PostgreSQL 9.6 两项任务：基于候选人表的数据库操作需求

数据表结构与初始化数据 #

以下是已创建并插入数据的candidates表的SQL语句：

create table candidates ( 
    id int primary key, 
    position varchar not null, 
    salary int not null 
); 

insert into candidates values (1, 'junior', 10500); 
insert into candidates values (2, 'senior', 15000); 
insert into candidates values (3, 'senior', 35000); 
insert into candidates values (4, 'junior', 8000); 
insert into candidates values (5, 'senior', 30000); 
insert into candidates values (6, 'senior', 25000); 
insert into candidates values (7, 'junior', 30000); 
insert into candidates values (8, 'senior', 50000); 
insert into candidates values (9, 'senior', 30000); 
insert into candidates values (10, 'junior', 7000); 
insert into candidates values (11, 'junior', 8000); 
insert into candidates values (12, 'senior', 33000); 
insert into candidates values (13, 'junior', 5000); 
insert into candidates values (14, 'senior', 47000); 
insert into candidates values (15, 'junior', 12000);

待完成任务 #

现有两项基于上述candidates表的PostgreSQL 9.6数据库任务需完成。

内容的提问来源于stack exchange，提问作者ikram irzayev