mysql从表中检索并操作当前行和上一行

Question

我看了几个地方，也许我只是没有正确地描述搜索。我发现了类似的问题，但没有人回答这个问题。

我有一个Sight Inventoryories表（用户走过存储区域并实际检查手头有多少产品）。该表处理多个位置。表结构（部分，仅需要的信息）是：

create table location_inventory (
    id                int unsigned not null auto_increment,
    location_id       int unsigned references location(location_id),
    inventory_item_id int unsigned references inventory_item (inventory_item_id),
    inventory_date    date comment 'Date the sight inventory was taken',
    quantity          decimal( 15,2 ) comment 'Number of items found during inventory',
    primary key ( id ),
    unique            (location_id,inventory_item_id,inventory_date)
);

它应该是对表单的查询：

select
    a.location_id location,
    a.inventory_item_id inventory_item,
    a.inventory_date curr_date,
    a.quantity curr_quant,
    b.inventory_date prev_date,
    b.quantity prev_quant,
    a.quantity - b.quantity num_used
from
    location_inventory a
    left outer join
       (
         select
            location_id,
            inventory_item_id,
            inventory_date,
            quantity
          from
            location_inventory
          where
           something
      ) b
      on ( location_id,inventory_item_id )
  where
      a.inventory_date between DATEA and DATEB

但我还没有把它付诸实践。

这是我缺少的整个子查询。我已经看到了几个答案，我们得到了前一个日期，但没有一个我可以实际检索上一行的其余值;它最终检索整个表中最近的条目的值。

Answer 1

当您选择仅显示表结构的一部分时，您可以省略我们可能需要的内容。下面我假设您有一列id作为每行的唯一标识符

SELECT
    a.location_id           location
  , a.inventory_item_id     inventory_item
  , a.inventory_date        curr_date
  , a.quantity              curr_quant
  , b.inventory_date        prev_date
  , b.quantity              prev_quant
  , a.quantity - b.quantity num_used
FROM (
    SELECT
        *
      , (SELECT id
         FROM location_inventory
         WHERE location_id = t.location_id
           and inventory_item_id = t.inventory_item_id
           and inventory_date < t.inventory_date
         ORDER BY inventory_date DESC
         LIMIT 1
        ) prev_id
    FROM location_inventory t
    ) a
LEFT OUTER JOIN location_inventory b ON a.prev_id = b.id
WHERE a.inventory_date BETWEEN DATEA AND DATEB

另一种方法是为每个所需值使用相关子查询：

SELECT
    a.location_id           location
  , a.inventory_item_id     inventory_item
  , a.inventory_date        curr_date
  , a.quantity              curr_quant
  , (SELECT inventory_date
         FROM location_inventory
         WHERE location_id = t.location_id and inventory_item_id = t.inventory_item_id and inventory_date < t.inventory_date
         ORDER BY inventory_date DESC
         LIMIT 1
        )                   prev_date
  , (SELECT quantity
         FROM location_inventory
         WHERE location_id = t.location_id and inventory_item_id = t.inventory_item_id and inventory_date < t.inventory_date
         ORDER BY inventory_date DESC
         LIMIT 1
        )                   prev_quant
  , a.quantity 
    - (SELECT quantity
         FROM location_inventory
         WHERE location_id = t.location_id and inventory_item_id = t.inventory_item_id and inventory_date < t.inventory_date
         ORDER BY inventory_date DESC
         LIMIT 1
        )
FROM location_inventory t
WHERE a.inventory_date BETWEEN DATEA AND DATEB

自版本8.0发布以来，MySQL支持lag()等窗口函数，这使得这更加容易和高效。

SELECT
    a.location_id           location
  , a.inventory_item_id     inventory_item
  , a.inventory_date        curr_date
  , a.quantity              curr_quant
  , lag(inventory_date,1) over(partiton by location_id,inventory_item_id order by inventory_date DESC)  prev_date
  , lag(quantity,1) over(partiton by location_id,inventory_item_id order by inventory_date DESC)        prev_quant
  , a.quantity 
    - lag(quantity,1) over(partiton by location_id,inventory_item_id order by inventory_date DESC)       num_used
FROM location_inventory a
WHERE a.inventory_date BETWEEN DATEA AND DATEB

最后注意：我不赞成依赖于查询中的序列的别名方案