How to Join to First Row in SQL Server

Joining the first row in SQL Server can be a common requirement in various scenarios, such as when we need to retrieve specific data associated with the first occurrence of a particular group or when you want to fetch additional details related to the first record in a result set.

In this article, We will learn about How to Join to first row in SQL Server by understanding various approaches along with the examples and so on.

How to Join First Row in SQL Server?

When working with SQL Server, we may encounter situations where we need to join to the first row of a related table. This could be to retrieve the earliest record for each group or to extract initial data for further analysis. To solve this problem, we'll explore three different approaches are as follows:

1. Using Subqueries
   
   
2. Using ROW_NUMBER()
   
   
3. Using CROSS APPLY

Let's set up an environment

To understand How to Join to first row in SQL Server we need a table on which we will perform various operations and queries. Here we will consider a table called EmployeeReviews which contains EmployeeID, ReviewDate, Rating, and department as Columns.

-- Create EmployeeReviews table
CREATE TABLE EmployeeReviews (
    EmployeeID INT,
    ReviewDate DATE,
    Rating INT,
    PRIMARY KEY (EmployeeID, ReviewDate)
);

-- Insert sample data into the EmployeeReviews table
INSERT INTO EmployeeReviews (EmployeeID, ReviewDate, Rating) VALUES
(1, '2024-01-01', 4),
(1, '2024-02-01', 5),
(2, '2024-01-15', 3),
(3, '2024-02-01', 4),
(3, '2024-03-01', 5),
(4, '2024-01-01', 5),
(4, '2024-02-01', 4);

Output:

1. Using Subqueries

This method uses a subquery to select the first row from the EmployeeReviews table for each employee. The subquery is joined to the Employee table using a LEFT JOIN to ensure that all employees are included in the result, even if they don't have a corresponding row in the EmployeeReviews table..

SELECT
    e.EmployeeID,
    e.Name,
    e.Salary,
    firstSalaryReview.ReviewDate,
    firstSalaryReview.Rating
FROM
    Employee e
    LEFT JOIN (
        SELECT TOP 1
            r.EmployeeID,
            r.ReviewDate,
            r.Rating
        FROM
            EmployeeReviews r
        ORDER BY
            r.ReviewDate
    ) firstSalaryReview ON e.EmployeeID = firstSalaryReview.EmployeeID;

Output:

Explanation:

• The subquery selects the first row from the EmployeeReviews table without any specific ordering. This can lead to unpredictable results because the database engine may return different rows each time the query is executed.
  
• The LEFT JOIN ensures that all rows from the Employee table are included in the result, even if there is no matching row in the subquery. However, since the subquery doesn't guarantee a specific row, the result may not always be meaningful.

2. Using ROW_NUMBER()

This method uses the ROW_NUMBER() window function to assign a unique row number to each row in the EmployeeReviews table, partitioned by EmployeeID and ordered by ReviewDate.

The main query then filters the results to include only rows where the row number is 1, effectively selecting the first row for each employee.

WITH RankedReviews AS (
    SELECT
        r.EmployeeID,
        r.ReviewDate,
        r.Rating,
        ROW_NUMBER() OVER (PARTITION BY r.EmployeeID ORDER BY r.ReviewDate) AS RowNum
    FROM
        EmployeeReviews r
)
SELECT
    e.EmployeeID,
    e.Name,
    e.Salary,
    rr.ReviewDate,
    rr.Rating
FROM
    Employee e
    LEFT JOIN RankedReviews rr ON e.EmployeeID = rr.EmployeeID AND rr.RowNum = 1;

Output:

Explanation:

• The ROW_NUMBER() function assigns a unique row number to each row in the EmployeeReviews table, partitioned by EmployeeID and ordered by ReviewDate. This ensures that the first row for each employee is assigned a row number of 1.
  
• The main query then left joins the Employee table with the RankedReviews common table expression (CTE) on EmployeeID and RowNum = 1. This ensures that only the first row for each employee is included in the result.

3. Using CROSS APPLY

This method uses the CROSS APPLY operator to apply a table-valued function (in this case, a subquery) to each row from the Employee table.

The subquery selects the first row from the EmployeeReviews table for each employee, and the CROSS APPLY ensures that only the first row is returned for each employee in the result set.

Consider the following example.

SELECT
    e.EmployeeID,
    e.Name,
    e.Salary,
    firstSalaryReview.ReviewDate,
    firstSalaryReview.Rating
FROM
    Employee e
    CROSS APPLY (
        SELECT TOP 1
            r.ReviewDate,
            r.Rating
        FROM
            EmployeeReviews r
        WHERE
            r.EmployeeID = e.EmployeeID
        ORDER BY
            r.ReviewDate
    ) firstSalaryReview;

Output:

Explanation:

• The CROSS APPLY operator is used to apply a table-valued function (in this case, a subquery) to each row from the Employee table. This allows us to join each employee with their first review from the EmployeeReviews table.
  
• The subquery selects the first row from the EmployeeReviews table for each employee, ordered by ReviewDate. The WHERE clause ensures that only rows for the current employee are considered.

CONCLUSION

Partitioning into first row of SQL Server can be implemented by different methods, including subqueries with ROW_NUMBER (), cross apply with TOP or Common Table Expressions (CTE). Different procedures will be good for various purposes and might be more appropriate depending on the particularities of your choosing and the difficulty of your information. As you learn these methods, you can appropriately apply them to your queries in order to easily get the necessary data from your SQL Server database.