选择表的第一行（具有int值的列，其值的总和小于输入

Question

在这种情况下我的输入为5，我想选择前两行，删除第一行并更新第二行，将值7代替{{1 }}

我尝试进行此查询，但这还不够：

10

Answer 1

您可以使用变量来获取感兴趣的行以及更新记录所需的信息：

SELECT *
FROM   (
    SELECT   `Qty in acquisto`,
             `Prezzo in acquisto`,
             @take := least(`Qty in acquisto`, @needed) as taken,
             `Qty in acquisto` - @take as adjusted_acquisto, 
             @needed := @needed - @take as still_needed
    FROM     book,
             (select @needed := 5) as init
    ORDER BY `Prezzo in acquisto` DESC) base
WHERE  taken + still_needed > 0

样本数据的输出是：

| Qty in acquisto | Prezzo in acquisto | taken | adjusted_acquisto | still_needed |
|-----------------|--------------------|-------|-------------------|--------------|
|               2 |               1000 |     2 |                 0 |            3 |
|              10 |                960 |     3 |                 7 |            0 |

请参阅SQL fiddle

在最里面的查询中，使用别名 init ，您可以传递所需的书籍数量（示例中为5）。

因此，在列adjusted_acquisto中，您可以找到执行删除和更新所需的值：

如果该值为0，则删除相应的记录。

该值不为0，用该值更新数量。

Answer 2

E.g：

 SELECT * FROM my_table;
+------+--------+
| id   | amount |
+------+--------+
|  800 |      8 |
|  900 |      3 |
|  950 |      4 |
|  960 |     10 |
| 1000 |      2 |
+------+--------+
SELECT n.id
     , GREATEST(amount-@x,0) new_amount
     , @x:=GREATEST(@x-amount,0) x 
  FROM my_table n
     , (SELECT @x:=5) vars 
 ORDER 
    BY id DESC;
+------+--------+------------+------+
| id   | amount | new_amount | x    |
+------+--------+------------+------+
| 1000 |      2 |          0 |    3 |
|  960 |     10 |          7 |    0 |
|  950 |      4 |          4 |    0 |
|  900 |      3 |          3 |    0 |
|  800 |      8 |          8 |    0 |
+------+--------+------------+------+