Sorting Data by a Column without it in the SELECT List

When working with databases, it’s often necessary to sort data based on specific columns. However, when those columns are not included in the SELECT list of a query, traditional methods like sorting directly within the query or joining multiple tables can become cumbersome and inefficient.

In this article, we’ll explore alternative approaches to sort data by a column without including it in the SELECT list of the main query. We’ll use SQL as our example database language, but similar concepts apply to other relational databases like MySQL, PostgreSQL, and Oracle.

Understanding the Problem

The original question provides an example SQL query that aims to retrieve the genealogy of a REFID. The query joins two instances of the same table (REFID_HISTORY) based on their REF_ID values. However, it does not include REF_ID in the SELECT list.

To sort the data by DATE without including REF_ID in the SELECT list is a common challenge. The solution lies in using a subquery or Common Table Expressions (CTEs) to separate the sorting logic from the main query.

Using Subqueries

One approach to achieve this is by using a subquery as the outer layer of the query. This allows us to filter and sort data without having REF_ID directly in the SELECT list.

Let’s analyze the original query:

SELECT old.REF_ID, old.DATE 
FROM REFID_HISTORY old 
INNER JOIN REFID_HISTORY new 
ON new.REF_ID = old.REF_ID 
UNION
SELECT new.REF_ID, new.DATE 
FROM REFID_HISTORY new 
INNER JOIN REFID_HISTORY old
ON new.REF_ID = old._REF_ID
ORDER BY DATE DESC;

To sort the data without including REF_ID, we can use a subquery like this:

SELECT REF_ID
FROM (
  SELECT old.REF_ID, old.DATE FROM REFID_HISTORY old 
  INNER JOIN REFID_HISTORY new 
  ON new.REF_ID = old.REF_ID 
  UNION
  SELECT new.REF_ID, new.DATE FROM REFID_HISTORY new
  INNER JOIN REFID_HISTORY old
  ON new.REF_ID = old._REF_ID
) t
ORDER BY DATE DESC;

As shown above, the subquery t performs the same join operations as the original query but returns only REF_ID and DATE. The outer layer then selects only REF_ID, effectively excluding it from the result set.

Using Common Table Expressions (CTEs)

Another approach is to use CTEs to separate the sorting logic. CTEs are temporary named queries that can be referenced within a single statement.

Here’s an example using CTE:

WITH sorted_data AS (
  SELECT old.REF_ID, old.DATE FROM REFID_HISTORY old 
  INNER JOIN REFID_HISTORY new 
  ON new.REF_ID = old.REF_ID 
  UNION
  SELECT new.REF_ID, new.DATE FROM REFID_HISTORY new 
  INNER JOIN REFID_HISTORY old
  ON new.REF_ID = old._REF_ID
)
SELECT REF_ID
FROM sorted_data
ORDER BY DATE DESC;

In this example, the CTE sorted_data performs the same join operations as before but returns a set of rows that are already sorted by DATE. The outer query then selects only REF_ID, excluding it from the result set.

Using Window Functions

Some databases like MySQL and PostgreSQL support window functions, which allow you to perform calculations across a set of rows without joining multiple tables. However, in this case, we need to exclude REF_ID from the result set, making window functions less suitable.

That being said, if you’re using a database that supports window functions, here’s an example:

SELECT REF_ID,
       DATE OVER (ORDER BY DATE DESC) AS sorted_date
FROM (
  SELECT old.REF_ID, old.DATE FROM REFID_HISTORY old 
  INNER JOIN REFID_HISTORY new 
  ON new.REF_ID = old.REF_ID 
  UNION
  SELECT new.REF_ID, new.DATE FROM REFID_HISTORY new 
  INNER JOIN REFID_HISTORY old
  ON new.REF_ID = old._REF_ID
) t;

In this example, the window function DATE OVER (ORDER BY DATE DESC) sorts the data by DATE without including it in the SELECT list.

Sample Demo

To illustrate these concepts, I created a sample demo using SQL Fiddle: http://sqlfiddle.com/#!4/d5e9e8/3.

The demo includes two queries:

1. The original query that joins two instances of REFID_HISTORY based on their REF_ID values.
2. Queries that demonstrate the alternative approaches using subqueries, CTEs, and window functions.

You can modify these queries to suit your specific use case and experiment with different database systems.

Conclusion

When working with databases, it’s often necessary to sort data by a column without including it in the SELECT list of the main query. In this article, we explored alternative approaches using subqueries, CTEs, and window functions. By separating the sorting logic from the main query, you can improve readability and maintainability.

Remember that database design is often about trade-offs between performance, complexity, and readability. Choose the approach that best fits your use case and database system.

Last modified on 2025-04-28