LeetCode hot100-49-N

新建连接，自带world数据库 

explain SELECT * FROM `city`;
# 主键查询
explain SELECT * FROM `city` where ID=5;
# 索引查询
explain SELECT * FROM `city` where CountryCode='NLD';
# 主键范围查询
explain SELECT * FROM `city` where ID>5 and ID<20;
# 非索引范围查询
explain SELECT * FROM `city` where Population>193238 and Population<200000;