如何将当前行与PostgreSQL中的下一行和上一行进行比较? [英] How to compare the current row with next and previous row in PostgreSQL?

问题描述

我想知道如何在SQL查询中与下一行或上一行进行逻辑比较来检索结果.我正在使用PostgreSQL.

I want to know how to retrieve results in a SQL query doing some logic comparison with the next or previous rows. I'm using PostgreSQL.

示例
假设我的数据库中有一个具有两个属性(有序位置和随机数)的表，我想检索介于偶数之间的奇数.我该怎么办?

Example
Supposing I have a table in my database with two attributes (ordered position and random numbers), I want to retrieve the odd numbers that are between even numbers. How can I do this?

实际用法
我想找到两个其他单词之间的单词，它们之间的名称为NAME(并且该单词不是名称).排序由句子和位置提供.

The real usage
I want to find words that are between two another words which have the category NAME (and the word is not a name). The ordering is provided by sentence and position.

修改 我想知道PostgreSQL的Window函数是否比查询更能解决此类问题.我听说过它们，但从未使用过.

Edit I want to know if the Window function of PostgreSQL are best solution for this kind of problem than doing queries. I heard about them, but never used.

推荐答案

这是我使用WINDOW functions的解决方案.我使用了lag和lead函数.两者都从与当前行偏移的行中的某一列返回一个值. lag返回，并且lead在偏移量中移至下一个.

This is my solution using WINDOW functions. I used the lag and lead functions. Both returns a value from a column from a row in offset from the current row. lag goes back and lead goes next in the offset.

SELECT tokcat.text
FROM (
    SELECT text, category, chartype, lag(category,1) OVER w as previousCategory, lead(category,1) OVER w as nextCategory
    FROM token t, textBlockHasToken tb
    WHERE tb.tokenId = t.id
    WINDOW w AS (
        PARTITION BY textBlockId, sentence
        ORDER BY textBlockId, sentence, position
    )
) tokcat
WHERE 'NAME' = ANY(previousCategory)
AND 'NAME' = ANY(nextCategory)
AND 'NAME' <> ANY(category)

---

简化版本:

---

Simplified version:

SELECT text
FROM (
    SELECT text
          ,category 
          ,lag(category) OVER w as previous_cat
          ,lead(category) OVER w as next_cat
    FROM   token t
    JOIN   textblockhastoken tb ON tb.tokenid = t.id
    WINDOW w AS (PARTITION BY textblockid, sentence ORDER BY position)
    ) tokcat
WHERE  category <> 'NAME'
AND    previous_cat = 'NAME'
AND    next_cat = 'NAME';

要点

不需要
• = ANY()，window函数将返回单个值
• 子查询中的一些冗余字段
• 无需按列排序，即您PARTITION BY-ORDER BY在 个分区内应用
• 请勿在不加引号的情况下使用大小写混合的标识符，这只会导致混乱. (更好的是:不要在PostgreSQL的 中使用大小写混合的标识符)

Major points

• = ANY() is not needed, the window function returns a single value
• some redundant fields in the subquery
• no need to order by columns, that you PARTITION BY - the ORDER BY applies within partitions
• Don't use mixed case identifiers without quoting, it only leads to confusion. (Better yet: don't use mixed case identifiers in PostgreSQL ever)

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