求知下面的selectamountfromaaamwheremonth=1andm.year=aaa.year中的aaa和m是什么?yearmonthamount199111.1199121.2199131.3199141.4199212.1199222.2199232.3199242.4查成

Question

求知下面的 select amount from aaa m where month=1 and m.year=aaa.year 中的aaa和m是什么?
year month amount
1991 1 1.1
1991 2 1.2
1991 3 1.3
1991 4 1.4
1992 1 2.1
1992 2 2.2
1992 3 2.3
1992 4 2.4
查成这样一个结果
year m1 m2 m3 m4
1991 1.1 1.2 1.3 1.4
1992 2.1 2.2 2.3 2.4
答案一、
select year,
(select amount from aaa m where month=1 and m.year=aaa.year) as m1,
(select amount from aaa m where month=2 and m.year=aaa.year) as m2,
(select amount from aaa m where month=3 and m.year=aaa.year) as m3,
(select amount from aaa m where month=4 and m.year=aaa.year) as m4
from aaa group by year

Answer 1

首先说明,如果此语句是再SQL Server中使用,必定报错.Oracle中没有这样试过,不做定论.但从语法上讲,From后面的aaa是表名,直接跟一个m只能是对表名称的转换,其类似于查询字段中常常用到的字段名称转换“Select A as B”,只不过表名称的转换时as可以省略.但是如果用了m转换表名称,则where条件里直接使用aaa.year是肯定会报错“找不到aaa”,除非查询中拿m表再和原来的aaa做一次联合查询.