How to create fast database queries

Archive for June 21st, 2010


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I'm finally back from my vacation. Tunisia's great: dates, Carthage, sea and stuff.

Now, to the questions.

Mahen asks:

Create a table called Group:

id prodname
1 X
1 Y
1 Z
2 A
2 B
2 C

The resultset should look like this:

id prodname
1 X,Y,Z
2 A,B,C

Can you please help me to solve the above problem using a recursive CTE?

This is out good old friend, GROUP_CONCAT. It's an aggregate function that returns all strings within a group, concatenated. It's somewhat different from the other aggregate functions, because, first, dealing with the concatenated string can be quite a tedious task for the groups with lots of records (large strings tend to overflow), and, second, the result depends on the order of the arguments (which is normally not the case for the aggregate functions). It's not a part of a standard SQL and as for now is implemented only by MySQL with some extra vendor-specific keywords (like ORDER BY within the argument list).

This functionality, however, is often asked for and I have written some articles about implementing this in PostgreSQL and Oracle.

Now, let's see how to do it in SQL Server.

Usually, SQL Server's FOR XML clause is exploited to concatenate the strings. To do this, we obtain a list of group identifiers and for each group, retrieve all it's product names with a subquery appended with FOR XML PATH(''). This makes a single XML column out of the recordset:
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Written by Quassnoi

June 21st, 2010 at 11:00 pm

Posted in SQL Server