【程序员笔试】+数据库+sql

http://zh.sqlzoo.net/wiki/SELECT_basics

练习mysql的网站

SQL Correlated Subqueries
https://www.geeksforgeeks.org/sql-correlated-subqueries/
[转]sql子查询 嵌套SELECT语句
https://www.cnblogs.com/wangshenhe/archive/2012/11/28/2792093.html

1. 基本
2. select查询结果作为范围
3. 子查询
4. sum等函数
5. 分组
6. 联接

---- 1

SELECT population FROM world WHERE name = 'France'
选出法国的人口

• where 过滤条件
  • 限定范围：< > = <> betwee
  • 字符串：like "%_string%"
    + 相关函数：concat(name,"name")拼接，replace

• select 显示视图
• from 指定查询的表

---

----- 2

• 查询比Russia大的城市
  SELECT name FROM world WHERE population > (SELECT population FROM world WHERE name='Russia')

• 查找最大的城市
  SELECT name FROM world WHERE population >= ALL(SELECT population FROM world WHERE population>0)

• 获得所有欧洲的gdp，然后找比这些gdp都高的城市
  SELECT name FROM world WHERE gdp >= ALL (SELECT gdp FROM world WHERE gdp>0 and continent like 'Europe') and continent not like 'Europe'

---

---- 3

• 对于每一个行，找到该行相同地区的所有区域面积，如果区域面积是这里面最大的，则保留该行。然后显示该行数据的地区，名字和区域。
  SELECT continent, name, area FROM world x WHERE area >= ALL (SELECT area FROM world y WHERE y.continent=x.continent AND area>0)

• 选择大于等于该州平均大小十倍的国家
  SELECT continent, name, area FROM world x WHERE area >= (SELECT AVG(area)10 FROM world y WHERE y.continent=x.continent
  AND area>0) order by continent*

• 列出人口<=25000000的地区
  select DISTINCT continent from world x where 25000000 >= All(select population from world y where y.continent=x.continent)

• 选出是其余城市三倍的城市
  select name, continent from world x where population > ALL(select population*3 from world y where y.continent=x.continent and y.name not like x.name)

---

---- 4
Using SUM, Count, MAX, DISTINCT and ORDER BY.

SUM: 求和
Count：计数
Max：最大
Distinct：去重
order by 排序

---

---- 5

• 按分组进行计数
  SELECT continent, COUNT(name)
  FROM world
  GROUP BY continent

• 计算总人数在500000000的地区
  SELECT continent, SUM(population)
  FROM world
  GROUP BY continent
  HAVING SUM(population)>500000000

---

---6

select name from casting join actor on(actor.id=casting.actorid)
where movieid=11768
作用于内键和外键相同的时候。

join 分类：

• outer
  • left：左表全有
  • right：右表全有
  • full
• inner：指定相等
• natural：默认相等
• cross：笛卡尔

---