Get records with max value for each group of grouped SQL results

sql

subquery

window-functions

sql-optimization

byAlex Kataev·Oct 16, 2024

To fetch the max-value records per group, use the window function ROW_NUMBER(). Organise your data by Value in descending order within each partition defined by GroupId, then filter the rows based on their row number (rn).

SELECT GroupId, Value
FROM (
    SELECT GroupId, Value,
    ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY Value DESC) as rn
    FROM Records
) tmp
WHERE rn = 1

Here, ROW_NUMBER() imparts a unique sequential ID to every row within each GroupId, ordered by the Value in a descending manner. The outer SQL query then selects records where this row number equals 1. In essence, this process enables selection of the record with the highest Value per GroupId.

Elaborating on max value retrieval

Complex data groupings sometimes require more nuanced methods to effectively retrieve max values from grouped SQL results, increasing the robustness and efficiency of your data processes.

Employing LEFT JOIN with a subquery

One dependable method to pinpoint the top record within each group employs a LEFT JOIN together with a subquery. This approach ensures that all potential matches are taken into account:

SELECT a.*
FROM Records a
LEFT JOIN Records b 
  ON a.GroupId = b.GroupId AND a.Value < b.Value
WHERE b.Value IS NULL;

In this query, the LEFT JOIN creates a relationship between two instances of the same table, based on the grouping criterion (GroupId). This efficiently finds a second record (b) with a higher value. The WHERE clause subsequently removes rows where such a record exists, thus leaving us solely with the highest values.

Handling ties within groups

You might encounter situations where top values are tied. In such cases, your query should define a way to prioritize identical values. For instance, you could select the record with the first alphabetical name when values are equal:

SELECT GroupId, Name, Value
FROM (
    SELECT GroupId, Name, Value,
    ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY Value DESC, Name ASC) as rn
    FROM Records
) tmp
WHERE rn = 1

This snippet extends the ORDER BY clause inside the ROW_NUMBER function to prioritize names alphabetically, thus resolving any ties in an order determined by the name.

Using DISTINCT ON in PostgreSQL

PostgreSQL provides a database-specific solution with DISTINCT ON, enabling succinct and straightforward max value retrieval:

SELECT DISTINCT ON (GroupId) GroupId, Name, Value
FROM Records
ORDER BY GroupId, Value DESC, Name;

This command fetches the first row for each GroupId following sorting by Value, ensuring the uniqueness and simplicity of your SQL queries.

SQL query optimization

When drafting SQL queries, strive for efficient data retrieval by considering the performance. You can optimize subqueries and window functions using indexes on your table columns, particularly for grouping and sorting attributes.

By incorporating these additional techniques, you can proficiently handle grouped SQL results in varying situations, ensuring accurate and efficient data retrieval.

explain-codes / Sql / Get records with max value for each group of grouped SQL results

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