How to Dynamically Generate Columns in SQL for Data Analysis?

Dynamicly create columns in SQL

Dynamic column generation in SQL allows for flexible data structure building based on dynamic input. Consider the following scenario:

• Customers table contains customer details (ID and name)
• CustomerRewards table contains reward types (TypeID and Description)
• Rewards table connecting customers and rewards

The goal is to dynamically create columns representing reward types and count the number of customer rewards in each type.

To do this, you can use SQL’s PIVOT function:

<code class="language-sql">SELECT
  name,
  [Bronze],
  [Silver],
  [Gold],
  [Platinum],
  [AnotherOne]
FROM
  (
    SELECT
      c.name,
      cr.description,
      r.typeid
    FROM
      customers AS c
    LEFT JOIN
      rewards AS r
      ON c.id = r.customerid
    LEFT JOIN
      customerrewards AS cr
      ON r.typeid = cr.typeid
  ) AS x
PIVOT
  (
    COUNT(typeid)
    FOR description IN ([Bronze], [Silver], [Gold], [Platinum], [AnotherOne])
  ) AS p;</code>

Here, the PIVOT function converts the rows into columns based on the description fields, showing the counts for each reward type.

For an unknown number of columns, you can use dynamic SQL:

<code class="language-sql">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 AS c
                LEFT JOIN
                  rewards AS r
                  ON c.id = r.customerid
                LEFT JOIN
                  customerrewards AS cr
                  ON r.typeid = cr.typeid
            ) AS x
            PIVOT 
            (
                COUNT(typeid)
                FOR description IN (' + @cols + ')
            ) AS p ';

EXECUTE (@query);</code>

Dynamic queries create columns based on values ​​in the customerrewards table.

To include the "Total" column, you can use the ROLLUP operator:

<code class="language-sql">SELECT
  name,
  SUM([Bronze]) AS Bronze,
  SUM([Silver]) AS Silver,
  SUM([Gold]) AS Gold,
  SUM([Platinum]) AS Platinum,
  SUM([AnotherOne]) AS AnotherOne
FROM
  (
    SELECT
      name,
      [Bronze],
      [Silver],
      [Gold],
      [Platinum],
      [AnotherOne]
    FROM
      (
        SELECT
          c.name,
          cr.description,
          r.typeid
        FROM
          customers AS c
        LEFT JOIN
          rewards AS r
          ON c.id = r.customerid
        LEFT JOIN
          customerrewards AS cr
          ON r.typeid = cr.typeid
      ) AS x
      PIVOT
      (
        COUNT(typeid)
        FOR description IN ([Bronze], [Silver], [Gold], [Platinum], [AnotherOne])
      ) AS p
  ) AS x1
GROUP BY
  name
WITH ROLLUP;</code>

By dynamically generating columns, you can efficiently handle changing data structures and present data in a comprehensive way.

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