针对Select Zoo题库平台，这次做的是select专题的练习SELECT within SELECT Tutorial和测试Nested SELECT Quiz。

一、SELECT within SELECT Tutorial：

SELECT within SELECT Tutorial​sqlzoo.net

题目描述：

1、Bigger than Russia:

2、Richer than UK:

3、Neighbours of Argentina and Australia:

4、Between Canada and Poland:

5、Percentages of Germany:

6、Bigger than every country in Europe:

7、Largest in each continent:

8、First country of each continent:

9、Difficult Questions1:

10、Difficult Questions2:

上面是select子查询部分所涉及的练习题，题目代码均能正常运行，获得预期成功的笑脸结果。后面是对应的测试题代码，也是能正常运行的代码，可供大家参考。

二、Nested SELECT Quiz：

Nested SELECT Quiz​sqlzoo.net

上面是练习题所涉及的表bbc

1. Select the code that shows the name, region and population of the smallest country in each region:

2.Select the code that shows the countries belonging to regions with all populations over 50000:

3.Select the code that shows the countries with a less than a third of the population of the countries around it:

4.Select the result that would be obtained from the following code:

5.Select the code that would show the countries with a greater GDP than any country in Africa (some countries may have NULL gdp values):

6.Select the code that shows the countries with population smaller than Russia but bigger than Denmark:

7.Select the result that would be obtained from the following code:

子查询相关知识点：

子查询就是在一个select查询语句中嵌套select查询语句。

一般可以在from子句中直接下定义视图的sql查询语句，外面会用一个括号括起来。这是普通的多行子查询，一般返回多行，也可以返回单行。

也可以结合使用in、any、all在where子句中直接书写子查询的sql查询语句，这种子查询是标量子查询。这种子查询是单一数值，可以结合比较运算符一起使用。

关联子查询时要使用两个表名，第二个表名的ID数据要和第一个表名的ID数据进行匹配。使用关联条件进行在每个组内比较。

sql里还有5个重要的聚合函数：

count(列名);求某列的行数，如count(*)

sum(列名);对某列数据求和。它只能对数据类型的列计算。

avg(列名);求某列数据的平均值。它只能对数值类型的列计算。

max(列名);求某列数据的最大值

min(列名);求某列数据的最小值