答案中的select amount from aaa m中的m是什么意思，为什么要加个m？

Question

面试题：怎么把这样一个表儿aaa
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

m是数据库写法的别名，这样在写表的字段的时候可以用别名代替表的全称，直接用别名.字段即可

Answer 2

表的别名 ， 在多表查询的时候用的很多