mysql - 从数据库中的多行中删除数量

Question

这是我的表结构:
table.id(自增；主键)
table.qty(可能的值:0 - 1000)

行数:

+----+-----+
| id | qty |
+----+-----+
|  1 |   0 |
|  2 |   5 |
|  3 |   5 |
|  4 |  10 |
+----+-----+

I want to decrease the quantity available by 12. Meaning, the new result should be:
Edit: I probably should have specified all these numbers are arbitrary. They could be anything.

+----+-----+
| id | qty |
+----+-----+
|  1 |   0 |
|  2 |   0 |
|  3 |   0 |
|  4 |   8 |
+----+-----+

Right now, I do the following:

1. SELECT id, qty FROM table WHERE qty > 0 LIMIT 12

2. Filter through all the data to figure out what qty to remove from each row

3. Use a PDO prepared statement to mass update the affected rows:

   UPDATE `table` SET qty = ? WHERE id = ?`
   

我的问题:有没有办法在 MySQL 中完成这一切？

Answer 1

SET @q = 12;

UPDATE `table`
SET qty = CONCAT(GREATEST(qty - @q, 0), LEFT(@q := @q - LEAST(qty, @q), 0))
ORDER BY id;

查看 sqlfiddle .