How to Convert Oracle SQL's KEEP DENSE_RANK to PostgreSQL Equivalent?

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Summary: Learn how to translate Oracle SQL's KEEP DENSE_RANK functionality into PostgreSQL using common table expressions and window functions.
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How to Convert Oracle SQL's KEEP DENSE_RANK to PostgreSQL Equivalent?

Migrating from Oracle to PostgreSQL can sometimes present challenges due to differences in SQL syntax and functionality between the two databases. One such feature that often needs to be translated is Oracle SQL's KEEP DENSE_RANK aggregate function. In this guide, we will explore how to emulate the KEEP DENSE_RANK functionality in PostgreSQL effectively.

Understanding KEEP DENSE_RANK in Oracle

The KEEP DENSE_RANK function in Oracle SQL is used to retain the first or last occurrence of values that share the same rank within a certain partition or group. This function allows you to retrieve the top or bottom ranked values within each partition based on the specified sorting criteria.

For example, suppose you have the following Oracle SQL query:

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This query retrieves, for each product, the minimum price by keeping the first occurrence of each price within its respective partition.

Emulating KEEP DENSE_RANK in PostgreSQL

PostgreSQL does not have a direct equivalent of the KEEP DENSE_RANK syntax. However, you can achieve similar functionality using Common Table Expressions (CTEs) and window functions. The following steps outline how to convert the Oracle SQL query to PostgreSQL.

Step-by-Step Conversion

Use a common table expression to assign dense ranks to the rows:

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Filter the results to keep only the first ranked occurrences:

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Complete PostgreSQL Query

Combining the above steps, the final PostgreSQL query looks like this:

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Conclusion

While PostgreSQL does not offer a direct equivalent to Oracle's KEEP DENSE_RANK function, you can still achieve the same result using CTEs and window functions. This workaround allows you to retain the desired values within each partition based on specified sorting criteria.

Understanding how to perform such conversions is crucial for a smooth migration from Oracle to PostgreSQL. Mastering these techniques will allow database professionals to harness the full power of PostgreSQL while staying true to the logic and functionality of their original Oracle SQL queries.