How to Dynamically Generate Columns in SQL Using the PIVOT Function?

Dynamically Generating Columns in SQL Using the PIVOT Function

This article addresses the challenge of dynamically creating columns in SQL to represent varying reward types in a customer rewards database. The reward types are not fixed and can change over time. We'll explore solutions using the SQL PIVOT function, demonstrating both static and dynamic approaches, and including the addition of a total column for comprehensive results.

The Problem:

We have database tables for customers, their rewards, and the types of rewards. The types of rewards are dynamic; they can be added or removed. The goal is to generate a result set with a column for each reward type, showing the count of that reward type for each customer.

Static Solution (Fixed Number of Reward Types):

If the reward types are known beforehand, a simple PIVOT query suffices:

<code class="language-sql">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;</code>

This query works well, but only if the reward types ("Bronze," "Silver," etc.) are predefined.

Dynamic Solution (Unknown Number of Reward Types):

For a dynamic number of reward types, we need 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 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);</code>

This code dynamically constructs the SQL query, building the column list from the customerrewards table. This allows for adaptation to any number of reward types.

Adding a Total Column:

To include a total count of rewards per customer, we can use the ROLLUP function (both static and dynamic versions are shown below):

Static Version with Total:

<code class="language-sql">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;</code>

Dynamic Version with Total:

<code class="language-sql">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);</code>

These dynamic SQL examples provide a robust and flexible solution for generating reports with a variable number of reward type columns, including a helpful total column. Remember to adapt table and column names to match your specific database schema.

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