How to use string_agg in SQL Server

Hello Dev! Have you ever needed to concatenate strings in SQL Server? If so, then you’re in the right place. In this article, we’ll show you how to use the string_agg function in SQL Server to concatenate strings from multiple rows into a single string. We’ll also go over some of the most common use cases and answer some frequently asked questions. So, let’s get started!

What is string_agg?

String_agg is a powerful function in SQL Server that lets you concatenate strings from multiple rows into a single string. It was introduced in SQL Server 2017 and is part of the Transact-SQL language. String_agg is often used to combine the results of a query into a single string, which can be useful for reporting or data analysis purposes.

To use string_agg, you’ll need to specify the delimiter that you want to use to separate the concatenated strings. String_agg will then concatenate the strings from each row together, separated by the delimiter. Let’s take a closer look at how this works.

Using string_agg

The syntax for string_agg is fairly straightforward. Here’s an example:

Column 1	Column 2
Row 1, Column 1	Row 1, Column 2
Row 2, Column 1	Row 2, Column 2
Row 3, Column 1	Row 3, Column 2

Let’s say you want to concatenate the values in Column 1 into a single string, separated by commas. Here’s how you would do it:

SELECT string_agg(Column1, ',') AS ConcatenatedValuesFROM MyTable

The result would be:

ConcatenatedValues
Row 1, Column 1, Row 2, Column 1, Row 3, Column 1

Common use cases

Reporting

String_agg can be very useful for creating reports that summarize data from multiple rows into a single line. For example, let’s say you have a table that contains customer orders, and you want to create a report that shows all of the products that each customer has ordered, separated by commas. Here’s how you could do it:

SELECT CustomerID, string_agg(ProductName, ',') AS OrderedProductsFROM OrdersINNER JOIN Products ON Orders.ProductID = Products.ProductIDGROUP BY CustomerID

The result would be a table that shows each customer’s ID and all of the products they’ve ordered, separated by commas.

Data analysis

String_agg can also be used for data analysis purposes. For example, let’s say you have a table that contains employee data, and you want to create a report that shows the departments that each employee has worked in, separated by commas. Here’s how you could do it:

SELECT EmployeeID, string_agg(DepartmentName, ',') AS DepartmentsWorkedInFROM EmployeeDepartmentsINNER JOIN Departments ON EmployeeDepartments.DepartmentID = Departments.DepartmentIDGROUP BY EmployeeID

The result would be a table that shows each employee’s ID and all of the departments they’ve worked in, separated by commas.

FAQ

What is the maximum length of the concatenated string?

The maximum length of the concatenated string is 8,000 characters. If the concatenated string exceeds this length, an error will be thrown.

Can I use string_agg with NULL values?

Yes, you can use string_agg with NULL values. If a NULL value is encountered, it will be treated as an empty string.

Can I specify a different delimiter?

Yes, you can specify a different delimiter by replacing the comma in the string_agg function with the delimiter of your choice.

Can I use string_agg in a subquery?

Yes, string_agg can be used in a subquery. However, you’ll need to use a derived table to do so. Here’s an example:

SELECT CustomerID, (SELECT string_agg(ProductName, ',')FROM OrdersINNER JOIN Products ON Orders.ProductID = Products.ProductIDWHERE Orders.CustomerID = Customers.CustomerID) AS OrderedProductsFROM Customers

The result would be the same as the previous example, but using a subquery instead of a join.

Are there any performance considerations when using string_agg?

Yes, there are some performance considerations when using string_agg. In general, you should avoid using string_agg for very large datasets, as it can be slow and resource-intensive. Additionally, you should make sure that your query is optimized, and that you’re only selecting the data that you actually need.

That’s it for our introduction to string_agg in SQL Server. We hope you found this article helpful, and that you’re now able to use string_agg to concatenate strings from multiple rows into a single string. If you have any questions or comments, please feel free to leave them below.