SQL allows the use of expressions in the select list to project data, as in the following example, which returns a list of books that cost more than 100.00 with an additional sales_tax column containing a sales tax figure calculated at 6% of the price.
SQL允许在选择列表中使用表达式来投影数据，如下例所示，它返回成本超过100.00的书籍列表，另外一列sales_tax包含以price的6％计算的销售税数据。