Using Common Table Expressions (CTEs) in SQL Server

Creating a New Column and Feeding it with Specific Data

In this article, we’ll explore how to modify an existing query using Common Table Expressions (CTEs) to create a new column in a table and feed it with specific data. We’ll delve into the details of CTEs, their benefits, and provide step-by-step instructions on how to achieve this task.

Understanding Common Table Expressions (CTEs)

A Common Table Expression (CTE) is a temporary result set that is defined within the execution of a single SQL statement. It’s similar to a view, but it’s not stored in the database and can be used only once in a query. CTEs are useful for simplifying complex queries, reducing code duplication, and improving readability.

Creating a New Column using CTE

Let’s start by understanding how we can modify the existing query to create a new column called Tag that will contain either 'dup' or an empty string ('') based on the row number.

The provided T-SQL query extracts all duplicate records from the t1 table based on the Email column. The output shows rows with duplicates marked by their corresponding ID, Name, Email, and a RowNum value greater than 1.

Our goal is to modify this query to create a new column called Tag, where:

• Rows with RowNum = 1 will have an empty string ('') in the Tag column.
• Other rows (RowNum > 1) will have 'dup' in the Tag column.

Using Case Expression

To achieve this, we can use a CASE expression within our query. Here’s how you can do it:

SELECT *, 
       CASE WHEN RowNum = 1 THEN '' ELSE 'dup' END AS Tag
FROM [CTE1]

In this query:

• We select all columns (*) from the CTE CTE1.
• The CASE expression checks whether the row number is equal to 1. If true, it assigns an empty string; otherwise, it assigns 'dup' to the Tag column.
• Finally, we return the modified table with the new Tag column.

Updating the Table

If you want to update the existing table (t1) with the same results, you can run a separate UPDATE statement on the CTE:

update CTE1 set 
  Tag=CASE WHEN rownum=1 THEN '' ELSE 'dup' END

This query updates the Tag column in the CTE with either an empty string or 'dup' based on the row number.

Changing Ordering Criteria

Currently, the ordering criteria are based solely on the Email column. However, it’s recommended to change this to be based on the ID column to consistently keep the highest/lowest ID of each group:

SELECT *, 
       CASE WHEN RowNum = 1 THEN '' ELSE 'dup' END AS Tag
FROM [CTE1] ORDER BY ID

By ordering the results by ID, we can ensure that the duplicates are correctly assigned to their corresponding IDs.

Using a Subquery

Alternatively, you can create a subquery within your main query:

SELECT *,
       CASE 
           WHEN RowNum = 1 THEN ''
           ELSE 'dup'
       END AS Tag
FROM (
    SELECT *, 
           ROW_NUMBER() OVER (PARTITION BY Email ORDER BY ID) as RowNum    
    FROM [t1]
) as CTE1

This approach is useful when you need to create the Tag column based on a different condition or want more control over your queries.

Best Practices

When working with Common Table Expressions:

• Keep them simple and focused on performing a specific task.
• Avoid using complex logic within CTEs; instead, use other query elements like CASE, JOIN, or subqueries to simplify the code.
• Use meaningful names for your CTEs to make your queries more readable.
• Be mindful of performance implications when using CTEs, as they can impact query execution times.

By following these guidelines and best practices, you’ll be able to effectively use Common Table Expressions in SQL Server to create new columns with specific data.

Last modified on 2024-05-25