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.
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?
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.
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.
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)
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';
= ANY()
is not needed, the window function returns a single valuePARTITION BY
- the ORDER BY applies within partitions