计算字段

1. 拼接字段
   select Concat(vend_name, '(', vend_contry, ')') as vend_title from vendors order by vend_name;
   结果： ACME （USA）

2. 删除右侧多余数据整理数据Ltrim(), Trim()
   select Concat( Rtrim(vend_name), '(', Rtrim(vend_contry), ')' ) from vendors order by vend_name;

3. select prod_id, quantity, item_price from orderitems where order_num = 20005;

4. select prod_id, quantity, item_price * quantity as expanded_price from orderitems where order_num = 20005;

select 3*2 6
select Trim('abc') abc
select Now() 利用Now函数返回当前日期和时间。