Скачать или смотреть Sorting Results with the UNION Operator in Oracle SQL

Discover how to sort results from multiple queries when using the `UNION` operator in Oracle SQL. This guide unpacks a common coding problem and provides a clear, step-by-step solution.
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How to Sort Results with the UNION Operator in Oracle SQL

When working with multiple SELECT statements in Oracle SQL, specifically using the UNION operator, you may encounter challenges when trying to sort the results. This can be especially true if you want to sort results of different queries that produce distinct data sets. In this post, we'll explore a specific problem related to sorting results obtained through the UNION operator, particularly regarding the HackerRank SQL challenge titled The PADS. Here’s how we can solve this.

The Problem at Hand

You are tasked with retrieving two sets of results from an OCCUPATIONS table:

An alphabetically ordered list of names, along with the first letter of each profession in parentheses.

A count of occurrences for each occupation in a specific format.

The challenge intensifies when you try to sort the combined results of both queries. A common error encountered in such SQL queries is the inability to sort before or directly within the UNION. Here's an example of the query that resulted in an error:

[[See Video to Reveal this Text or Code Snippet]]

A Clear Solution

To efficiently sort the results from both queries, you can utilize UNION ALL and introduce an identifier for each query. Here’s a step-by-step breakdown:

Step 1: Use Subqueries

By wrapping your UNION in parentheses, you allow for an ordering mechanism on the outer query level, rather than attempting to sort the two individual queries directly.

Step 2: Assign Identifiers

Assigning an identifier to each query helps maintain the order when combined. You can do this by introducing an additional column in both queries.

Step 3: Use UNION ALL

Using UNION ALL instead of UNION ensures that all duplicate rows are included in the result. This is critical if you expect similar names and occupations, and want to show all occurrences.

Here's how your revised SQL query would look:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Query

Inner Queries:

The first inner query selects names along with the first letter of each occupation and assigns it the identifier 1.

The second query counts the occurrences of each occupation and is tagged with 2.

Outer Query:

It selects the info column and sorts by qry first (ensuring that the names come before the counts) and then by info (which organizes the results within those groups).

Conclusion

With these steps, you can seamlessly sort results from multiple queries using the UNION operator in Oracle SQL. By using identifiers, subqueries, and UNION ALL, you can avoid errors and achieve the desired output structure. Remember, careful attention to these details can make a significant difference in your SQL queries, especially when dealing with sorts and unions. Happy querying!