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How to simulate the GROUP_CONCAT function

MySQL 4.1 adds a number of new aggregate functions, among them GROUP_CONCAT. Earlier versions require you to build and compile the function as a C++ extension, which I’m told is easy. However, that is usually not possible in a shared hosting environment.

It is possible to use user variables to write some queries without GROUP_CONCAT. The key is to understand how MySQL implements selects. As the following example shows, it really loops through each row one at a time, evaluating the expression for each. This example uses the same fruits table as in my article on simulating the ROW_NUMBER function:

set @result = '';
select @result := concat(@result, variety, ' ') 
from fruits where type = 'apple';
+------------------------------------------+
| @result := concat(@result, variety, ' ') |
+------------------------------------------+
| fuji                                     |
| fuji gala                                |
| fuji gala limbertwig                     |
+------------------------------------------+
select @result;
+-----------------------+
| @result               |
+-----------------------+
| fuji gala limbertwig  |
+-----------------------+

It is possible to get similar functionality from Microsoft SQL Server 2000 with a local variable.

declare @result varchar(8000);
set @result = '';
select @result = @result + name + ' '
    from master.dbo.systypes;
select rtrim(@result);

The result is the string “image text uniqueidentifier tinyint smallint int smalldatetime real money datetime float sql_variant ntext bit decimal numeric smallmoney bigint varbinary varchar binary char timestamp nvarchar nchar sysname.” It is necessary to initialize the string to “ before the select, because NULL concatenates to NULL.

Posted on Wed, Sep 28, 2005. Approximately 300 Words.

Databases