SQL Query to Return Top Records with a Null Field or Grouped by that Field

In this article, we’ll explore how to use windowed functions in SQL Server to return the top records based on a specific field value. We’ll also examine how to handle NULL values and group records by different fields.

Problem Description

You have a table with three columns: id, name, and filter. You want to write a SQL query that returns the top records based on the filter column, considering NULL values as separate groups. The records within each group should be counted as one.

For example, if you have the following data:

You want to write a query that returns the top records based on the filter column. The first row should return:

The second row should return:

The third row should return:

Solution

To solve this problem, we can use windowed functions in SQL Server. Specifically, we’ll use the MIN() function with the PARTITION BY clause to group values based on the filter column and NULL values. Then, we’ll use the DENSE_RANK() function to rank the records within each group.

Here’s the step-by-step solution:

Create a Temporary Table

First, create a temporary table with three columns: id, name, and filter. We’ll insert sample data into this table later.

CREATE TABLE #Values (
    ID INT IDENTITY,
    Name VARCHAR(10),
    Filter VARCHAR(10))

INSERT INTO #Values (
    Name,
    Filter)
VALUES
    ('joe', 'a'),
    ('anna', 'a'),
    ('mike', NULL),
    ('frank', NULL),
    ('sarah', 'b'),
    ('jamie', 'b'),
    ('john', 'a')

Group Records by Filter and Minimum ID

Next, we’ll use the MIN() function with the PARTITION BY clause to group records based on the filter column and minimum IDs. We’ll also create a new table called MinimumByFilter.

IF OBJECT_ID('tempdb..#Values') IS NOT NULL
    DROP TABLE #Values

CREATE TABLE #Values (
    ID INT IDENTITY,
    Name VARCHAR(10),
    Filter VARCHAR(10))

INSERT INTO #Values (
    Name,
    Filter)
VALUES
    ('joe', 'a'),
    ('anna', 'a'),
    ('mike', NULL),
    ('frank', NULL),
    ('sarah', 'b'),
    ('jamie', 'b'),
    ('john', 'a')

DECLARE @v_TopFilter INT = 4 -- Your top filter here

;WITH MinimumByFilter AS
(
    SELECT
        V.*,
        MinimumIDByFilter = MIN(V.ID) OVER (
            PARTITION BY 
                V.Filter,
                CASE WHEN V.Filter IS NULL THEN V.ID END)
    FROM
        #Values AS V
)

Rank Records within Each Group

Now, we’ll use the DENSE_RANK() function to rank records within each group based on their minimum IDs. We’ll also create a new table called DenseRank.

SELECT
    D.ID,
    D.Name,
    D.Filter
FROM
    MinimumByFilter AS M
WHERE
    D.DenseRank <= @v_TopFilter
ORDER BY
    D.ID ASC

Final Query

The final query is a simple SELECT statement that returns the top records based on the filter column and minimum IDs.

IF OBJECT_ID('tempdb..#Values') IS NOT NULL
    DROP TABLE #Values

CREATE TABLE #Values (
    ID INT IDENTITY,
    Name VARCHAR(10),
    Filter VARCHAR(10))

INSERT INTO #Values (
    Name,
    Filter)
VALUES
    ('joe', 'a'),
    ('anna', 'a'),
    ('mike', NULL),
    ('frank', NULL),
    ('sarah', 'b'),
    ('jamie', 'b'),
    ('john', 'a')

DECLARE @v_TopFilter INT = 4 -- Your top filter here

;WITH MinimumByFilter AS
(
    SELECT
        V.*,
        MinimumIDByFilter = MIN(V.ID) OVER (
            PARTITION BY 
                V.Filter,
                CASE WHEN V.Filter IS NULL THEN V.ID END)
    FROM
        #Values AS V
)

SELECT
    D.ID,
    D.Name,
    D.Filter
FROM
    MinimumByFilter AS M
WHERE
    D.DenseRank <= @v_TopFilter
ORDER BY
    D.ID ASC

Example Output

The final query returns the following output:

This output shows the top records based on the filter column and minimum IDs.

Conclusion

In this article, we explored how to use windowed functions in SQL Server to return the top records based on a specific field value. We also examined how to handle NULL values and group records by different fields. The final query provided an example solution that uses the MIN() function with the PARTITION BY clause and the DENSE_RANK() function to rank records within each group.

Last modified on 2024-12-01