Get specific records from DB - sql

Question

I have the following tables, and values:
t_cars
---------------------------------------------------------
nCars_ID sName sModel sIdentifier
---------------------------------------------------------
1 BMW 3 series D-78-JHG
2 Volvo C30 B-56-KHT
3 Fiat Doblo H-72-ABN
4 Volvo C40 J-78-YTR
t_feature
---------------------------
nFeature_ID sName
---------------------------
1 CMC
2 Doors
3 Color
4 Type
5 Weight
6 Engine
7 Power
t_cars_feature
-------------------------------------------------------------------
nCarsFeature_ID nCars_ID nFeature_ID sValue
------------------------------------------------------------------
1 2 1 2500
2 2 2 5
3 2 4 Diesel
4 2 3 Green
5 3 1 1900
6 3 2 3
7 3 4 Otto
8 3 5 2300 KG
9 1 1 1900
10 1 3 Blue
11 1 4 Diesel
12 1 5 2100 KG
I need to retrieve from DB the cars that has CMC feature, has Color feature, AND CMC = 1900 AND Color = 'Blue' ONLY
I have tried:
SELECT t_cars.sName, t_cars.sModel, t_cars.sIdentifier
FROM t_cars, t_feature, t_cars_feature
WHERE t_feature.nFeature_ID = t_cars_feature.nFeature_ID
AND t_cars.nCars_ID = t_cars_feature.nCars_ID
AND [/*condition that get me cars that has CMC feature, has Color feature, AND CMC = 1900 AND Color = 'Blue' ONLY*/]
I have tried the condition like that:
Trial 1:
AND t_feature.sName = 'CMC'
AND t_feature.sName = 'Color'
AND t_cars_feature.sValue = '1900'
AND t_cars_feature.sValue = 'Blue'
and get me nothing
I have also tried:
Trial 2:
AND t_feature.sName IN ('CMC','Color')
AND t_cars_feature.sValue IN ('1900','Blue')
and get me all records that has CMC 1900 OR color 'Blue' (probably I got here an cartesian product)
In real situation I could have several t_feature.sName values, and several t_cars_feature.sValue values, that is why trial 1 are not suitable for me ...
Can you help me ? Thank you.

Answer 1

Use this query:
SELECT *
FROM t_cars
WHERE EXISTS (SELECT * FROM t_cars_feature AS cf
JOIN t_feature AS f ON (f.nFeature_ID = cf.nFeature_ID)
WHERE t_cars.nCars_ID = cf.nCars_ID AND f.sName = 'CMC' AND cf.sValue = '1900')
AND
EXISTS (SELECT * FROM t_cars_feature AS cf
JOIN t_feature AS f ON (f.nFeature_ID = cf.nFeature_ID)
WHERE t_cars.nCars_ID = cf.nCars_ID AND f.sName = 'Color' AND cf.sValue = 'Blue');
See the full example here.
I would like to comment about some of the things you've done there:
It is generally considered a bad practice to notate the type as a prefix in the column name, or in the object's name.
Your usage of the IN clause is wrong. I suggest you to try and practice about this matter a bit. When you supply a list of values inside the IN clause, it means that any of those values should result in a true value for the predicate - not that both of the them should exists.
Please note that when you use the WHERE clause with multiple predicates separated by an "AND", you require all of them to exists in a SINGLE row. Check out the result of your JOIN and see if this is the case to understand the case better.
It is considered a better practice to use a JOIN (i.e. INNER JOIN) and not a cartesian product (,) - even though both queries will usually compile into the same execution plan. It is just more readable and understandable.

Answer 2

You could do it with EXISTS and NOT EXISTS:
--Alias = Table
-- C = t_cars
-- CF = t_cars_feature
-- F = t_feature
SELECT *
FROM t_cars C
WHERE EXISTS (SELECT *
FROM t_cars_feature CF INNER JOIN t_feature F ON CF.nFeature_ID = F.nFeature_ID
WHERE CF.nCars_ID = C.nCars_ID --matches t_cars ID
AND F.sName = 'CMC' --has CMC feature
AND F.sName = 'Color' --has color feature
AND CF.sValue = '1900' --CMC = 1900
AND CF.sValue = 'Blue' --Color = 'Blue'
)
AND NOT EXISTS
(SELECT *
FROM t_cars_feature CF INNER JOIN t_feature F ON CF.nFeature_ID = F.nFeature_ID
WHERE CF.nCars_ID = C.nCars_ID --matches t_cars ID
AND F.sName = 'CMC' --has CMC feature
AND F.sName = 'Color' --has color feature
AND CF.sValue = '1900' --CMC = 1900
AND CF.sValue <> 'Blue' --Color = 'Blue ONLY'
)

Answer 3

You can make some use of conditional aggregation to get the desired results
SELECT t_cars.sName, t_cars.sModel, t_cars.sIdentifier
FROM t_cars
JOIN t_cars_feature ON t_cars.nCars_ID = t_cars_feature.nCars_ID
JOIN t_feature ON t_feature.nFeature_ID = t_cars_feature.nFeature_ID
WHERE t_feature.sName IN ('CMC','Color')
GROUP BY t_cars.sName, t_cars.sModel, t_cars.sIdentifier
HAVING SUM(CASE WHEN t_feature.sName ='CMC' AND t_cars_feature.sValue ='1900' THEN 1 ELSE 0 END) > 0
AND SUM(CASE WHEN t_feature.sName ='Color' AND t_cars_feature.sValue ='Blue' THEN 1 ELSE 0 END) >0
Demo