN id = (1*(3/3)) THEN quantity-1
END
WHERE id in (1)
the record will be updated 3 times by the same query
why I need This ?
because my software accepts that the user can add the same (Product ID) many times in the same invoice so I need the query to update the quantity many times in the same statement for one (Product ID)

Answer 1

I think this might actually be what you want. I have id=1 in my table 3 times, so mcount is set equal to 3. The row for ID=2 was not updated because of the where condition.
update table1
join (select id, count(*) as ct from table1 group by id) as ct_tbl
set mcount = ct_tbl.ct
where table1.id=1
mysql> select * from table1;
+------+--------+
| id | mcount |
+------+--------+
| 1 | 3 |
| 1 | 3 |
| 1 | 3 |
| 2 | 0 |
+------+--------+
4 rows in set (0.00 sec)
I don't want to hijack the other answer because it's totally correct but I think it needs more explanation.
UPDATE `table1` SET `mcount` = CASE
WHEN id = 1 THEN 1
WHEN id = 1 THEN 2
END
WHERE id in (1)
that is the equivalent of... (psuedo)
in the table table1
where the id = 1
if id = 1 then set mcount = 1
if id = 1 then set mcount = 2
Because of the where statement, we already know that we will only select rows where id=1. Those if statements will just overwrite each other. I really don't know what you're after but normally...
the following will add one to mcount
UPDATE `table1`
SET `mcount` = mcount + 1;
WHERE id in (1)
the following will toggle mcount values
UPDATE `table1`
SET `mcount` = CASE WHEN mcount = 1 THEN 2
WHEN mcount = 2 THEN 1 END
WHERE id in (1)
the following will toggle mcount values
UPDATE `table1`
SET `mcount` = CASE WHEN other_field = 'Y' THEN 1
WHEN other_field = 'N' THEN 2 END
WHERE id in (1)
working with multiple ID values...
UPDATE `table1`
SET `mcount` = CASE WHEN id = 1 THEN 1
WHEN id = 2 THEN 2
WHEN id = 3 THEN 3 END
WHERE id in (1,2,3)
which is the same as...
UPDATE `table1`
SET `mcount` = id
WHERE id in (1,2,3)
now the real question is... if ID=1 then what should mcount be?
also....
set #row:=0;
update table1
set mcount = #row:=#row+1
where id = 1;
mysql> select * from table1;
+------+--------+
| id | mcount |
+------+--------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 0 |
+------+--------+
4 rows in set (0.00 sec)

Answer 2

Your statement has the same condition for each case, always will return 1 (first statement), the second statement will be ignored because it is the same condition.

Answer 3

Thank You ... I found the trick by myself
UPDATE `table1` SET `quantity` = CASE
WHEN id = (1*(1/1)) THEN quantity-1
WHEN id = (1*(2/2)) THEN quantity-1
WHEN id = (1*(3/3)) THEN quantity-1
END
WHERE id in (1)
the record will be updated 3 times by the same query