Home Database Mysql Tutorial How to Dynamically Generate Columns with Count in SQL for Data Mining?

How to Dynamically Generate Columns with Count in SQL for Data Mining?

Jan 09, 2025 pm 03:27 PM

How to Dynamically Generate Columns with Count in SQL for Data Mining?

Use SQL to dynamically generate columns

This article discusses a common problem in the field of data mining: dynamically creating columns based on dynamic data. This challenge arises when data needs to be presented in a user-friendly format, especially when a count of values ​​is required in each dynamically generated column.

Problem Statement

We have three tables: Customers, CustomerRewards and Rewards. The goal is to generate a new table that shows each customer's name and the number of rewards they have in each reward type (e.g. Bronze, Silver, Gold, etc.). However, reward types are dynamic, meaning that new types can be added or removed over time.

Solution: Use the PIVOT function

Static PIVOT:

If the number of reward types is known in advance, we can use a hard-coded PIVOT function. For example:

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select name, [Bronze], [Silver], [Gold], [Platinum], [AnotherOne]

from

(

  select c.name,

    cr.description,

    r.typeid

  from customers c

  left join rewards r

    on c.id = r.customerid

  left join customerrewards cr

    on r.typeid = cr.typeid

) x

pivot

(

  count(typeid)

  for description in ([Bronze], [Silver], [Gold], [Platinum], [AnotherOne])

) p;

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Dynamic PIVOT:

If the number of reward types may vary, we can use dynamic SQL to perform PIVOT:

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DECLARE @cols AS NVARCHAR(MAX),

    @query  AS NVARCHAR(MAX)

 

select @cols = STUFF((SELECT ',' + QUOTENAME(description)

                    from customerrewards

                    group by description, typeid

                    order by typeid

            FOR XML PATH(''), TYPE

            ).value('.', 'NVARCHAR(MAX)')

        ,1,1,'')

 

set @query = 'SELECT name,' + @cols + ' from

             (

                select c.name,

                  cr.description,

                  r.typeid

                from customers c

                left join rewards r

                  on c.id = r.customerid

                left join customerrewards cr

                  on r.typeid = cr.typeid

            ) x

            pivot

            (

                count(typeid)

                for description in (' + @cols + ')

            ) p '

 

execute(@query)

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Contains total column

To include the total column we can use ROLLUP:

STATIC ROLLUP:

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select name, sum([Bronze]) Bronze, sum([Silver]) Silver,

  sum([Gold]) Gold, sum([Platinum]) Platinum, sum([AnotherOne]) AnotherOne

from

(

  select name, [Bronze], [Silver], [Gold], [Platinum], [AnotherOne]

  from

  (

    select c.name,

      cr.description,

      r.typeid

    from customers c

    left join rewards r

      on c.id = r.customerid

    left join customerrewards cr

      on r.typeid = cr.typeid

  ) x

  pivot

  (

    count(typeid)

    for description in ([Bronze], [Silver], [Gold], [Platinum], [AnotherOne])

  ) p

) x

group by name with rollup

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Dynamic ROLLUP:

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DECLARE @cols AS NVARCHAR(MAX),

    @colsRollup AS NVARCHAR(MAX),

    @query  AS NVARCHAR(MAX)

 

select @cols = STUFF((SELECT ',' + QUOTENAME(description)

                    from customerrewards

                    group by description, typeid

                    order by typeid

            FOR XML PATH(''), TYPE

            ).value('.', 'NVARCHAR(MAX)')

        ,1,1,'')

 

select @colsRollup

      = STUFF((SELECT ', Sum(' + QUOTENAME(description) + ') as ' + QUOTENAME(description)

                    from customerrewards

                    group by description, typeid

                    order by typeid

            FOR XML PATH(''), TYPE

            ).value('.', 'NVARCHAR(MAX)')

        ,1,1,'')

 

 

set @query

          = 'SELECT name, ' + @colsRollup + '

             FROM

             (

                SELECT name,' + @cols + ' from

                 (

                    select c.name,

                      cr.description,

                      r.typeid

                    from customers c

                    left join rewards r

                      on c.id = r.customerid

                    left join customerrewards cr

                      on r.typeid = cr.typeid

                ) x

                pivot

                (

                    count(typeid)

                    for description in (' + @cols + ')

                ) p

              ) x1

              GROUP BY name with ROLLUP'

 

execute(@query)

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