sql - How to merge rows on specific condition?

Sql - How to merge rows on specific condition?

If you want to merge rows in SQL based on a specific condition, you can use the CASE statement and aggregate functions like MAX, MIN, or SUM. Here's an example where rows are merged based on a common identifier:

Assuming you have a table named your_table with columns like id, column1, and column2, and you want to merge rows with the same id by concatenating values:

SELECT id, MAX(column1) AS merged_column1, MAX(column2) AS merged_column2 FROM your_table GROUP BY id;

This query groups rows by the id column and uses the MAX function to select the maximum value for each column within each group. Adjust the aggregate function based on your specific requirements.

If you want to concatenate values, and assuming you are using MySQL, you can use the GROUP_CONCAT function:

SELECT id, GROUP_CONCAT(column1) AS merged_column1, GROUP_CONCAT(column2) AS merged_column2 FROM your_table GROUP BY id;

This query concatenates values for column1 and column2 within each group defined by the id column.

Adjust the query based on your specific conditions and the structure of your table. Note that different databases might have different functions for string concatenation or aggregation.

Examples

  1. Using GROUP BY with Aggregation Functions:

    • SELECT Column1, MAX(Column2) as MergedColumn FROM YourTable GROUP BY Column1;
    • Description: Merges rows with the same value in Column1 by using the GROUP BY clause and applying an aggregation function (e.g., MAX) to other columns.

  2. Using CASE Statement for Conditional Merging:

    • SELECT Column1, CASE WHEN MAX(Column2) > 50 THEN 'High' ELSE 'Low' END AS MergedColumn FROM YourTable GROUP BY Column1;
    • Description: Uses a CASE statement within the SELECT statement to conditionally merge rows based on a specified condition.

  3. Using CONCATENATION for String Columns:

    • SELECT Column1, STRING_AGG(Column2, ', ') AS MergedColumn FROM YourTable GROUP BY Column1;
    • Description: Merges string columns using the STRING_AGG function, concatenating values with a specified delimiter.

  4. Using COALESCE for Merging with Priority:

    • SELECT Column1, COALESCE(MAX(Column2), MAX(Column3), MAX(Column4)) AS MergedColumn FROM YourTable GROUP BY Column1;
    • Description: Uses COALESCE to merge rows with priority, selecting the first non-null value among multiple columns.

  5. Using ROW_NUMBER() Window Function:

    • SELECT Column1, Column2, ROW_NUMBER() OVER (PARTITION BY Column1 ORDER BY SomeColumn) AS RowNum INTO #TempTable FROM YourTable; SELECT Column1, MAX(Column2) AS MergedColumn FROM #TempTable WHERE RowNum = 1 GROUP BY Column1;
    • Description: Uses ROW_NUMBER() to assign a row number within each partition and selects the first row for each group.

  6. Using Self-Join with Conditions:

    • SELECT t1.Column1, t1.Column2, t2.Column2 AS MergedColumn FROM YourTable t1 LEFT JOIN YourTable t2 ON t1.Column1 = t2.Column1 AND t2.SomeCondition;
    • Description: Performs a self-join with conditions to merge rows based on specified criteria.

  7. Using GROUP_CONCAT for Multiple Values:

    • SELECT Column1, GROUP_CONCAT(Column2) AS MergedColumn FROM YourTable GROUP BY Column1;
    • Description: Uses GROUP_CONCAT to merge multiple values into a single column for each group.

  8. Using Pivot for Dynamic Column Merging:

    • SELECT Column1, [Condition1] AS MergedColumn1, [Condition2] AS MergedColumn2 FROM YourTable PIVOT ( MAX(Column2) FOR [SomeCondition] IN ([Condition1], [Condition2]) ) AS PivotTable;
    • Description: Uses PIVOT to dynamically merge rows based on different conditions as columns.

  9. Using Subquery with NOT EXISTS:

    • SELECT Column1, Column2 FROM YourTable t1 WHERE NOT EXISTS ( SELECT 1 FROM YourTable t2 WHERE t2.Column1 = t1.Column1 AND t2.SomeCondition );
    • Description: Uses a subquery with NOT EXISTS to filter out rows for merging based on a specified condition.

  10. Using CROSS APPLY for Dynamic Merging:

    • SELECT Column1, MergedColumn FROM YourTable CROSS APPLY ( SELECT TOP 1 Column2 AS MergedColumn FROM YourTable t2 WHERE t2.Column1 = YourTable.Column1 ORDER BY SomeColumn ) AS Subquery;
    • Description: Uses CROSS APPLY to dynamically merge rows based on conditions specified in a subquery.

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