Reputation: 1463
I want to do a special request on my database (PostgreSQL v9.4.5), but I don't manage to do it.
In order to simply, let's say I have the following table AvgTemperatures, representing different averages of temperature taken in different cities, and calculated on different length of time (counted in months) :
id | city | avg | months
----+-----------+------+--------
1 | New-York | 20 | 3 <--- average temperate over the last 3 months
2 | New-York | 19 | 6 <--- average temperate over the last 6 months
3 | New-York | 15 | 12 <--- etc
4 | New-York | 15 | 24
5 | Boston | 13 | 3
6 | Boston | 18 | 8
7 | Boston | 17 | 12
8 | Boston | 16 | 15
9 | Chicago | 12 | 2
10 | Chicago | 14 | 12
11 | Miami | 28 | 1
12 | Miami | 25 | 4
13 | Miami | 21 | 12
14 | Miami | 22 | 15
15 | Miami | 20 | 24
Now, imagine that I want to select all the rows concerning the measures in a city where at least one average has been over 19 degrees. In this case I want :
id | city | avg | months
----+-----------+------+--------
1 | New-York | 20 | 3
2 | New-York | 19 | 6
3 | New-York | 15 | 12
4 | New-York | 15 | 24
11 | Miami | 28 | 1
12 | Miami | 25 | 4
13 | Miami | 21 | 12
14 | Miami | 22 | 15
15 | Miami | 20 | 24
I could do something like :
SELECT *
FROM AvgTemperatures
WHERE MIN(avg) OVER (PARTITION BY city) > 16
But :
********** Erreur **********
ERROR: window functions not allowed in WHERE clause
What's more, I cannot use GROUP BY
as in :
SELECT *
FROM AvtTemperatures
GROUP BY city
HAVING MIN(avg) > 16
because I will lose information due to the aggregation (by the way this query is not valid because of the "SELECT *").
I'm pretty sure I can use the OVER PARTITION BY
to solve that, but I don't know how. Does someone have an idea ?
Upvotes: 6
Views: 36261
Reputation: 176189
"All-at-Once Operations" means that all expressions in the same logical query process phase are evaluated logically at the same time.
And great chapter Impact on Window Functions:
Suppose you have:
CREATE TABLE Test ( Id INT) ;
INSERT INTO Test VALUES ( 1001 ), ( 1002 ) ;
SELECT Id
FROM Test
WHERE Id = 1002
AND ROW_NUMBER() OVER(ORDER BY Id) = 1;
All-at-Once operations tell us these two conditions evaluated logically at the same point of time. Therefore, SQL Server can evaluate conditions in WHERE clause in arbitrary order, based on estimated execution plan. So the main question here is which condition evaluates first.
Case 1:
If ( Id = 1002 ) is first, then if ( ROW_NUMBER() OVER(ORDER BY Id) = 1 )
Result: 1002
Case 2:
If ( ROW_NUMBER() OVER(ORDER BY Id) = 1 ), then check if ( Id = 1002 )
Result: empty
So we have a paradox.
This example shows why we cannot use Window Functions in WHERE clause. You can think more about this and find why Window Functions are allowed to be used just in SELECT and ORDER BY clauses!
To get what you want you can wrap windowed function with CTE/subquery
as in Gordon answer:
;WITH cte AS
(
SELECT t.*, MAX(AVG) OVER (PARTITION BY city) AS average
FROM avgTemperatures t
)
SELECT *
FROM cte
where average > 19
ORDER BY id;
Output:
╔═════╦══════════╦═════╦═════════╗
║ id ║ city ║ avg ║ months ║
╠═════╬══════════╬═════╬═════════╣
║ 1 ║ New-York ║ 20 ║ 3 ║
║ 2 ║ New-York ║ 19 ║ 6 ║
║ 3 ║ New-York ║ 15 ║ 12 ║
║ 4 ║ New-York ║ 15 ║ 24 ║
║ 11 ║ Miami ║ 28 ║ 1 ║
║ 12 ║ Miami ║ 25 ║ 4 ║
║ 13 ║ Miami ║ 21 ║ 12 ║
║ 14 ║ Miami ║ 22 ║ 15 ║
║ 15 ║ Miami ║ 20 ║ 24 ║
╚═════╩══════════╩═════╩═════════╝
Upvotes: 9
Reputation: 125474
The simplest solution is to use the bool_or
aggregate function
select id, city, avg, months
from avttemperatures
where city in (
select city
from avttemperatures
group by 1
having bool_or(avg > 19)
)
order by 2, 4
;
id | city | avg | months
----+----------+-----+--------
11 | Miami | 28 | 1
12 | Miami | 25 | 4
13 | Miami | 21 | 12
14 | Miami | 22 | 15
15 | Miami | 20 | 24
1 | New-York | 20 | 3
2 | New-York | 19 | 6
3 | New-York | 15 | 12
4 | New-York | 15 | 24
The test table:
create table avttemperatures (
id int, city text, avg int, months int
);
insert into avttemperatures (id, city, avg, months) values
( 1,'New-York',20,3),
( 2,'New-York',19,6),
( 3,'New-York',15,12),
( 4,'New-York',15,24),
( 5,'Boston',13,3),
( 6,'Boston',18,8),
( 7,'Boston',17,12),
( 8,'Boston',16,15),
( 9,'Chicago',12,2),
( 10,'Chicago',14,12),
( 11,'Miami',28,1),
( 12,'Miami',25,4),
( 13,'Miami',21,12),
( 14,'Miami',22,15),
( 15,'Miami',20,24);
Upvotes: 4
Reputation: 4523
No need to aggregate if you only want to know if at least one exists:
SELECT id, city, avg, months
FROM avgtemperatures t
WHERE EXISTS ( SELECT 42
FROM avgtemperatures x
WHERE x.city = t.city
AND x.avg > 19
)
ORDER BY city,months DESC
;
Note: avg
is a bad name for a column.
Upvotes: 3
Reputation: 1271013
Use a subquery to get the maximum and then a where
:
select t.*
from (select t.*, max(avg) over (partition by city) as maxavg
from avgTemperatures t
) t
where maxavg > 19;
An alternative is to do this in the where
clause:
select t.*
from avgTemperatures t
where t.city in (select t2.city from avgTemperatures t2 where t2.avg > 19);
Upvotes: 1
Reputation:
You need to wrap this in a derived table to be able to use in the where clause:
select *
from (
SELECT t.*, MIN(avg) OVER (PARTITION BY city) as city_avg
FROM AvgTemperatures t
) x
WHERE city_avg > 16
Upvotes: 1