How to Achieve SQL Server's TOP n WITH TIES Functionality in PostgreSQL?

PostgreSQL Equivalent for TOP n WITH TIES

When dealing with data retrieval in SQL Server, the TOP n WITH TIES clause allows the retrieval of the first n rows of a table, including any rows that tie for the last position. This feature ensures that all rows with equivalent values are returned, eliminating the need for additional queries.

In PostgreSQL, there is no direct equivalent to the WITH TIES clause. However, there are two approaches that achieve the same result:

Using a Rank Subquery

WITH cte AS (
    SELECT *, RANK() OVER (ORDER BY <column_name>) AS rnk
    FROM tablename
)
SELECT *
FROM cte
WHERE rnk <= n;

This method assigns a rank to each row in the table, with ties receiving the same rank. The LIMIT clause is then used to return only the rows with ranks up to and including n.

Using Array Aggregation

Another method involves using array aggregation and the UNNEST function to group and return tied rows:

SELECT UNNEST(
    ARRAY_AGG(DISTINCT nums) OVER (
        ORDER BY nums DESC
        ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
    )
) AS nums
FROM Numbers
ORDER BY nums DESC
LIMIT n;

This approach aggregates all distinct values of nums into an array, which is then unnested to return the tied rows. The LIMIT clause ensures that only the top n values are returned.

PostgreSQL 13 and Beyond

It's worth noting that PostgreSQL 13 introduced the WITH TIES clause, directly equivalent to SQL Server's implementation. Therefore, if you're using PostgreSQL 13 or later, you can use the following syntax:

SELECT TOP n WITH TIES nums
FROM Numbers
ORDER BY nums DESC;

This simplifies the process of retrieving tied rows in PostgreSQL, eliminating the need for alternative methods.

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