Converting a Column to Multiple Rows Using SQL Server

In this article, we’ll explore how to convert a column in a SQL Server table into multiple rows using a single query. We’ll cover the basics of SQL and provide an example to illustrate this concept.

Understanding SQL Tables

A SQL table is a collection of data organized into rows and columns. Each row represents a single record or entry, while each column represents a field or attribute of that record. In our case, we have a table called StudesntMarkList with several columns:

• StudentID
• StudentName
• Performance
• class
• Section
• Several subject-specific fields (e.g., subject1, subject2, etc.)

The Problem

The problem at hand is to transform this table so that each value in the long list of columns becomes a separate row. In other words, we want to convert the following data:

into the following:

The Solution

To achieve this transformation, we’ll use a technique called CROSS APPLY in combination with the VALUES function.

Understanding CROSS APPLY

CROSS APPLY is a SQL Server feature that allows you to join two tables together when one of them has only one column. In our case, we want to apply some logic to each row of the table and generate new rows as a result.

The Solution Query

Here’s the query that achieves the desired transformation:

SELECT StudentID,
       v.*
FROM StudesntMarkList 
CROSS APPLY (VALUES 
('StudentName',StudentName,''),
('Performance',Performance,''),
('class',class,''),
('subject1',subject1,subjectmark1),
('subject2',subject2,subjectmark2),
('subject3',subject3,subjectmark3),
('subject4',subject4,subjectmark4),
('subject5',subject5,subjectmark5),
('subject6',subject6,subjectmark6)

) v(att,attval,Val)

Let’s break this query down:

• CROSS APPLY is used to join the StudesntMarkList table with a derived table created by the VALUES function.
• The VALUES function returns multiple values as separate rows. In our case, we have nine values (one for each subject field).
• Each value returned by VALUES has three parts: an attribute name (att), a value name (attval), and a value itself (Val). We’re interested in the value part.
• The SELECT statement then selects all columns from the derived table (v) using the attribute names as aliases.

Explanation and Example

Here’s a step-by-step explanation of how this query works:

1. The CROSS APPLY operator applies the logic defined by the VALUES function to each row of the StudesntMarkList table.
2. For each row, it generates nine new rows using the values returned by VALUES.
3. Each new row represents a separate attribute-value pair (e.g., StudentName, StudentID, etc.).
4. The SELECT statement then selects all columns from these derived tables, using the attribute names as aliases.

Let’s take a closer look at how this works with our example data:

When we run the query, it generates the following nine new rows:

As you can see, each value from the original table has been converted into a separate row.

Conclusion

In this article, we’ve seen how to convert a column in a SQL Server table into multiple rows using the CROSS APPLY function and the VALUES clause. This technique is useful when you need to transform data in a flexible and efficient way.

Last modified on 2024-09-18