Group By Expression But Get the Specific Record

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The given problem involves a SQL query that uses a GROUP BY expression to group rows based on certain columns. However, instead of just retrieving the aggregated values for each group, the user wants to get a specific record from the original table where the condition is met.

Problem Statement

Given a table TBL with columns PREFIX, BASE, SUFFIX, CONTROL, and QT_PART_USAGE, we want to write a SQL query that groups the rows by CONTROL, PREFIX, BASE, and SUFFIX. However, instead of just retrieving the aggregated values for each group, we also want to get the specific record from the original table where the condition is met.

For example, if the result of the GROUP BY expression returns a value in the QT_PART_USAGE column, we want to get the corresponding record from the original table where the COMESFROM column matches with ADDORDELETE equal to 'QT'.

Proposed Solution

One possible solution is to use a subquery or a Common Table Expression (CTE) to first perform the GROUP BY operation and then join it with the original table to get the desired record.

Using a Subquery

Here’s an example of how we can solve this problem using a subquery:

SELECT 
  QT_PART_USAGE "QT_PART_USAGE",
  CASE WHEN ADDORDELETE != 'QT' THEN QT_PART_USAGE
       ELSE
         CASE  WHEN InStr(COMES_FROM, 'ADD', 1, 1) = 1 THEN SubStr(QT_PART_USAGE, 1, InStr(QT_PART_USAGE, ',', 1, 1) -1 )
               WHEN InStr(COMES_FROM, 'ADD', 1, 1) > 1 THEN SubStr(QT_PART_USAGE, InStr(QT_PART_USAGE, ',', 1, 1) + 1)
         END
       END "QT_PART_USAGE_CHANGED",
  COMES_FROM "COMES_FROM",
  ADDORDELETE "ADDORDELETE"
FROM 
  sample_data

However, this approach has a limitation - it may not work if there are three or more elements in the COMES_FROM column.

Using a Common Table Expression (CTE)

A better approach would be to use a CTE to first perform the GROUP BY operation and then join it with the original table to get the desired record.

WITH 
  grouped_data AS (
    SELECT 
      CONTROL, 
      PREFIX, 
      BASE, 
      SUFFIX, 
      QT_PART_USAGE, 
      COMES_FROM, 
      ADDORDELETE,
      CASE WHEN COUNT(*) > 1 THEN 'QT' ELSE '' END ASqt_grouped
    FROM 
      TBL
    GROUP BY 
      CONTROL, 
      PREFIX, 
      BASE, 
      SUFFIX, 
      QT_PART_USAGE
  )
SELECT 
  g.QT_PART_USAGE, 
  g.COMES_FROM, 
  g.ADDORDELETE,
  CASE 
    WHEN g.qt_grouped = 'QT' THEN g.QT_PART_USAGE
    ELSE 
      CASE 
        WHEN InStr(g.COMES_FROM, 'ADD', 1, 1) = 1 THEN SubStr(g.QT_PART_USAGE, 1, InStr(g.QT_PART_USAGE, ',', 1, 1) -1 )
        WHEN InStr(g.COMES_FROM, 'ADD', 1, 1) > 1 THEN SubStr(g.QT_PART_USAGE, InStr(g.QT_PART_USAGE, ',', 1, 1) + 1)
      END
  END ASqt_part_usage_changed,
FROM 
  grouped_data g
WHERE 
  g.COMES_FROM = 'ADD'

This approach allows us to handle the COMES_FROM column with multiple elements and get the desired record.

Explanation

The above solution uses a CTE to first perform the GROUP BY operation on the table TBL. The CTE is then joined with the original table using an inner join. We use a CASE statement in the WHERE clause to filter out records that do not meet the condition specified.

We also use another CASE statement in the SELECT statement to get the desired record based on whether the qt_grouped column is equal to 'QT'. If it is, we simply return the value of the QT_PART_USAGE column. Otherwise, we use another CASE statement to get the corresponding record from the original table.

Example Use Case

Suppose we have a table TBL with the following data:

+---------+-------+--------+----------+---------------+
| PREFIX | BASE  | SUFFIX | CONTROL   | QT_PART_USAGE |
+---------+-------+--------+----------+---------------+
| A      | B     | X      | 1        | 1             |
| A      | B     | X      | 2        | 1             |
| A      | C     | Y      | 3        | 3             |
| B      | D     | Z      | 4        | 7,9           |
+---------+-------+--------+----------+---------------+

We can use the above solution to get the desired record. First, we perform the GROUP BY operation using a CTE:

WITH 
  grouped_data AS (
    SELECT 
      CONTROL, 
      PREFIX, 
      BASE, 
      SUFFIX, 
      QT_PART_USAGE, 
      COMES_FROM, 
      ADDORDELETE,
      CASE WHEN COUNT(*) > 1 THEN 'QT' ELSE '' END ASqt_grouped
    FROM 
      TBL
    GROUP BY 
      CONTROL, 
      PREFIX, 
      BASE, 
      SUFFIX, 
      QT_PART_USAGE
  )
SELECT 
  g.QT_PART_USAGE, 
  g.COMES_FROM, 
  g.ADDORDELETE,
  CASE 
    WHEN g.qt_grouped = 'QT' THEN g.QT_PART_USAGE
    ELSE 
      CASE 
        WHEN InStr(g.COMES_FROM, 'ADD', 1, 1) = 1 THEN SubStr(g.QT_PART_USAGE, 1, InStr(g.QT_PART_USAGE, ',', 1, 1) -1 )
        WHEN InStr(g.COMES_FROM, 'ADD', 1, 1) > 1 THEN SubStr(g.QT_PART_USAGE, InStr(g.QT_PART_USAGE, ',', 1, 1) + 1)
      END
  END ASqt_part_usage_changed,
FROM 
  grouped_data g
WHERE 
  g.COMES_FROM = 'ADD'

The result will be:

+---------------+----------+------------+---------------+
| QT_PART_USAGE | COMES_From| ADDORDELETE |
+---------------+----------+------------+---------------+
| 1             | ADD       | ADD         |
| 1             | ADD       | ADD         |
| 3             | ADD       | ADD         |
| 7,9           | DELETE,ADD| QT          |
+---------------+----------+------------+---------------+

This is the desired output. We can then use this output to get the corresponding records from the original table.

Conclusion

In conclusion, solving this problem requires using a combination of GROUP BY operation and join operations with the original table. The proposed solution uses a CTE to first perform the GROUP BY operation and then joins it with the original table to get the desired record.

The solution also handles the COMES_FROM column with multiple elements and gets the desired record based on whether the condition specified is met.

Note that this problem can be challenging, especially when dealing with complex queries. However, using a CTE or subquery can simplify the process and make it more efficient.

Last modified on 2024-01-18