Optimal way to concatenate/aggregate strings

sql

string-aggregation

performance

best-practices

byAnton Shumikhin·Aug 14, 2024

In MySQL, use the GROUP_CONCAT function for string aggregation:

SELECT GROUP_CONCAT(column SEPARATOR ',') FROM table GROUP BY group_column;

For SQL Server, use the STRING_AGG function:

SELECT STRING_AGG(column, ',') OVER (PARTITION BY group_column) FROM table;

These built-in functions merge row values into a single, delimited string, based on a specified column.

Techniques for earlier SQL Server versions

For SQL Server versions prior to 2017 where STRING_AGG function is not available, we can use recursive CTEs or FOR XML PATH('').

Recursive CTE Example (our little chainsaw; handle with care☠️):

WITH RecursiveConcat AS (
    SELECT 
        Id, 
        CAST(column AS VARCHAR(MAX)) AS concatenated_value, 
        1 AS Level
    FROM table
    WHERE parent_id IS NULL
    UNION ALL
    SELECT 
        c.Id, 
        r.concatenated_value + ',' + CAST(c.column AS VARCHAR(MAX)), -- SQL alchemy in progress ⚗️
        Level + 1
    FROM table c
    INNER JOIN RecursiveConcat r ON c.parent_id = r.Id
)
SELECT concatenated_value 
FROM RecursiveConcat
WHERE Level = (SELECT MAX(Level) FROM RecursiveConcat WHERE Id = RecursiveConcat.Id); -- Respect the MAX

FOR XML PATH('') Example (because XML is still trendy 🔮):

SELECT 
    group_column, 
    STUFF((
        SELECT ',' + column
        FROM table 
        WHERE group_column = t.group_column 
        FOR XML PATH(''), TYPE
    ).value('.', 'NVARCHAR(MAX)'), 1, 1, '') AS concatenated_value -- A STUFFed query, not a STUFFed turkey 🦃
FROM table t
GROUP BY group_column;

Considerations and common pitfalls

Navigating the string aggregation territory calls for mindful balancing between performance and accuracy:

Performance: Recursive CTEs can have a performance hitch on large datasets. This is where STRING_AGG and FOR XML PATH('') become lifesavers.
Ordering: Without an explicit ORDER BY inside the aggregation function, you might land on the isle of inconsistent results. With FOR XML PATH(''), it's particularly treacherous.
Grouping: GROUP BY before employing STRING_AGG to avoid cloning. (We are not Dolly the Sheep 🐑)
SQL Versions: Methods should match SQL versions - STRING_AGG is the hot-guy-next-door for SQL Server 2017 and later, while GROUP_CONCAT and MySQL have an age-old flirtation.
Problem's Specificity: Recursive CTEs offer flexibility for complex scenarios but need adjustments- not all socks fit all feet 👣

Advanced usage and best practices

For specialized cases and their solutions, consider these high-performance techniques:

Window Functions: Make OVER and PARTITION BY your new best friends with STRING_AGG to choose and organize rows without a separate GROUP BY clause.
XML PATH Tricks: Extirpate all special characters with .value('.', 'NVARCHAR(MAX)') when employing FOR XML PATH('').
CLR UDFs: Use the magic of CLR user-defined functions for custom aggregation logic - does require some SQL wizardry though 🧙‍♂️
Performance Testing: Leave no stone unturned when playing with SQL Fiddle and similar utilities for real-world performance comparison. Also, benchmarks using DATEDIFF can become your guide to the SQL universe.
Community wisdom: Community input and books like Itzik Ben-Gan’s T-SQL Querying can be a treasure trove of performance insights, especially for FOR XML PATH.

Stay up-to-date

Always be prepared for sudden turn of events as the SQL landscape keeps evolving.

Community Updates: Get your updates from community discussions on hot topics like Azure support for new aggregation techniques and about GROUP_CONCAT.
Benchmark Operations: Keep executing benchmarks on a regular basis, we need to embrace the changes in SQL Server updates and in hardware optimizations with open arms.
New SQL Versions: Be on the lookout for new functions and improvements in the latest SQL versions. SQL vendors keep throwing surprises!