Oracle学习第六课

伪列

rowid: 当前数据在数据库中的物理位置.

特点: 不存在任何表中,能查询出来.

-- 查看员工的工号,名字,薪资,及其物理位置id
select employee_id,first_name,salary,rowid
from employees;

rownum: 对满足查询条件的,查询结果进行编号(从1开始)

特点:

对满足where条件的查询结果进行标序号.

序号一定从1开始,且连续整数值.

rownum做条件判断: <n <=n >=1 =1

-- 查看员工的工号,名字,薪资, 每行数据添加编号(1-开始)
select employee_id,frist_name,salary,rownum
from employees;

-- 查看薪资大于10000的员工信息: 工号,名字,薪资,序号 rownum.
select employee_id,first_name,salary,rownum
from employees
where salary>10000;


-- 查看员工表中,前10个员工信息(工号,名字,薪资,序号)[序号<=10]

select employee_id,first_name,salary,rownum
from employees
where rownum <= 10;

-- 查看员工表中,第11~第20个员工信息(工号,名字,薪资,序号)
select employee_id,first_name,salary,rownum
from employees
where rownum >10 and rownum <=20; [×]

子查询

概念: 一个查询语句执行过程,需要用到另一条查询sql(子查询)的执行结果?
-- 具有x薪资的员工信息(工号,名字,薪资)?
 select * from employees where salary = X; --x 是最高薪资?
 select max(salary) from employees; -- x

1. 子查询结果是1行1列的值

将子查询的结果作为主查询的条件判断.

-- 具有最高薪资的员工信息(工号,名字,薪资)?
思路:
1. 获得员工的最高薪资   maxSalary
   select max(salary) from employees;--maxSalary;

2. 获得薪资为maxSalary的员工信息(工号,名字,薪资)
   select employee_id,first_name,salary from employees where salary = (maxSalary);
3. 合并SQL
    select employee_id,first_name,salary from employees where salary = (select max(salary) from employees);
    
-- 查询薪资数大于平均工资员工信息(工号,名字,薪资)
1. 获得平均工资 avgSalary
    select avg(salary) from employees;--avgSalary
2. 获得薪资>avgSalary的员工信息?
    select * from employees where salary > (avgSalary);
3. 合并
    select * from employees where salary > (select avg(salary) from employees);

2. 子查询的结果是n行1列的值

将子查询的结果作为主查询的条件判断,且一般使用多值枚举判断 in

-- 查询和姓 'King' 的在同一部门的员工信息?
1. 获得king所在的部门id: --deptId
    select department_id from employees where last_name ='King';--DeptId
2. 查询部门id为deptId的员工信息?
    select * from employees where department_id in (DeptId)
3. 合并:
 select * from employees where department_id in (select department_id from employees where last_name ='King');

3. 子查询的结果是n行n列

主查询将子查询的结果作为临时表,再次查询.

-- 查看员工信息(工号,名字,薪资),按照薪资降序排序的前10名员工信息?
select employee_id,first_name,salary
from employees
where rownum <=10
order by salary desc;[错误: 按理先排序后取前10个,实际sql执行顺序先获取前10后排序]

人工思路:
1. 先按照薪资降序排序---排序后的结果   
    select employee_id,first_name,salary from employees order by salary desc; -- EMP1 表-- n行n列的结果  临时表
2. 对排序后的结果取前10
    select * 
    from (EMP1) 
    where rownum <=10;
3. 合并
    select * from
        (select employee_id,first_name,salary from employees order by salary desc) 
    where rownum <=10;

分页查询[重点]

-- 1. 查看员工表中,第11~第20个员工信息(工号,名字,薪资,序号)
    思路:
    1. 获得拥有序号效果的列的EMP1表?[对表添加一个rownum的RN的序号列]
    select employee_id,first_name,salary,rownum as rn
    from employees;--EMP1
    2. 查询EMP1中RN的值为11~20之间的员工信息?[对步骤1的查询临时表,再次查询]
    select employee_id,first_name,salary,rn
    from (emp1)
    where rn >=11 and rn <=20;
    3. 合并
    select employee_id,first_name,salary,rn
    from 
        (select employee_id,first_name,salary,rownum as rn from employees)
    where rn >=11 and rn <=20;

-- 2. 查询员工信息按照薪资降序排序后的第11条~第20条员工信息?[重要, 笔试, 100%开发]

    1. 按照薪资降序排序:
        select * from employees order by salary desc;-- t1
    2. 对t1表 添加RN序号列
        select employee_id,first_name,salary,rownum rn
        from (t1);--t2
    3. 获取RN 11~20之间的员工信息?
       select employee_id,first_name,salary,rn
       from (t2)
       where rn >=11 and rn <=20;
        
    4. 合并
        select employee_id,first_name,salary,rn
        from
            (select employee_id,first_name,salary,rownum rn
                 from 
                    (select * from employees order by salary desc))
        where rn >=11 and rn<=20;