user1105595 user1105595 - 7 months ago 33
SQL Question

SQL select elements where sum of field is less than N

Given that I've got a table with the following, very simple content:

# select * from messages;
id | verbosity
----+-----------
1 | 20
2 | 20
3 | 20
4 | 30
5 | 100
(5 rows)


I would like to select N messages, which sum of verbosity is lower than Y (for testing purposes let's say it should be 70, then correct results will be messages with id 1,2,3).
It's really important to me, that solution should be database independent (it should work at least on Postgres and SQLite).

I was trying with something like:

SELECT * FROM messages GROUP BY id HAVING SUM(verbosity) < 70;


However it doesn't seem to work as expected, because it doesn't actually sum all values from verbosity column.

I would be very grateful for any hints/help.

Answer
SELECT m.id, sum(m1.verbosity) AS total
FROM   messages m
JOIN   messages m1 ON m1.id <= m.id
WHERE  m.verbosity < 70    -- optional, to avoid pointless evaluation
GROUP  BY m.id
HAVING SUM(m1.verbosity) < 70
ORDER  BY total DESC
LIMIT  1;

This assumes a unique, ascending id like you have in your example.


In modern Postgres - or generally with modern standard SQL (but not in SQLite):

Simple CTE

WITH cte AS (
   SELECT *, sum(verbosity) OVER (ORDER BY id) AS total
   FROM   messages
   )
SELECT *
FROM   cte
WHERE  total <= 70
ORDER  BY id;

Recursive CTE

Should be faster for big tables where you only retrieve a small set.

WITH RECURSIVE cte AS (
   (  -- parentheses required
   SELECT id, verbosity, verbosity AS total
   FROM   messages
   ORDER  BY id
   LIMIT  1
   )

   UNION ALL 
   SELECT c1.id, c1.verbosity, c.total + c1.verbosity 
   FROM   cte c
   JOIN   LATERAL (
      SELECT *
      FROM   messages
      WHERE  id > c.id
      ORDER  BY id
      LIMIT  1
      ) c1 ON  c1.verbosity <= 70 - c.total
   WHERE c.total <= 70
   )
SELECT *
FROM   cte
ORDER  BY id;

All standard features, except for LIMIT.

Strictly speaking, there is no such thing as "database-independent". There are various SQL-standards, but no RDBMS complies completely. LIMIT works for PostgreSQL and SQLite (and some others). Use TOP 1 for SQL Server, rownum for Oracle. Here's a comprehensive list on Wikipedia.

The SQL:2008 standard would be:

...
FETCH  FIRST 1 ROWS ONLY

... which PostgreSQL supports - but hardly any other RDBMS.

The pure alternative that works with more systems would be to wrap it in a subquery and

SELECT max(total) FROM <subquery>

But that is slow and unwieldy.

SQL Fiddle.

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