Complex Query duplicating Result (same id, different columns values) - sql

Question

I have this query, working great:
SELECT * FROM
(
select
p.id,
comparestrings('marco', pc.value) as similarity
from
unit u, person p
inner join person_field pc ON (p.id = pc.id_person)
inner join field c ON (pc.id_field = c.id AND c.flag_name = true)
where ( u.id = 1 ) AND p.id_unit = u.id
) as subQuery
where
similarity is not null
AND
similarity > 0.35
order by
similarity desc;
Let me explain the situation.
TABLES:
person ID as column.
field a table that represents a column, like name, varchar (something like that)
person_field represents the value of that person and that field.. Like this:
unit not relevant for this question
Eg.:
Person id 1
Field id 1 {name, eg)
value "Marco Noronha"
So the function "comparestrings" returns a double from 0 to 1, where 1 is exact ('Marco' == 'Marco').
So, I need all persons that have similarity above 0.35 and i also need its similarity.
No problem, the query works fine and as it was suppost to. But now I have a new requirement that, the table "person_field" will contain an alteration date, to keep track of the changes of those rows.
Eg.:
Person ID 1
Field ID 1
Value "Marco Noronha"
Date - 01/25/2013
Person ID 1
Field ID 1
Value "Marco Tulio Jacovine Noronha"
Date - 02/01/2013
So what I need to do, is consider ONLY the LATEST row!!
If I execute the same query the result would be (eg):
1, 0.8
1, 0.751121
2, 0.51212
3, 0.42454
//other results here, other 'person's
And lets supose that the value I want to bring is 1, 0.751121 (witch is the lattest value by DATE)
I think I should do something like order by date desc limit 1...
But if I do something like that, the query will return only ONE person =/
Like:
1, 0.751121
When I really want:
1, 0.751121
2, 0.51212
3, 0.42454

Answer 1

You can use DISTINCT ON(p.id) on the sub-query:
SELECT * FROM
(
select
DISTINCT ON(p.id)
p.id,
comparestrings('marco', pc.value) as similarity
from
unit u, person p
inner join person_field pc ON (p.id = pc.id_person)
inner join field c ON (pc.id_field = c.id AND c.flag_name = true)
where ( u.id = 1 ) AND p.id_unit = u.id
ORDER BY p.id, pc.alt_date DESC
) as subQuery
where
similarity is not null
AND
similarity > 0.35
order by
similarity desc;
Notice that, to make it work I needed to add ORDER BY p.id, pc.alt_date DESC:
p.id: required by DISTINCT ON (if you use ORDER BY, the first fields must be exactly the same as DISTINCT ON);
pc.alt_date DESC: the alter date you mentioned (we order desc, so we get the oldest ones by each p.id)
By the way, seems that you don't need a sub-query at all (just make sure comparestrings is marked as stable or immutable, and it'll be fast enough):
SELECT
DISTINCT ON(p.id)
p.id,
comparestrings('marco', pc.value) as similarity
FROM
unit u, person p
inner join person_field pc ON (p.id = pc.id_person)
inner join field c ON (pc.id_field = c.id AND c.flag_name = true)
WHERE ( u.id = 1 ) AND p.id_unit = u.id
AND COALESCE(comparestrings('marco', pc.value), 0.0) > 0.35
ORDER BY p.id, pc.alt_date DESC, similarity DESC;

Answer 2

Change the reference to person to a subquery as in the following example (the subquery is the one called p):
. . .
from unit u cross join
(select p.*
from (select p.*,
row_number() over (partition by person_id order by alterationdate desc) as seqnum
from person p
) p
where seqnum = 1
) p
. . .
This uses the row_number() function to identify the last row. I've used an additional subquery to limit the result just to the most recent. You could also include this in an on clause or a where clause.
I also changed the , to an explicit cross join.