Getting one-to-many string results as columns in a combined query

In this article, we’ll explore how to use SQL Server’s pivot function to get one-to-many string results as columns in a combined query. We’ll also delve into the concept of unpivoting and show you how to achieve the desired result using two different approaches.

Understanding the problem

We have two tables: TableA and TableB. TableA has an ID column, a Name column, and we want to select the corresponding data from TableB based on the Name in TableA.

Here’s an example of what the tables might look like:

Table A

Table B

Our goal is to get the following result:

The current solution

The current solution uses a combination of subqueries and conditional statements to achieve the desired result. Here’s an example of how we might implement this:

SELECT 
    A.ID, 
    A.Name, 
    'EyeColor' = (SELECT B.Value FROM B WHERE B.ItemID = A.ID AND B.Setting = 'EyeColor'),
    'Nickname' = (SELECT B.Value FROM B WHERE B.ItemID = A.ID AND B.Setting = 'Nickname')
FROM TableA as A
WHERE (some criteria that filters on values in Table A)

While this solution works, it can be cumbersome and may not scale well to larger result sets.

Unpivoting the data

One approach to achieving the desired result is by unpivoting the data from TableB. Unpivot is a SQL Server function that allows us to transform rows into columns.

To unpivot the data, we first need to create a temporary table that contains all the unique values for each setting:

declare @tmp as table(ItemID int, EyeColor varchar(20), NickName varchar(20))

insert into @tmp(ItemID, EyeColor,NickName)
Values(1001    , 'Blue','Billy')
,( 1002    ,'Green'  , 'Trish')

select * from @tmp

Then, we use the UNPIVOT function to transform the rows into columns:

select ItemID,Variables
from
(select ItemID,EyeColor,NickName from @tmp) source
UNPIVOT
(Variables for Setting in (EyeColor,NickName)) as unpvt

-- Result:
ItemID EyeColor    NickName
 1001   Blue    Billy
 1002   Green   Trish

Grouping and aggregating the data

To get the final result, we need to group the data by ItemID and aggregate the values for each setting. We can use the GROUP BY clause along with conditional statements to achieve this:

declare @tmp2 as table(ID int, ItemID int, Setting varchar(20), Data varchar(20))

 insert into @tmp2(ID,ItemID, Setting, Data)
 Values(1,1001, 'EyeColor','Blue')
 ,(2,1001,'NickName','Billy')
 ,(3,1002,'EyeColor','Green')   
 ,(4,1002,'NickName','Trish')   

 select ItemID, Max(Case When Setting='EyeColor' then Data End) EyeColor,
    Max(Case When Setting='NickName' then Data End) NickName
    from @tmp2
    group by ItemID

-- Result:
ItemID    EyeColor    NickName
  1001      Blue    Billy
  1002     Green    Trish

Combining the results

To get the final result, we need to combine the data from both approaches. We can use a SELECT statement with multiple UNION clauses:

SELECT ItemID, Name, EyeColor, NickName FROM TableA
UNION ALL
SELECT ItemID, Name, MAX(Case When Setting='EyeColor' then Data End) AS EyeColor,
    Max(Case When Setting='NickName' then Data End) AS NickName
FROM @tmp2
GROUP BY ItemID

-- Result:
ItemID | Name      | EyeColor | NickName 
 1001 | William   | Blue     | Billy   
 1002 | Patricia  | Green    | Trish    

Conclusion

In this article, we explored how to use SQL Server’s pivot function to get one-to-many string results as columns in a combined query. We also delved into the concept of unpivoting and showed you how to achieve the desired result using two different approaches.

By understanding the differences between these approaches, you can choose the best method for your specific use case and improve the performance and efficiency of your SQL queries.

Last modified on 2023-09-21