MySQL计算字段
计算字段 数据库中存放的表是按列存放，但是有时客户机想获得的信息是若干列之间的组合，或者求和的。这个组合或者求和的动作可以放在客户机应用程序来做，但是在数据库中实现更为高效。这个新计算出来的结果就称为字段。 例如我们需要vend的name后面用括号
计算字段
数据库中存放的表是按列存放，但是有时客户机想获得的信息是若干列之间的组合，或者求和的值。这个组合或者求和的动作可以放在客户机应用程序来做，但是在数据库中实现更为高效。这个新计算出来的结果就称为字段。
例如我们需要vend的name后面用括号说明所属国家。则可以实现如下，使用MySQL的Concat拼接函数实现拼接：
但是这仅仅是得到了一个值，没有相应的名称，客户机上的应用程序没法引用这个结果。因此使用别名来命令这个新获得的字段以供客户机应用程序调用，别名使用as 后接别名名称：
除了合并多个列，有时候还需要进行计算，如计算出某订单中同类物品的总价，先看一下订单：
我们要做得就是计算同一类物品的总价，新生成一列：
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