sql - 联合操作后如何获取记录数..？

Question

我有这样的查询:

select author1 
from books_group 
where category='cse' 
union 
select author2 
from books_group 
where category='cse' 
union 
select author3 
from books_group 
where category='cse'

上面的查询合并了来自三个选择命令的所有记录..

我的任务是计算执行上述sql命令后我们拥有的记录数...

我尝试了下面的查询，但它给出了一个错误..

"select count(*) from (select author1 from books_group where category='cse' union select author2 from books_group where category='cse' union select author3 from books_group where category='cse') "

那么union运算后如何获取记录数..？？？

Answer 1

你很接近，你需要为你的子选择指定一个别名:

select
    count(*)
from
    (
    select author1 from books_group where category='cse' union
    select author2 from books_group where category='cse' union
    select author3 from books_group where category='cse'
    ) a