average-population-of-each-continent

题目如下：

这道题主要考联合查询。

Given the CITY and COUNTRY tables, query the names of all the continents (COUNTRY.Continent) and their respective average city populations (CITY.Population) rounded down to the nearest integer.

Note:CITY.CountryCode and COUNTRY.Code are matching key columns.

Input Format

答案如下：

SELECT SON,PARENT ,c.p FROM (SELECT A.N as son ,B.N as parent FROM BST A RIGHT JOIN BST B on A.P=B.N)
tmp  join BST C ON tmp.parent =c.n