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To create a single string from multiple rows in MySQL, you can use the GROUP_CONCAT function. This function allows you to concatenate values from multiple rows into a single string, separated by a specified delimiter. Here is an example of how you can use this function:

SELECT GROUP_CONCAT(column_name ORDER BY column_name ASC SEPARATOR ',')
FROM table_name
GROUP BY group_by_column;

In this example, the GROUP_CONCAT function will concatenate the values in the column_name column, ordered alphabetically, and separated by a comma. The results will be grouped by the group_by_column column.

To create a single string from multiple rows in T-SQL, you can use the FOR XML PATH clause in conjunction with the STUFF function. Here is an example of how you can use this approach:

SELECT STUFF(
    (SELECT ',' + column_name
     FROM table_name
     FOR XML PATH(''))
    , 1, 1, ''
)
FROM table_name
GROUP BY group_by_column;

In this example, the STUFF function is used to remove the leading comma from the concatenated string. The FOR XML PATH clause is used to concatenate the values in the column_name column, separated by a comma. The results will be grouped by the group_by_column column.

We can also use the STRING_AGG() function to create a single string from multiple rows in T-SQL. This function allows you to concatenate values from multiple rows into a single string, separated by a specified delimiter. Here is an example of how you can use this function:

SELECT STRING_AGG(column_name, ',')
FROM table_name
GROUP BY group_by_column;

In this example, the STRING_AGG function will concatenate the values in the column_name column, separated by a comma. The results will be grouped by the group_by_column column.

Note that the STRING_AGG() function is only available in T-SQL starting with SQL Server 2017. If you are using an older version of SQL Server, you will need to use the FOR XML PATH and STUFF approach described above.