There is a table World

+-----------------+------------+------------+--------------+---------------+
| name            | continent  | area       | population   | gdp           |
+-----------------+------------+------------+--------------+---------------+
| Afghanistan     | Asia       | 652230     | 25500100     | 20343000      |
| Albania         | Europe     | 28748      | 2831741      | 12960000      |
| Algeria         | Africa     | 2381741    | 37100000     | 188681000     |
| Andorra         | Europe     | 468        | 78115        | 3712000       |
| Angola          | Africa     | 1246700    | 20609294     | 100990000     |
+-----------------+------------+------------+--------------+---------------+

A country is big if it has an area of bigger than 3 million square km or a population of more than 25 million.

Write a SQL solution to output big countries‘ name, population and area.

For example, according to the above table, we should output:

+--------------+-------------+--------------+
| name         | population  | area         |
+--------------+-------------+--------------+
| Afghanistan  | 25500100    | 652230       |
| Algeria      | 37100000    | 2381741      |
+--------------+-------------+--------------+

题意：找到表中人口大于2500 0000或者面积大于300 0000 平方公里的国家。
解题思路：
　　数据库的题解题思路就比较单一了，首先知道要什么之后设定查询条件，这里有两个那么就是where 条件一 or 条件二，观察结果，表头是name，population,
area, 那么select World.name World.population World.area，没要求按照什么顺序排序所以就可以忽略排序部分，所以连起来代码如下：

1 SELECT name,population,area
2 FROM World
3 WHERE World.population>25000000 
4     OR World.area>3000000;

　　当然也可以使用union方式，代码要稍微多一点，理论上运行会比where略快。union相当于使用两个索引同时进行查找，而or相当于从一个索引查完之后去另一个索引查。理论上是这样但是你实际运行的时候，oj对你的响应不一定那么快，所以两个可能不一定谁快谁慢

1 SELECT name, population, area
2 FROM World
3 WHERE area > 3000000 
4 UNION
5 SELECT name, population, area
6 FROM World
7 WHERE population > 25000000

　　简单的问题，可以用来对数据库查询进行初步的掌握。

LeetCode解题思路：595. Big Countries