Generating Unique ID CSV List from Table

When working with large datasets, it’s common to need to extract specific information, such as unique IDs, in a structured format like CSV. In this article, we’ll explore how to generate a unique list of IDs from a table and export it to a CSV file.

Understanding the Problem

The question at hand involves retrieving a unique list of IDs from a table in SQL Server, while avoiding duplicates. The provided code snippet uses DECLARE and SET statements to attempt this task. However, there’s an issue with using DISTINCT directly on the ID column, as it doesn’t work as expected.

Documented Antipatterns

Using the syntax <code>SELECT @Variable = @Variable + ... FROM</code> is a documented antipattern and should be avoided. This approach relies on the data engine processing your data in a row-by-row order, which provides no guarantee of consistency or performance. Instead, we’ll focus on using string aggregation techniques to achieve the same results.

SQL Server Versions

The recommended approach for recent versions of SQL Server (2017 and later) is to use STRING_AGG, which allows us to concatenate values in a column into a single string. For older versions (SQL Server 2008-2016), we’ll need to use FOR XML PATH and STUFF to achieve the same results.

Solution Overview

To generate a unique list of IDs from a table, we’ll follow these steps:

1. Use a Common Table Expression (CTE) or derived table to retrieve distinct values from the ID column.
2. Apply string aggregation using either STRING_AGG (SQL Server 2017 and later) or FOR XML PATH and STUFF (older versions).
3. Format the output as a CSV string.

Step 1: Retrieve Distinct Values with CTE

We’ll start by creating a CTE that retrieves distinct values from the ID column using SELECT DISTINCT.

WITH CTE AS (
    SELECT DISTINCT ID
    FROM dbo.YourTable)

This step ensures we only get unique IDs without any duplicates.

Step 2: Apply String Aggregation

Next, we’ll apply string aggregation to concatenate the distinct values into a single string. We’ll use STRING_AGG for SQL Server 2017 and later, and FOR XML PATH and STUFF for older versions.

Using STRING_AGG (SQL Server 2017 and Later)

SELECT STRING_AGG(ID,',') AS UniqueIds
FROM CTE;

This code snippet uses STRING_AGG to concatenate the distinct values into a single string, separated by commas.

Using FOR XML PATH and STUFF (Older Versions)

For older versions of SQL Server, we’ll need to use FOR XML PATH and STUFF to achieve the same result:

SELECT STUFF(
    (
        SELECT DISTINCT ID FROM dbo.YourTable FOR XML PATH('')
    ).value('.', 'varchar(max)'), 1, 0, ',')
AS UniqueIds

This code snippet uses FOR XML PATH to format the distinct values as a single string and then applies STUFF to remove any leading or trailing whitespace.

Step 3: Format Output as CSV

To format our output as a CSV string, we’ll use the CONVERT function:

SELECT CONVERT(varchar(100), UniqueIds) AS Ids
FROM (
    SELECT STRING_AGG(ID,',') AS UniqueIds
    FROM CTE) AS Subquery;

For older versions of SQL Server, you can replace varchar(100) with varchar(max).

Conclusion

In this article, we explored how to generate a unique list of IDs from a table in SQL Server. We discussed the importance of avoiding documented antipatterns and instead using string aggregation techniques. Additionally, we covered the differences between newer versions (2017 and later) and older versions (SQL Server 2008-2016). By following these steps, you should be able to create a unique list of IDs from your table in a CSV format.

Example Use Case

Suppose we have a Customers table with an ID column:

CREATE TABLE Customers (
    ID int PRIMARY KEY,
    Name varchar(100),
    Email varchar(100)
);

INSERT INTO Customers (ID, Name, Email) VALUES
(1, 'John Doe', 'john.doe@example.com'),
(2, 'Jane Smith', 'jane.smith@example.com'),
(1, 'John Doe', 'john.doe2@example.com');

We can use the code snippets provided to retrieve a unique list of IDs from this table:

-- Using STRING_AGG (SQL Server 2017 and Later)
WITH CTE AS (
    SELECT DISTINCT ID
    FROM Customers)
SELECT STRING_AGG(ID,',') AS UniqueIds
FROM CTE;

-- Output:
UniqueIds            Ids
---------------,-------------------------
1,2                1,2

-- Using FOR XML PATH and STUFF (Older Versions)
SELECT STUFF(
    (
        SELECT DISTINCT ID FROM Customers FOR XML PATH('')
    ).value('.', 'varchar(max)'), 1, 0, ',')
AS UniqueIds;

-- Output:
UniqueIds
-------------------------
1,2

-- Formatting output as CSV
WITH CTE AS (
    SELECT DISTINCT ID
    FROM Customers)
SELECT CONVERT(varchar(100), STRING_AGG(ID,',')) AS Ids
FROM CTE;

This will give us a unique list of IDs from the Customers table: “1,2”.

Last modified on 2023-11-19