Ordering with Union: A SQL Solution for Oracle Databases

When working with SQL queries that involve union operations, it’s not uncommon to encounter scenarios where the ordering of results is not straightforward. In this article, we’ll explore a technique for ordering with union in SQL, specifically tailored for use cases involving Oracle databases.

Background and Problem Statement

The provided Stack Overflow question illustrates a common issue that arises when working with union queries: how to ensure that the first query’s result appears as the top row in the output. This can be particularly challenging when there are no inherent ordering rules or constraints defined on the columns involved.

The Challenge of Union Ordering

To understand why this is a problem, let’s take a closer look at the example provided:

SELECT NM, DEP_CD FROM EMP WHERE DEP_CD='1100' AND (SELECT COUNT(1) FROM BBS_TABLE WHERE UP_DEP_CD = '1100') > 0
UNION
SELECT NM, DEP_CD FROM EMP WHERE DEP_CD '1110'

In this query, the first SELECT statement is wrapped within another query using a subquery. However, Oracle’s union operator performs an implicit ordering based on the columns of the individual queries being joined.

The Solution: Using RANK or DENSE_RANK

To achieve our goal of placing the top result from the first query at the top of the output, we can use the RANK function to assign a ranking value to each row within a partition. We’ll demonstrate how this works in both Oracle and standard SQL.

Assigning Ranking Values with RANK

Here’s an example that illustrates how we can use RANK to solve our problem:

SELECT NM, DEP_CD, 1 AS rnk FROM (
    SELECT NM, DEP_CD, 1 AS rnk FROM EMP WHERE DEP_CD='1100' AND (SELECT COUNT(1) FROM BBS_TABLE WHERE UP_DEP_CD = '1100') > 0
    UNION ALL
    SELECT NM, DEP_CD, 2 AS rnk FROM EMP WHERE DEP_CD='1110'
)
ORDER BY 3 ASC;

In this code snippet:

• We use a subquery to wrap both the original queries.
• Within that query, we use RANK to assign a ranking value of 1 to rows in the first partition (DEP_CD='1100') and 2 to rows in the second partition (DEP_CD='1110').
• We then order by column 3 (which is actually an alias for rnk, representing the rank value) using an ascending sort.

Note that the UNION ALL operator is used instead of UNION to preserve duplicate rows in the results. If we were to use UNION instead, the resulting set would be missing the duplicate rows from each table.

Using Outer Queries for Clean Output

Another approach is to create an outer query that selects only those columns needed and orders by our custom rank column. Here’s how it works:

SELECT x.NM, x.DEP_CD FROM (
    SELECT NM, DEP_CD, 1 AS rnk FROM EMP WHERE DEP_CD='1100' AND (SELECT COUNT(1) FROM BBS_TABLE WHERE UP_DEP_CD = '1100') > 0
    UNION ALL
    SELECT NM, DEP_CD, 2 AS rnk FROM EMP WHERE DEP_CD='1110'
)x
ORDER BY x.rnk ASC;

This outer query is more flexible because it doesn’t have to include every column from both tables.

Oracle-Specific Considerations

While the above solution works for standard SQL and most databases, we need to take into account Oracle-specific features like aliasing when ordering by a subquery’s column in an outer query.

To handle the case where we don’t want our final output to have the rank field:

SELECT x.NM, x.DEP_CD FROM (
    SELECT NM, DEP_CD, 1 AS rnk FROM EMP WHERE DEP_CD='1100' AND (SELECT COUNT(1) FROM BBS_TABLE WHERE UP_DEP_CD = '1100') > 0
    UNION ALL
    SELECT NM, DEP_CD, 2 AS rnk FROM EMP WHERE DEP_CD='1110'
)x
ORDER BY x.rnk ASC;

By choosing to include the rank column when ordering, we can safely exclude it from our final output:

SELECT x.NM, x.DEP_CD
FROM (
    SELECT NM, DEP_CD, 1 AS rnk FROM EMP WHERE DEP_CD='1100' AND (SELECT COUNT(1) FROM BBS_TABLE WHERE UP_DEP_CD = '1100') > 0
    UNION ALL
    SELECT NM, DEP_CD, 2 AS rnk FROM EMP WHERE DEP_CD='1110'
)x
ORDER BY x.rnk ASC;

However, if we truly wish to omit it from the output, there’s an alternative.

SELECT x.NM, x.DEP_CD
FROM (
    SELECT NM, DEP_CD,
           1 AS rnk FROM EMP WHERE DEP_CD='1100' AND (SELECT COUNT(1) FROM BBS_TABLE WHERE UP_DEP_CD = '1100') > 0
    UNION ALL
    SELECT NM, DEP_CD,
           2 AS rnk FROM EMP WHERE DEP_CD='1110'
)x
ORDER BY x.rnk ASC;

To omit the rank column from our output:

SELECT x.NM, x.DEP_CD
FROM (
    SELECT NM, DEP_CD, RANK() OVER (ORDER BY (CASE WHEN DEP_CD = '1100' THEN 1 ELSE 2 END)) AS rnk FROM EMP
)x
ORDER BY x.rnk ASC;

This uses Oracle’s built-in RANK function with an OVER clause to assign the rank values.

Conclusion

In conclusion, when dealing with union operations in SQL and trying to ensure that the first query’s result is at the top of the output, we can use a variety of strategies, including assigning ranking values using functions like RANK, DENSE_RANK, or ROW_NUMBER.

We’ve examined Oracle-specific considerations and how these can be leveraged when working with standard SQL.

By understanding these techniques, you’ll be better equipped to handle complex union queries that require special ordering logic.

Last modified on 2025-03-08