Sql Query - Concatenating Results into One String

sql

string-aggregation

performance-optimization

sql-best-practices

byAlex Kataev·Jan 22, 2025

Concatenate rows into a single string using STRING_AGG in SQL Server 2017 and later:

SELECT STRING_AGG(column, ', ') AS combined
FROM table;

For older versions of SQL, use a blend of FOR XML PATH and STUFF:

SELECT STUFF((
    SELECT ', ' + column
    FROM table
    FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)'), 1, 2, '') AS combined; -- Don't talk STUFF about me!

Change column and table to your actual column name and table name.

Navigating special characters and ordering

When table values have special characters or need to be ordered, you might hit obstacles:

To decode XML entities back into usual characters, use .value. For instance, < transmutes back to <.
Use the ORDER BY clause in your subquery to sort the values ensuring a consistent order.
For special characters, consider manually encoding. You may construct a user-defined function or reference well-known encoding solutions like @KM's.

Managing string lengths and performance

When concatenating data, the resultant string can be sizable. Use VARCHAR(MAX) or NVARCHAR(MAX) to avoid size restrictions.

Now let's discuss performance:

FOR XML PATH & STUFF may have a performance penalty compared to functions designed specifically for aggregation like STRING_AGG.
In SQL, set-based solutions are typically faster than loops or cursors.
Use STRING_AGG in SQL Server 2017 and later for simplicity and better performance.

Dealing with null values and numeric concatenation

Nulls can be a hiccup in concatenation, but there are smooth fixes:

Using COALESCE or ISNULL, replace NULL with an empty string or a placeholder.

Got numeric values to concatenate? Perform a casting operation:

Using CAST or CONVERT, turn numeric values into string, ensuring your result isn't skewered due to incompatible data types. You can't mix apples with oranges, can you?

Peculiarities with large datasets

Working with sizable datasets? Here are some tips:

Chunking: Break queries into parts to flee the dreaded resource overconsumption.
Harness the power of temp tables or table variables to store intermediary results when dealing with complex string operations.

Troubleshooting concatenation

Column existence: Always make sure all to-be-concatenated columns exist in your table structure.
Optimization: Continuously test your solutions with different datasets and use execution plans to fine-tune performance.

Advanced issues and solutions

XML escape inconveniences: FOR XML PATH likes to escape XML characters (<, >, &).
Dynamic concatenation: Dynamic SQL and system tables can bend the concatenation rules to your will.