子查询（7种类型）

where型子查询：
查出每个栏目最新的商品(以good_id为最大为最新商品)：
goods货物表，good_id表的主键,cat_id栏目的编号

select cat_id,good_id,good_name from goods where good_id in(selct max(good_id) from goods group by cat_id);

form型子查询：
查出每个栏目最新的商品(以good_id为最大为最新商品)：

select * from (select cat_id,good_id,good_name from goods order by cat_id asc, good_id desc) as tep group by cat_id;

from和where型综合练习：
查出挂科2门及以上同学的平均分：

思路讲解：

-- 1.先求出挂科两门以上及两门的同学
-- select name,count(*) as gk from stu where score<60 group by name having gk>=2;
-- 2.去除多余的一行
-- select name from (select name,count(*) as gk from stu where score<60 group by name having gk>=2)as tmp;
--3.最终结果
select name ,avg(score) as '平均分' from stu where name in(select name from (select name,count(*) as gk from stu where score<60 group by name having gk>=2)as tmp)
group by name;

查询结果：

其余5种：
department表：

employee表：

in子查询：查询年龄为20岁的员工部门

 select * from department where did in(SELECT did from employee where age=20);

exists子查询:查询是否存在年龄大于21岁的员工

select * from department where EXISTS (SELECT did from employee where age>21);

any子查询：查询满足条件的部门

select * from department where did> any (SELECT did from employee );

all子查询：查询满足条件的部门

select * from department where did> all(SELECT did from employee );

比较运算符子查询：查询赵四是哪个部门的

select * from department where did= all(SELECT did from employee where name='赵四'); 

总结：
  where型子查询：指把内部查询的结果作为外层查询的比较条件。
  from型子查询：把内层的查询结果当成临时表，供外层sql再次查询。
  in子查询：内层查询语句仅返回一个数据列，这个数据列的值将供外层查询语句进行比较。
  exists子查询：把外层的查询结果，拿到内层，看内层是否成立，简单来说后面的返回true,外层（也就是前面的语句）才会执行，否则不执行。
  any子查询：只要满足内层子查询中的任意一个比较条件，就返回一个结果作为外层查询条件。
  all子查询：内层子查询返回的结果需同时满足所有内层查询条件。
  比较运算符子查询：子查询中可以使用的比较运算符如 “>” “<” “= ” “!=”