How do I structure my SQL query to prevent the return of duplicate rows with related data? - sql

Question

I need some help with an SQL Query. I have a database table that has related data with other tables. When I query the table it returns the duplicate rows for every row of related data i.e.
|-------------| |-------------| |-------------|
| Cars | | Options | | Value |
|-------------| ------> |-------------| ------> |-------------|
| CarId | | OptionsId | | ValueId |
| CarMake | | OptionName | | CostValue |
| CarModel | | Confirmed | | CarId |
|-------------| | CarId | | OptionsId |
|-------------| |-------------|
|
|
---------------> |-------------|
| Warranty |
|-------------|
| WarrantyId |
| WarrantyType|
| CarId |
|-------------|
The query that I have made, which was designed in the query builder of SSMS (because of this it is not using aliases and has the 3 stage naming convention, this will be changed) is as follows:
SELECT dbo.Cars.CarId,
dbo.Cars.Make,
dbo.Cars.Model,
dbo.Options.OptionName,
dbo.Warranty.WarrantyType,
dbo.Value.CostValue
FROM dbo.Cars
LEFT JOIN dbo.Options ON dbo.Cars.CarId = dbo.Options.CarId
LEFT JOIN Value ON Options.OptionsId = Value.OptionsId
LEFT JOIN dbo.Warranty on dbo.Cars.CarId = dbo.Warranty.CarId
Executing this query as it stands returns my data, however, for cars with multiple options I receive duplicate rows i.e.
Id | Make | Model | Option Name | Warranty Type | Value
27 | Ford | Fiesta | Heated Seats | Static | 500
27 | Ford | Fiesta | Front Fog Lights | Static | 400
I've been looking around for possible answers to this question and found that the proposed solution is to use the keyword DISTINCT or to create a subquery. I added DISTINCT to my query but the same data was returned, probably because the options are both distinct in their own right, I don't know I'm guessing.
I'm happy to use a subquery but not sure how to apply that to my above query code. All I want to do here is return one single row for each car with the highest option value i.e.
27 | Ford | Fiesta | Heated Seats | Static | 500
Can anyone help me write this query? I think I've included everything in this question but if I can offer more, please let me know.

Answer 1

Instead of joining the table Value which gives you multiple rows,
you must join this query:
SELECT
dbo.Value.CarId,
dbo.Value.OptionsId,
MAX(dbo.Value.CostValue) AS CostValue
FROM dbo.Value
GROUP BY dbo.Value.CarId, dbo.Value.OptionsId
which you will give you from the table Value for each car the option with the max value.
So try this:
SELECT dbo.Cars.CarId,
dbo.Cars.Make,
dbo.Cars.Model,
dbo.Options.OptionName,
v.CostValue,
dbo.Warranty.WarrantyType
FROM dbo.Cars
LEFT JOIN dbo.Options ON dbo.Cars.CarId = dbo.Options.CarId
INNER JOIN (
SELECT
dbo.Value.CarId,
dbo.Value.OptionsId,
MAX(dbo.Value.CostValue) AS CostValue
FROM dbo.Value
GROUP BY dbo.Value.CarId, dbo.Value.OptionsId
) AS v ON Options.OptionsId = v.OptionsId
LEFT JOIN dbo.Warranty on dbo.Cars.CarId = dbo.Warranty.CarId

Answer 2

you can try like below by using window function
with cte as(
SELECT dbo.Cars.CarId,
dbo.Cars.Make,
dbo.Cars.Model,
dbo.Options.OptionName,
Value.CostValue,
row_number() over(partition by dbo.Cars.CarId,
dbo.Cars.Make,
dbo.Cars.Model order by Value.CostValue desc) rn
FROM dbo.Cars
LEFT JOIN dbo.Options ON dbo.Cars.CarId = dbo.Options.CarId
LEFT JOIN Value ON Options.OptionsId = Value.OptionsId
LEFT JOIN dbo.Warranty on dbo.Cars.CarId = dbo.Warranty.CarId
) select * from cte where rn=1