MySQL UPDATE with OUT parameter

mysql.cs

// Make sure to set AllowUserVariables=true in connection string

private string SqlLogUpdate => _sqlLogUpdate ?? (_sqlLogUpdate = $@"
Update {_tableName} 
Set DuplicateCount = DuplicateCount + @DuplicateCount,
   GUID = (@newGUID := GUID) -- set the variable here
Where ErrorHash = @ErrorHash; -- AND ...
Select @newGUID; -- still have to select it to get it back";

protected override bool LogError(Error error)
{
    using (var c = GetConnection())
    {
        if (Settings.RollupPeriod.HasValue && error.ErrorHash.HasValue)
        {
            var queryParams = GetUpdateParams(error);
            if (Guid.TryParse(c.QueryFirst<string>(SqlLogUpdate, queryParams), out var guid)
            {
              // if we found an exception that's a duplicate, jump out
              error.GUID = guid;
              return true;
            }
        }

        // ...
    }
}