select 2*3 from dual;
--求年薪
select ename,sal, sal*12 from emp;
--别名
select ename,sal, sal*12 as "年薪" from emp;
select ename,sal, sal*12 "年薪" from emp;
--员工一年总收入
select * from emp;
select ename,sal, sal*12+comm as "年总收入" from emp;
--注意：含有任何null值的数学表达式最后的结果都为null
--字符串连接符 ||
select ename||'员工姓名' from emp;
--处理一个字符串中含有单引号的情况 :使用2个单引号表示一个单引号
select ename||'员工''姓名' from emp;
--含有任何null值的字符串表达式中，null被当作空字符串处理
select empno, ename||mgr from emp;
--求员工每个级别顶薪
select hisal*12 as "员工年顶薪" from salgrade;
/* distinct 消除重复值 */
--求emp表中的部门列表
select distinct deptno from emp;
select deptno, job from emp;
select distinct deptno, job from emp;--去除deptno, job 组合起来的重复值
--null值
--求奖金为null的员工信息
select * from emp where comm is null;
--求奖金不为null的员工信息
select * from emp where comm is not null;
/* in */
--求员工编号在某一个集合内的值
select * from emp where empno in (7369,7499,7521)
select * from emp where empno not in (7369,7499,7521)
/* oracle 日期 */
select * from emp;
--求入职日期在1982-1-23后入职的人
select * from emp where hiredate > '23-1月-82';
/* like */
--查询员工 名字第二个字面为A的人
select * from emp where ename like '_A%';
--查询名字中含有 % 等通配符的数据时，使用转义字符 \ (\%)
select * from emp where ename like '%\%%';
--查询1987年入职的人
select * from emp where hiredate>='1-1月-87' and hiredate<='31-12月-87';
select * from emp where hiredate like '%87%'
/* order by*/
select * from emp order by deptno asc,sal desc;
/*常用函数*/
--lower()
select empno, Lower(ename) from emp;
--查询名字带有 “A“ 或 "a"的员工
select * from emp where ename like '%A%' or ename like '%a%';
select * from emp where Lower(ename) like '%a%';
--Upper(); 小写转大写
select Upper('wepull') from dual;
--Substr
select Substr('wepull',2,3) from dual; --epu
select Substr('wepull',2) from dual; --epull
select Substr(ename,2) from emp;
--Chr() 某个ASCII码值对应的字符
select Chr(97) from dual; -- a
-- Ascii()函数
--例子：求一个字符的ASCII码值
select Ascii('a') from dual; --97
--常用数值函数
select round(23.652) from dual --24
select round(23.652,2) from dual --23.65
select round(23.652,-1) from dual --20
select round(493.652,-2) from dual --500
--to_char()
select to_char(456123.45,'L999,999.99') from dual-- ￥456,123.00
--to_char()对日期的转换
select sysdate from dual;
select to_char(sysdate,'YYYY/MM/DD') from dual;
--to_date()函数
--例子:将1985年2月14日8:00以后入职的雇员信息取出
select * from emp where hiredate > to_date('1985-02-14 8:00','YYYY-MM-DD HH:MI')
--to_number()函数
--求薪水<于1200的人
select * from emp where sal<to_number('$1,200','$9,999')
--nvl 处理空值
select ename, sal*12+comm as "一年总收入" from emp
select ename, sal*12+nvl(comm,0) as "一年总收入" from emp
---聚合函数/组函数
select * from emp;
--emp表中deptno数量
select count(deptno) from emp;
select count(distinct deptno) as "部门总数" from emp;
--group by
--求每个部门的最大工资
select deptno ,max(sal) from emp group by deptno;
--求每个部门中职位的最大工资
select deptno ,job,max(sal) from emp group by deptno , job;
---求每个部门工资最高人信息
select ename,deptno,sal from emp where (deptno,sal)in
(select deptno ,max(sal) from emp group by deptno);
--方法二
select * from emp join
(select deptno ,max(sal) max_sal from emp group by deptno ) t
on (emp.deptno=t.deptno and emp.sal=t.max_sal)
--having
select deptno, avg(sal) from emp group by deptno having avg(sal)>2500
---子查询
--求工资最高的人的信息
select * from emp where sal=(
select max(sal) from emp
);
--rownum（分页）
--查询emp表前5条记录
select ename,rownum from emp where rownum<=5;
--查询emp表第5条以后记录
select * from (select empno, ename,rownum as r from emp)
where r>5;
--rownum 与排序
select empno,ename,sal ,rownum from emp order by sal;
select empno,ename,sal ,rownum from
(select empno,ename,sal from emp order by sal)
上司
--自连接
--求emp表中员工的姓名和对应的上司（mgr）
select * from emp;
select e1.ename, e2.ename
from emp e1,emp e2
where e1.mgr=e2.empno;
--交叉连接
select ename from emp;
select dname from dept;
select ename,dname from emp ,dept; --结果为笛卡尔乘积
--SQL1999 语法
select ename,dname from emp cross join dept;
--连接查询
create table a(
id int primary key,
name nvarchar2(20)
);
select * from a;
insert into a values (1,'凤姐');
insert into a values (3,'犀利哥');
insert into a values (5,'春哥');
insert into a values (6,'曾哥');
insert into a values (7,'平平');
create table b(
id int primary key,
info nvarchar2(20)
)
insert into b values (1,'深圳');
insert into b values (2,'武汉');
insert into b values (6,'成都');
select * from a;
select * from b;
select * from a left outer join b on a.id=b.id;--左外连接
select * from a right outer join b on (a.id=b.id);--右外连接
select * from a full join b on (a.id=b.id);--全连接 （左外连接+右外连接）
select * from a inner join b on (a.id=b.id) --内连接
--非等值连接
--查询出雇员名字和薪水等级（2个表连接查询）
select * from emp;
select * from salgrade;
select e.ename ,e.sal ,s.grade from emp e
join salgrade s
on (e.sal between s.losal and s.hisal)
--查询出雇员名字，及对应的部门名称，薪水等级（3个表连接查询）
select e.ename, d.dname, s.grade from emp e
join dept d on(e.deptno=d.deptno)
join salgrade s on (e.sal between s.losal and s.hisal)
--练习
--求部门中薪水最高的人名。
select * from emp e;
select e.deptno, max(e.sal) from emp e group by deptno ;
select e.ename ,t.deptno,e.sal from emp e
join (select e.deptno, max(e.sal) max_sal from emp e group by deptno) t
on (e.deptno=t.deptno and e.sal =max_sal)
--求部门平均薪水等级？
select deptno, avg(sal) from emp e group by deptno;
select * from salgrade;
select t.deptno, t.avg_sal, s.grade from salgrade s
join (select deptno, avg(sal) avg_sal from emp e group by deptno) t
on(t.avg_sal between s.losal and s.hisal );
--求哪些人是经理
select * from emp;
select * from emp where mgr in (select mgr from emp);
select * from emp where mgr in (select distinct mgr from emp);
--平均薪水最高的部门编号与薪水
--第一步：先求出部门平均薪水
select e.deptno ,avg(sal) from emp e group by e.deptno;
--第二步：求出平均薪水最高值
select max(t.avg_sal)
from (select e.deptno ,avg(sal) avg_sal from emp e group by e.deptno) t
--综合
select deptno ,avg_sal from
(select e.deptno ,avg(sal) avg_sal from emp e group by e.deptno) t
where t.avg_sal =
( select max(t.avg_sal)
from (select e.deptno ,avg(sal) avg_sal from emp e group by e.deptno) t
)
--方法二：
select deptno ,avg_sal from
(select e.deptno ,avg(sal) avg_sal from emp e group by e.deptno) t
where avg_sal =(select max(avg(sal)) from emp group by deptno )
--注意 :组函数嵌套只能嵌套2层
select * from emp;
--比普通员工的最高薪水还要高的经理人名称
-- 1.先求普通员工的最高薪水？
select max(sal) from emp e
where e.empno not in (select mgr from emp where mgr is not null )
--2.求比上面的值要大，并且还是经理人
select e.ename,e.sal from emp e
where e.empno in (select distinct mgr from emp where mgr is not null )
and e.sal>
( select max(sal) from emp e
where e.empno not in (select distinct mgr from emp where mgr is not null )
);
--复制表
create table emp as select * from scott.emp where 1<>1;--只复制结构
create table emp as select * from scott.emp ;--复制数据
create table dept as select * from scott.dept ;
create table salgrade as select * from scott.salgrade ;
create table bonus as select * from scott.bonus ;
select * from emp
---Orcle 高级部分
--视图
select * from scott.emp;
create view emp_view
as
select empno, ename,sal from scott.emp;
select * from emp_view;
drop view emp_view;
--查询每个部门的名称、总人数、平均工资、最低工资的员工名字。
select dept.dname from demp;
select deptno, count(empno) ,avg(sal),min(sal) from emp group by deptno ;
--综合： 3张表连接查询
select d.dname, t.deptno, t.c, t.avg_sal,t.min_sal ,e.ename
from dept d
join
(select deptno, count(empno) c ,avg(sal) avg_sal,min(sal) min_sal from emp group by deptno ) t
on (d.deptno=t.deptno)
join emp e
on (e.sal=t.min_sal and e.deptno=t.deptno)
--把上面结果封装成视图
create or replace view deptInfo_view
as
select d.dname, t.deptno, t.c, t.avg_sal,t.min_sal ,e.ename
from dept d
join (select deptno, count(empno) c ,avg(sal) avg_sal,min(sal) min_sal from emp group by deptno ) t
on (d.deptno=t.deptno)
join emp e
on (e.sal=t.min_sal and e.deptno=t.deptno)
--
select * from deptinfo_view;
---------
--外键约束
drop table t_type;
create table t_type(
id int primary key,
typeName varchar2(20)
)
drop table t_book;
create table t_book(
id int primary key,
name varchar2(20) ,
tid int ,
constraint FK_tid foreign key (tid) references t_type(id)--字段级的外键约束，不用加foreign key
)
--
select * from user_tables;
select table_name from user_tables;
insert into t_type values (1,'言情类');
insert into t_type values (2,'计算机类');
insert into t_type values (3,'言武侠类');
select * from t_book t
insert into t_book values (1,'小磊子的情书',1);
insert into t_book values (2,'凤姐的韵事',1);
insert into t_book values (3,'干露露的视频',2);
insert into t_book values (4,'java自学成才',2);
insert into t_book values (5,'刘德华的情歌',3);
--------
--视图中使用DML的规定（不重要）
create or replace view book_view
as
select id,name from t_book;
--
select * from t_book
select * from book_view;
delete from book_view where id=4;
--只读视图
create or replace view book_view
as
select id,name from t_book
with read only;
--序列
create or replace sequence my_sec;
select my_sec.nextval from dual;
select my_sec.currval from dual;
create sequence book_sec;
create table book(
id number,
name varchar2(50)
)
alter table book add constraint PK_id primary key (id)
select * from book;
insert into book values (book_sec.nextval,'111');
insert into book values (book_sec.nextval,'222');
insert into book values (book_sec.nextval,'333');
insert into book values (book_sec.nextval,'444');
create sequence se_1
increment by 50 --次增长的幅度 50
start with 100 --从100开始
maxvalue 400 --最大400
minvalue 50
cycle --到最大值后循环
cache 2;
select se_1.nextval from dual
--索引
create index idx_book on book(name);
drop index idx_book;
--同义词
t_book
create synonym t for t_book
select * from t;
drop synonym t;
--- PL/SQL
begin
dbms_output.put_line('hello word');
end;
--变量赋值
declare
v_name varchar2(20):='wepull';
begin
dbms_output.put_line(v_name);
end;
--复合类型变量
--记录类型
declare
--定义一个记录类型
type book_rec is record(
id int,
name varchar2(20)
);
b_rec book_rec; --声明一个变量是记录类型
begin
select id ,name into b_rec from wepull.t_book where id=1;
dbms_output.put_line(b_rec.id||b_rec.name);
end;
--%type
declare
v_id wepull.t_book.id%type ;--定义的变量v_id类型与t_book中的id列的类型一致
v_name wepull.t_book.name%type;
begin
/*
v_id:=1;
v_name:='wepull';
*/
select id ,name into v_id,v_name from wepull.t_book where id=1;
dbms_output.put_line(v_id);
dbms_output.put_line(v_name);
end;
--rowtype
declare
v_book wepull.t_book%rowtype ;--定义的变量v_book类型与t_book中列的类型一致
begin
select * into v_book from wepull.t_book where id=1;
dbms_output.put_line('id: '||v_book.id);
dbms_output.put_line('name: '||v_book.name);
dbms_output.put_line('tid: '||v_book.tid);
end;
--嵌套表
select * from t_book;
select * from wepull.t_book;
declare
type book_table_type is table of wepull.t_book%rowtype;--定义类型
v_book book_table_type:=book_table_type();
begin
v_book.extend(5);
select * into v_book(1) from wepull.t_book where id=1;
dbms_output.put_line(v_book(1).id||' '||v_book(1).name);
select * into v_book(2) from wepull.t_book where id=2;
dbms_output.put_line(v_book(2).id||' '||v_book(2).name);
end;
/
--流程控制语句
--if
--单个if
--1.查询用户输入的id的图书，如果图书的tid=1 则输出图书类型为言情类
declare
v_id wepull.t_book.id%type:=&v_id;
v_tid wepull.t_book.tid%type;
begin
select id,tid into v_id, v_tid from wepull.t_book where id=v_id;
if (v_tid=1) then
dbms_output.put_line('此id的图书为言情类');
end if;
end;
/
--if else
--1.查询用户输入的id的图书，如果图书的tid=1 则输出图书类型为言情类，否则输出不是言情类
declare
v_id wepull.t_book.id%type:=&v_id;
v_tid wepull.t_book.tid%type;
begin
select id,tid into v_id, v_tid from wepull.t_book where id=v_id;
if (v_tid=1) then
dbms_output.put_line(v_id|| ' :此id的图书为言情类');
else
dbms_output.put_line(v_id|| ' :此id的图书不是言情类');
end if;
end;
/
--
--if elsif
--1.查询用户输入的id的图书，
--如果图书的tid=1 则输出图书类型为言情类，
--如果图书的tid=2 则输出图书类型为计算机类
--如果图书的tid=3 则输出图书类型为言武侠类
select * from wepull.t_type;
declare
v_id wepull.t_book.id%type:=&v_id;
v_tid wepull.t_book.tid%type;
begin
select id,tid into v_id, v_tid from wepull.t_book where id=v_id;
if (v_tid=1) then
dbms_output.put_line(v_id|| ' :此id的图书为言情类');
elsif(v_tid=2)then
dbms_output.put_line(v_id|| ' :此id的图书为计算机类');
elsif(v_tid=3)then
dbms_output.put_line(v_id|| ' :此id的图书为武侠类');
end if;
end;
/
--case
declare
v_id wepull.t_book.id%type:=&v_id;
v_tid wepull.t_book.tid%type;
v_typeName wepull.t_type.typeName%type;
begin
select tid into v_tid from wepull.t_book where id = v_id;
v_typeName :=
case v_tid
when 1 then '言情类'
when 2 then '计算机类'
when 3 then '武侠类'
else '其他类'
end;
dbms_output.put_line('id: '||v_id);
dbms_output.put_line('tid: '||v_tid);
dbms_output.put_line('typeName: '||v_typeName);
end;
/
--
--case 搜索方式
declare
v_id wepull.t_book.id%type:=&v_id;
v_tid wepull.t_book.tid%type;
v_typeName wepull.t_type.typeName%type;
begin
select tid into v_tid from wepull.t_book where id = v_id;
v_typeName :=
case
when v_tid=1 then '言情类'
when v_tid=2 then '计算机类'
when v_tid=3 then '武侠类'
else '其他类'
end;
dbms_output.put_line('id: '||v_id);
dbms_output.put_line('tid: '||v_tid);
dbms_output.put_line('typeName: '||v_typeName);
end;
/
--循环
--循环1---Loop
--求 1+2+....+100=？
declare
v_i number:=0;
v_sum number:=0;
begin
loop
v_i:=v_i+1;
v_sum:=v_sum+v_i;
exit when(v_i=100);
end loop;
dbms_output.put_line('1+2+....+100='||v_sum);
end;
/
--循环1---while
--求 1+2+....+100=？
declare
v_i number:=0;
v_sum number:=0;
begin
while(v_i<100) loop
v_i:=v_i+1;
v_sum:=v_sum+v_i;
end loop;
dbms_output.put_line('1+2+....+100='||v_sum);
end;
/
--循环1---for
--求 1+2+....+100=？
declare
v_i number:=0;
v_sum number:=0;
begin
for v_i in 1..100
loop
v_sum:=v_sum+v_i;
end loop;
dbms_output.put_line('1+2+....+100='||v_sum);
end;
/
--
declare
v_i number:=0;
v_sum number:=0;
begin
for int in 1..100
loop
v_i:=v_i+1;
v_sum:=v_sum+v_i;
end loop;
dbms_output.put_line('1+2+....+100='||v_sum);
end;
/
--goto
declare
v_i number:=0;
v_sum number:=0;
begin
for v_i in 1..10
loop
dbms_output.put_line('当前i为： '||v_i);
if(v_i=5) then
goto endloop;
end if;
end loop;
<<endloop>>
dbms_output.put_line('此时v_i=5,退出了循环体');
end;
--null语句
--goto
declare
v_i number:=0;
v_sum number:=0;
begin
for v_i in 1..10
loop
dbms_output.put_line('当前i为： '||v_i);
if(v_i=5) then
goto endloop;
end if;
end loop;
<<endloop>>
null;
end;
--游标
--使用游标取出emp表总前10人的信息。
select * from scott.emp;
declare
--第一步：声明游标
cursor emp_cursor is select * from scott.emp where rownum<=10;
emp_record scott.emp%rowtype;
begin
--第二步：打开游标
open emp_cursor;
--第三步：从游标中取数据
loop
fetch emp_cursor into emp_record;--每取出一条数据，指针自动下移
exit when(emp_cursor%notfound);--最后次取数据失败
dbms_output.put_line(emp_record.empno|| ' ' ||emp_record.ename);
end loop;
--第四步：关闭游标
close emp_cursor;
end;
/
--错误例子
declare
--第一步：声明游标
cursor emp_cursor is select * from scott.emp where rownum<=10;
emp_record scott.emp%rowtype;
begin
--第二步：打开游标
open emp_cursor;
--第三步：从游标中取数据
loop
fetch emp_cursor into emp_record;--每取出一条数据，指针自动下移
dbms_output.put_line(emp_record.empno|| ' ' ||emp_record.ename);
exit when(emp_cursor%notfound);--最后次取数据失败
end loop;
--第四步：关闭游标
close emp_cursor;
end;
/
--使用游标取出emp表总前10人的信息。 while
declare
--第一步：声明游标
cursor emp_cursor is select * from scott.emp where rownum<=10;
emp_record scott.emp%rowtype;
begin
--第二步：打开游标
open emp_cursor;
--第三步：从游标中取数据
fetch emp_cursor into emp_record;
while(emp_cursor%found)loop
dbms_output.put_line(emp_record.empno|| ' ' ||emp_record.ename);
fetch emp_cursor into emp_record;
end loop;
--第四步：关闭游标
close emp_cursor;
end;
/
--使用游标取出emp表总前10人的信息。 for 必须掌握
declare
--第一步：声明游标
cursor emp_cursor is select * from scott.emp where rownum<=10;
emp_record scott.emp%rowtype;
begin
for emp_rec in emp_cursor loop
dbms_output.put_line(emp_rec.empno|| ' ' ||emp_rec.ename);
end loop;
end;
/
--使用游标取出emp表总指定job 和 deptno的员工信息。
select * from scott.emp
declare
--第一步：声明游标
cursor emp_cursor(v_job scott.emp.job%type,v_deptno scott.emp.deptno%type)
is select * from scott.emp where job=v_job and deptno=v_deptno;
emp_record scott.emp%rowtype;
begin
for emp_rec in emp_cursor('CLERK',30) loop
dbms_output.put_line(emp_rec.empno|| ' ' ||emp_rec.ename);
end loop;
end;
/
--模糊查询
select PATINDEX('%abb%','abcaabbeeabb') from dual;
select * from scott.emp where regexp_like(ename, '[AS]MITH');--SMITH
select * from scott.emp where ename like 'SMITH';
--游标的update操作
--更新指定图书的名字
declare
cursor book_cursor is select * from t_book for update;
--v_id t_book.id%type:=v_id;
v_name t_book.name%type:=&v_name;
begin
for book_rec in book_cursor loop
if(book_rec.tid=1) then
update t_book set name=v_name where current of book_cursor;
end if;
end loop;
end;
/
--隐式游标（了解）
--更新指定图书的名字，如果该图书没有找到，则打印”查无此图书”。
declare
v_name t_book.name%type:=&v_name;
v_id t_book.id%type:=&v_id;
begin
update t_book set name=v_name where id=v_id;
if(sql%found) then
dbms_output.put_line('修改成功： '||v_id||' '||v_name);
elsif(sql%notfound) then
dbms_output.put_line('没有指定id的图书 ');
end if;
end;
/
-------------存储过程
create or replace procedure p1
as
v_name t_book.name%type:='言情';
v_id t_book.id%type:=1;
begin
update t_book set name=v_name where id =v_id;
if(sql%found) then
dbms_output.put_line('修改成功');
end if;
end;
/
--调用存储过程
exec p1;
select * from t_book;
begin
p1;
end;
/
--带参数的存储过程
--求1...N的和 （N为入参）
create or replace procedure p2(v_number in number)
as
v_sum number:=0;
v_i number:=0;
begin
for v_i in 0..v_number loop
v_sum:=v_sum+v_i;
end loop;
dbms_output.put_line(v_sum);
end;
--调用
begin
p2(100);
end;
/
--带参数的存储过程
--求1...N的和 （v_number为入参,v_sum为出参）
create or replace procedure p3(v_number in number,v_sum out number)
as
begin
v_sum :=0;
for v_i in 0..v_number loop
v_sum:=v_sum+v_i;
end loop;
dbms_output.put_line('存储过程中打印：'||v_sum);
end;
--调用
declare
v_sum number;
begin
p3(100,v_sum);
dbms_output.put_line('调用打印：'||v_sum);
end;
/
--函数 无参
create or replace function my_fun
return date
as
v_date date;
begin
select sysdate into v_date from dual;
return v_date;
end;
/
--调用
select my_fun() from dual;
---
declare
v_date date;
begin
v_date:=my_fun();
dbms_output.put_line(v_date);
end;
/
--函数 带参数
--根据id查询出图书名字
create or replace function getName(v_id t_book.id%type)
return t_book.name%type
as
v_name t_book.name%type;
begin
select name into v_name from t_book where id = v_id;
return v_name;
end;
/
--调用
declare
v_name t_book.name%type;
begin
v_name:=getName(2);
dbms_output.put_line(v_name);
end;
/
--触发器
create table book_log(
username varchar2(10),
action varchar2(10),
adate date
)
select * from a;
create or replace trigger trigger_book
after insert or update or delete on scott.a
begin
if inserting then
insert into book_log values (user,'insert',sysdate);
elsif updating then
insert into book_log values (user,'update',sysdate);
elsif deleting then
insert into book_log values (user,'delete',sysdate);
end if;
end;
/
--测试
select * from a;
select * from book_log;
insert into a values (8,'龙磊的铃声');
update a set name='龙磊' where id = 8;
delete from a set name='龙磊' where id = 8;
--行触发器
create or replace trigger trigger_book
after insert or update or delete on scott.a for each row
begin
if inserting then
insert into book_log values (user,'insert',sysdate);
elsif updating then
insert into book_log values (user,'update',sysdate);
elsif deleting then
insert into book_log values (user,'delete',sysdate);
end if;
end;
/
--
update a set name=name||'2';
delete from book_log;
select * from book_log;
select * from a;
select * from emp;
--异常处理（了解）
declare
v_empno scott.emp.empno%type:=&v_empno;
v_emp_rec emp%rowtype;
begin
select * into v_emp_rec from scott.emp where scott.emp.empno=v_empno;
dbms_output.put_line(v_emp_rec.empno||' '||v_emp_rec.ename);
exception
when no_data_found then
dbms_output.put_line('没有此编号的员工 ');
end;
/
declare
v_deptno scott.emp.deptno%type:=&v_deptno;
v_emp_rec emp%rowtype;
begin
select * into v_emp_rec from scott.emp where scott.emp.deptno=v_deptno;
dbms_output.put_line(v_emp_rec.empno||' '||v_emp_rec.ename);
exception
when no_data_found then
dbms_output.put_line('没有此编号的员工 ');
when too_many_rows then
dbms_output.put_line('select 语句返回了多行数据 ');
end;
with e as 用法：
其实就是把一大堆重复用到的sql语句放在with as里面，取一个别名，后面的查询就可以用它，这样对于大批量的sql语句起到一个优化的作用，而且清楚明了。 with e as (select * from smc_sql ) select * from e where e.id=2; with as优点 增加了sql的易读性，如果构造了多个子查询，结构会更清晰； 更重要的是：“一次分析，多次使用”，这也是为什么会提供性能的地方，达到了“少读”的目标