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1.常见的数据库对象有哪些?
表(table) 视图(view) 序列(sequence) 索引(index) 同义词(synonym)
存储过程(procedure) 存储函数(function) 触发器(trigger)
2.表:数据的主要存储方式,由行和列组成。后面重点说
视图:存储起来的select语句。
对视图中数据的DML操作,会导致创建视图使用的表中的数据的修改。
create view emp_vu
as
select department_id,avg(salary) dept_avg_sal
from employees
group by department_id;
--with read only
select * from emp_vu;
序列:提供了一系列有规律的数值,常用来作为表的主键的值
create sequence emp_id_seq
start with 1001
increment by 1
maxvalue 10000
--minvalue
--cycle/nocycle
--cache/nocache
1)nextval / currval
select emp_id_seq.currval from dual;
select emp_id_seq.nextval from dual;
create table emp(
id number(10),
name varchar2(15)
)
insert into emp
values(emp_id_seq.nextval,'BB');
select * from emp;
裂缝:①多个表共用一个序列②出现回滚③出现异常
索引(index):当使用索引作用的列作为查询条件进行查询时,可以提高查询的效率。
--如何创建索引:①自动创建(声明为主键或唯一性约束的列) ②手动创建
create index emp_sal
on employees(salary);
3.重点:表
DDL:CREATE TABLE;ALTER TABLE;TRUNCATE TABLE;DROP TABLE;RENAME .. TO ..
不可回滚,即意味着:自动提交
--1.创建表
--1.1“白手起家”
create table dept(
dept_id number(10),
dept_name varchar2(15),
location_id varchar2(10),
birth Date
)
select * from dept;
--1.2基于现有的表,创建
create table emp1
as
select employee_id id,last_name name,hire_date,salary
from employees
--where department_id = 80;
where 1=2;
select * from emp1;
--1)对现有的表的复制/空表
create table emp_copy
as
select * from employees
--where 1=2;
select * from emp_copy;
--2.修改表
--2.1增加一个列
ALTER table emp
add(salary number(10,2) default 2000);
select * from emp;
--2.2修改现有的列
alter table emp
modify(salary number(15,2));
insert into emp(id,name)
values(1004,'CC');
--2.3重命名现有的列
alter table emp
rename column salary to sal;
--2.4删除现有的列
alter table emp
drop column sal;
--3.重命名现有的表
rename emp to employee;
select * from employee;
--4.清空表
truncate table employee;
rollback;
--5.删除表
drop table employee;
DML:增、删、改、查
--增insert into ...
--1.一条一条的添加
select * from dept;
insert into dept
values(,,,);
insert into dept(dept_id,location_id,dept_name)
values(,,);
--2.导入数据
insert into dept(dept_id,location_id,dept_name)
select department_id,location_id,department_name
from departments;
alter table dept
modify(dept_name varchar2(20));
--删
delete from dept
where dept_id < 40;
--改
update dept
set location_id = 1700
where dept_id = 20;
commit;
--查询(重中之重)
select .... --分组函数(count / max / min / avg / sum)
from ...,....--多表连接
where ...--过滤条件和 多表的连接条件(若不写,会出现笛卡尔积的错误)
group by ...--凡是在select中出新了分组函数,则没有使用分组函数的列要作为group by的条件
having avg(..) ...--分组函数作为过滤条件,要使用having
order by ... asc/desc;
二、企业SQL考核真题
数据表:
dept:deptno(primary key), dname, loc
emp:empno(primary key), ename, job, mgr(references emp(empno)), sal,
deptno(references dept(deptno))
DEPT deptno(primary key)dname loc |
EMP empno(primary key) ename job mgr(references emp(empno)) sal deptno(references dept(deptno)) |
1 列出emp表中各部门的部门号,最高工资,最低工资
select max(sal) as 最高工资,min(sal) as 最低工资,deptno from emp group by deptno;2 列出emp表中各部门job 含’REP’的员工的部门号,最低工资,最高工资
select max(sal) as 最高工资,min(sal) as 最低工资,deptno as 部门号 from emp where job like '%REP%' group by deptno;3 对于emp中最低工资小于7000的部门中job为'SA_REP'的员工的部门号,最低工资,最高工资
select max(sal) as 最高工资,min(sal) as 最低工资,deptno as 部门号 from emp b where job='SA_REP' and 7000>(select min(sal) from emp a where a.deptno=b.deptno) group by b.deptno
4写出对上题的另一解决方法
(请补充)select deptno,min(sal),max(sal)
from emp
where job = 'SA_REP' and deptno in (
select deptno
from emp
--group by deptno
having min(sal) < 7000
)
group by deptno
5根据部门号由高而低,工资由低而高列出每个员工的姓名,部门号,工资
select deptno as 部门号,ename as 姓名,sal as 工资 from emp order by deptno desc,sal asc6 列出'Abel'所在部门中每个员工的姓名与部门号
select ename,deptno from emp where deptno = (select deptno from emp where ename = 'Abel')
(法二)
select ename,deptno
from emp e1
where exists (
select 'x'
from emp e2
where e1.deptno = e2.deptno
and e2.ename = 'Abel'
)
7 列出每个员工的姓名,工作,部门号,部门名
select ename,job,emp.deptno,dept.dname from emp,dept where emp.deptno=dept.deptno8 列出emp中工作为'SH_CLERK'的员工的姓名,工作,部门号,部门名
select ename,job,dept.deptno,dname from emp,dept where dept.deptno=emp.deptno and job='SH_CLERK'9 对于emp中有管理者的员工,列出姓名,管理者姓名(管理者外键为mgr)
select a.ename as 姓名,b.ename as 管理者 from emp a,emp b where a.mgr is not null and a.mgr=b.empno10 对于dept表中,列出所有部门名,部门号,同时列出各部门工作为'SH_CLERK'的员工名与工作
select dname as 部门名,dept.deptno as 部门号,ename as 员工名,job as 工作 from dept,emp where dept.deptno = emp.deptno(+) and job = 'SH_CLERK'11 对于工资高于本部门平均水平的员工,列出部门号,姓名,工资,按部门号排序
select a.deptno as 部门号,a.ename as 姓名,a.sal as 工资 from emp a where a.sal>(select avg(sal) from emp b where a.deptno=b.deptno) order by a.deptno
(法二)select e.deptno,ename,sal
from emp e,(select deptno,avg(sal) avg_sal from emp group by deptno) b
where e.sal > b.avg_sal and e.deptno = b.deptno
12 对于emp,列出各个部门中工资高于本部门平均水平的员工数和部门号,按部门号排序
select count(a.sal) as 员工数,a.deptno 部门号 from emp a where a.sal>(select avg(sal) from emp b where a.deptno=b.deptno) group by a.deptno order by a.deptno13. 对于emp中工资高于本部门平均水平,人数多于1人的,列出部门号,高于部门平均工资的人数,按部门号排序
select *
from(
select deptno,count(*) count_num
from emp e
where sal > (
select avg(sal)
from emp e1
where e.deptno = e1.deptno
)
group by deptno
) e1
where e1.count_num > 1
order by e1.deptno
14 对于emp中工资高于本部门平均水平,且其人数多于3人的,列出部门号,部门人数,按部门号排序
select count(a.empno) as 员工数,a.deptno as 部门号,avg(sal) as 平均工资 from emp a where (select count(c.empno) from emp c where c.deptno=a.deptno and c.sal>(select avg(sal) from emp b where c.deptno=b.deptno))>3 group by a.deptno order by a.deptno
(法二)
select m.deptno,count(ee1.empno)
from(
select e1.deptno,count(empno) count_num
from emp e1
where e1.sal >
(select avg(sal) from emp e2 where e1.deptno = e2.deptno)
group by e1.deptno
) m,emp ee1
where m.count_num > 3 and m.deptno = ee1.deptno
group by m.deptno
15 对于emp中低于自己工资至少5人的员工,列出其部门号,姓名,工资,以及工资少于自己的人数
select a.deptno,a.ename,a.sal,(select count(b.ename) from emp as b where b.sal<a.sal) as 人数 from emp as a where (select count(b.ename) from emp as b where b.sal<a.sal)>5
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