code: func (statement *Statement) Cols(columns ...string) *Statement {
  	cols := col2NewCols(columns...)
  	for _, nc := range cols {
  		statement.columnMap.add(nc)
  	}
  
  	newColumns := statement.colmap2NewColsWithQuote()
  
  	statement.ColumnStr = strings.Join(newColumns, ", ")
  	statement.ColumnStr = strings.Replace(statement.ColumnStr, statement.Engine.quote("*"), "*", -1)
  	return statement
  }
docs: // Cols generate "col1, col2" statement