Get Top Row per Group in SQL

Here's the basic syntax:

SELECT
    column1,
    column2,
    -- ... other columns
    ROW_NUMBER() OVER (PARTITION BY grouping_column ORDER BY ordering_column) AS RowNumber
FROM
    your_table
GROUP BY
    grouping_column
HAVING
    RowNumber = 1;

Explanation:

1. GROUP BY grouping_column: This clause groups the data based on the specified column(s).
2. ROW_NUMBER() OVER (PARTITION BY grouping_column ORDER BY ordering_column):
   • PARTITION BY grouping_column: This partitions the result set into groups based on the same grouping column used in the GROUP BY clause.
   • ORDER BY ordering_column: This specifies the order within each group. The ROW_NUMBER() function assigns a sequential number to each row within the group, starting from 1.
3. HAVING RowNumber = 1: This filters the result set to only include rows where the RowNumber is 1, effectively selecting the top row from each group.

Example:

Suppose you have a table named Orders with columns OrderID, CustomerID, and OrderDate. To get the first order for each customer, you would use:

SELECT
    CustomerID,
    MIN(OrderID) AS FirstOrderID
FROM
    Orders
GROUP BY
    CustomerID;

However, if you want to get the entire row for the first order, you can use the ROW_NUMBER() approach:

SELECT
    OrderID,
    CustomerID,
    OrderDate
FROM
    (
        SELECT
            OrderID,
            CustomerID,
            OrderDate,
            ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY OrderDate) AS RowNumber
        FROM
            Orders
    ) AS RankedOrders
WHERE
    RowNumber = 1;

Understanding "Get Top 1 Row of Each Group" in SQL

Problem: We want to select only the first (or top) row from each group of data.

Solution: Use the ROW_NUMBER() function in combination with a PARTITION BY clause.

Table Structure:

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    CustomerID INT,
    OrderDate DATE
);

Query:

SELECT 
    OrderID,
    CustomerID,
    OrderDate
FROM (
    SELECT 
        OrderID,
        CustomerID,
        OrderDate,
        ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY OrderDate) AS RowNum
    FROM Orders
) AS RankedOrders
WHERE RowNum = 1;

1. Subquery:
2. Outer Query:

Breakdown of the ROW_NUMBER() function:

• PARTITION BY CustomerID: Groups the rows by CustomerID.
• ORDER BY OrderDate: Orders the rows within each group by OrderDate.
• ROW_NUMBER(): Assigns a sequential number to each row within its partition.

Key Points:

• The PARTITION BY clause defines the groups.
• The ORDER BY clause determines the order within each group.
• The ROW_NUMBER() function assigns a rank to each row.
• The outer query filters for rows with a RowNum of 1 to get the top row of each group.

Alternative Methods for "Get Top 1 Row of Each Group" in SQL

While the ROW_NUMBER() approach is commonly used, there are a few other methods that can be employed to achieve the same result:

Using GROUP BY and MIN() or MAX():

For scenarios where you only need a specific column from the top row (e.g., the earliest or latest date), you can directly use aggregation functions:

SELECT
    CustomerID,
    MIN(OrderDate) AS EarliestOrderDate
FROM
    Orders
GROUP BY
    CustomerID;

This will return the CustomerID and the earliest OrderDate for each customer.

Using CTE (Common Table Expression):

A CTE can be used to create a temporary result set, which can then be filtered to get the top row of each group:

WITH RankedOrders AS (
    SELECT
        OrderID,
        CustomerID,
        OrderDate,
        ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY OrderDate) AS RowNum
    FROM
        Orders
)
SELECT
    OrderID,
    CustomerID,
    OrderDate
FROM
    RankedOrders
WHERE
    RowNum = 1;

This approach is essentially the same as the original example but uses a CTE for better readability.

Using DISTINCT ON (PostgreSQL-specific):

If you're using PostgreSQL, the DISTINCT ON clause can be used to select the first row that is distinct based on a specific expression:

SELECT DISTINCT ON (CustomerID)
    OrderID,
    CustomerID,
    OrderDate
FROM
    Orders
ORDER BY
    CustomerID,
    OrderDate;

This will return the first row for each CustomerID based on the OrderDate ordering.

Choosing the Right Method:

The best method depends on your specific requirements:

• ROW_NUMBER(): Versatile and can be used to retrieve multiple columns from the top row.
• GROUP BY and MIN()/MAX(): Efficient for simple scenarios where only a specific column is needed.
• CTE: Improves readability and can be used in more complex queries.
• DISTINCT ON (PostgreSQL): A concise alternative for PostgreSQL-specific use cases.