How to get multiple fields from one criteria using SQL group by? - sql

Question

This query could return the wrong name, because the names I want to query are the ones of the smallest animals of each species. How could I get the right a.name
SELECT
a.name,
MIN(a.size)
FROM animal a
LEFT JOIN species s ON s.idSpecies = a.idAnimal
GROUP BY s.id

Answer 1

One approach for this, is to first find the smallest size of animal per species (as you have done), although I am assuming that species can never be null since an animal must belong to a species, it also does not require a join to species at this point:
SELECT a.IDSpecies, MIN(a.Size) AS Size
FROM Animal AS a
GROUP BY a.IDSpecies
Now you can join the result of this query back to your main query to filter the results.
SELECT a.Name AS AnimalName,
a.Size,
s.Name AS SpeciesName
FROM Animal AS a
INNER JOIN Species AS s
ON s.ID = a.IDSpecies
INNER JOIN
( SELECT a.IDSpecies, MIN(a.Size) AS Size
FROM Animal AS a
GROUP BY a.IDSpecies
) AS ma
ON ma.IDSpecies = a.IDSpecies
AND ma.Size = a.Size;
Another way of doing this, is to use NOT EXISTS:
SELECT a.Name AS AnimalName,
a.Size,
s.Name AS SpeciesName
FROM Animal AS a
INNER JOIN Species AS s
ON s.ID = a.IDSpecies
WHERE NOT EXISTS
( SELECT 1
FROM Animal AS a2
WHERE a2.IDSpecies = a.IDSpecies
AND a2.Size < a.Size
);
So you start with a simple select, then use NOT EXISTS to remove any animals, where a smaller animal exists in the same species.
Since MySQL will optimize LEFT JOIN/IS NULL better than NOT EXISTS, then the better way of writing the query in MySQL is:
SELECT a.Name AS AnimalName,
a.Size,
s.Name AS SpeciesName
FROM Animal AS a
INNER JOIN Species AS s
ON s.ID = a.IDSpecies
LEFT JOIN Animal AS a2
ON a2.IDSpecies = a.IDSpecies
AND a2.Size < a.Size
WHERE a2.ID IS NULL;
The concept is exactly the same as NOT EXISTS, but does not require a correlated subquery.

Answer 2

Following the simple example of Group By :
SELECT C.CountryName Country, SN.StateName, COUNT(CN.ID) CityCount
FROM Table_StatesName SN
JOIN Table_Countries C ON C.ID = SN.CountryID
JOIN Table_CityName CN ON CN.StateID = SN.ID
GROUP BY C.CountryName, SN.StateName ORDER BY SN.StateName
OUTPUT :
Country StateName CityCount
Australia Australian Capital Territory 219
Australia New South Wales 2250
Australia Northern Territory 218
Australia Queensland 2250
Australia South Australia 1501
Australia Tasmania 613
Australia Victoria 2250

Answer 3

Get all rows with size = smallest size. Use a correlated sub-quert to find that smallest size:
SELECT *
FROM animal a
LEFT JOIN species s ON s.idSpecies = a.idAnimal
where a.size = (select min(a2.size)
from animal a2
LEFT JOIN species s2 ON ss.idSpecies = as.idAnimal
where s.id = s2.id)