MySQL联盟情况

Question

我正在尝试学习数据库（特别是MySQL），我正在考虑购买的教科书中的问题（没有提供解决方案手册）。我已经实现了问题的A部分，但我很困惑我将如何做B部分。除了使用UNION之外，B部分需要获得与部分A相同的结果。我想知道有人能解释吗？

由于提前

部分在我看来，答案是：

SELECT Country, COUNT(City) from country LEFT OUTER JOIN city 
ON city.CountryId = country.CountryId group by Country; 

B部分：写A部分查询作为UNION？

Answer 1

select c1.countrynm,count(c2.citynm) 
from country c1 
join city c2 
on c2.countryid=c1.countryid 
group by c1.countrynm 
union select countrynm,0 from country where 7=3; 

mysql> select c1.countrynm,count(c2.citynm) 
-> from country c1 
-> join city c2 
-> on c2.countryid=c1.countryid 
-> group by c1.countrynm 
-> union select countrynm,0 from country where 7=3; 
+-----------+------------------+ 
| countrynm | count(c2.citynm) | 
+-----------+------------------+ 
| France |    2 | 
| Germany |    1 | 
+-----------+------------------+ 

Answer 2

试试这个

select country from country group by country 
union 
select count(city) from city group by city 

，而使用“联盟”，你将不会导致单独column.You将获得导致同一列