How can SQL be used to concatenate columns, pivot data, and sum counts for unique ID and month combinations?

Concatenating Columns and Pivoting Data with SQL

Problem:

You have a database with columns representing ID, type, subtype, count, and month. How can you convert this data into a new format where the type and subtype are concatenated into new columns and the count is summed for each unique combination of ID and month?

Solution:

To achieve this transformed output, you can utilize the PIVOT operator in SQL Server 2005. The PIVOT operator allows you to dynamically transpose rows into columns.

The following code snippet demonstrates how to use PIVOT to achieve the desired result:

DECLARE @sql AS varchar(max)
DECLARE @pivot_list AS varchar(max) -- Leave NULL for COALESCE technique
DECLARE @select_list AS varchar(max) -- Leave NULL for COALESCE technique

SELECT @pivot_list = COALESCE(@pivot_list + ', ', '') + '[' + PIVOT_CODE + ']'
        ,@select_list = COALESCE(@select_list + ', ', '') + 'ISNULL([' + PIVOT_CODE + '], 0) AS [' + PIVOT_CODE + ']'
FROM (
    SELECT DISTINCT [TYPE] + '_' + SUBTYPE AS PIVOT_CODE
    FROM stackoverflow_159456
) AS PIVOT_CODES

SET @sql = '
;WITH p AS (
    SELECT ID, [MONTH], [TYPE] + ''_'' + SUBTYPE AS PIVOT_CODE, SUM([COUNT]) AS [COUNT]
    FROM stackoverflow_159456
    GROUP BY ID, [MONTH], [TYPE] + ''_'' + SUBTYPE
)
SELECT ID, [MONTH], ' + @select_list + '
FROM p
PIVOT (
    SUM([COUNT])
    FOR PIVOT_CODE IN (
        ' + @pivot_list + '
    )
) AS pvt
'

EXEC (@sql)

Implementation Details:

1. Generate Dynamic Pivot List: The code dynamically creates a list of pivot columns based on the unique combinations of type and subtype in the original table.
2. Build PIVOT Query: The main PIVOT query first creates a common table expression (CTE) named "p" that groups the data by ID, month, and the concatenated type and subtype. It then sums the count for each group.
3. Transposing Rows into Columns: The PIVOT operator is applied to the CTE "p" to transpose the rows into columns. The pivot column list is dynamically generated using the @pivot_list variable.
4. Handling Null Values (Optional): By using the COALESCE function, the ISNULL() condition in the @select_list variable can be used to replace null values in the pivoted columns with zeros.

Executing the @sql variable will return the desired table with concatenated columns and summed counts. This solution allows for efficient and dynamic aggregation of complex data.

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