針對(duì)Select Zoo題庫平臺(tái)，這次做的是select專題的練習(xí)SELECT within SELECT Tutorial和測試Nested SELECT Quiz。

一、SELECT within SELECT Tutorial：

SELECT within SELECT Tutorial?sqlzoo.net

題目描述：

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子查詢部分所涉及的練習(xí)題，題目代碼均能正常運(yùn)行，獲得預(yù)期成功的笑臉結(jié)果。后面是對(duì)應(yīng)的測試題代碼，也是能正常運(yùn)行的代碼，可供大家參考。

二、Nested SELECT Quiz：

Nested SELECT Quiz?sqlzoo.net

上面是練習(xí)題所涉及的表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:

子查詢相關(guān)知識(shí)點(diǎn)：

子查詢就是在一個(gè)select查詢語句中嵌套select查詢語句。

一般可以在from子句中直接下定義視圖的sql查詢語句，外面會(huì)用一個(gè)括號(hào)括起來。這是普通的多行子查詢，一般返回多行，也可以返回單行。

也可以結(jié)合使用in、any、all在where子句中直接書寫子查詢的sql查詢語句，這種子查詢是標(biāo)量子查詢。這種子查詢是單一數(shù)值，可以結(jié)合比較運(yùn)算符一起使用。

關(guān)聯(lián)子查詢時(shí)要使用兩個(gè)表名，第二個(gè)表名的ID數(shù)據(jù)要和第一個(gè)表名的ID數(shù)據(jù)進(jìn)行匹配。使用關(guān)聯(lián)條件進(jìn)行在每個(gè)組內(nèi)比較。

sql里還有5個(gè)重要的聚合函數(shù)：

count(列名);求某列的行數(shù)，如count(*)

sum(列名);對(duì)某列數(shù)據(jù)求和。它只能對(duì)數(shù)據(jù)類型的列計(jì)算。

avg(列名);求某列數(shù)據(jù)的平均值。它只能對(duì)數(shù)值類型的列計(jì)算。

max(列名);求某列數(shù)據(jù)的最大值

min(列名);求某列數(shù)據(jù)的最小值

總結(jié)

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