This article gives an overview of the FIRST and LAST analytic functions. If you are new to analytic functions you should probably read this introduction to analytic functions first.
Most of the time I find myself using FIRST_VALUE and LAST_VALUE Analytic Functions in preference to FIRST and LAST. Pick which feels best for your use case.
Content
Related articles.
Setup
The examples in this article require the following table.
--drop table emp purge;
create table emp (
empno number(4) constraint pk_emp primary key,
ename varchar2(10),
job varchar2(9),
mgr number(4),
hiredate date,
sal number(7,2),
comm number(7,2),
deptno number(2)
);
insert into emp values (7369,'SMITH','CLERK',7902,to_date('17-12-1980','dd-mm-yyyy'),800,null,20);
insert into emp values (7499,'ALLEN','SALESMAN',7698,to_date('20-2-1981','dd-mm-yyyy'),1600,300,30);
insert into emp values (7521,'WARD','SALESMAN',7698,to_date('22-2-1981','dd-mm-yyyy'),1250,500,30);
insert into emp values (7566,'JONES','MANAGER',7839,to_date('2-4-1981','dd-mm-yyyy'),2975,null,20);
insert into emp values (7654,'MARTIN','SALESMAN',7698,to_date('28-9-1981','dd-mm-yyyy'),1250,1400,30);
insert into emp values (7698,'BLAKE','MANAGER',7839,to_date('1-5-1981','dd-mm-yyyy'),2850,null,30);
insert into emp values (7782,'CLARK','MANAGER',7839,to_date('9-6-1981','dd-mm-yyyy'),2450,null,10);
insert into emp values (7788,'SCOTT','ANALYST',7566,to_date('13-JUL-87','dd-mm-rr')-85,3000,null,20);
insert into emp values (7839,'KING','PRESIDENT',null,to_date('17-11-1981','dd-mm-yyyy'),5000,null,10);
insert into emp values (7844,'TURNER','SALESMAN',7698,to_date('8-9-1981','dd-mm-yyyy'),1500,0,30);
insert into emp values (7876,'ADAMS','CLERK',7788,to_date('13-JUL-87', 'dd-mm-rr')-51,1100,null,20);
insert into emp values (7900,'JAMES','CLERK',7698,to_date('3-12-1981','dd-mm-yyyy'),950,null,30);
insert into emp values (7902,'FORD','ANALYST',7566,to_date('3-12-1981','dd-mm-yyyy'),3000,null,20);
insert into emp values (7934,'MILLER','CLERK',7782,to_date('23-1-1982','dd-mm-yyyy'),1300,null,10);
commit;FIRST Analytic Function
The FIRST analytic function can be used to return the first value from an ordered sequence. Say we want to display the salary of each employee, along with the lowest salary within their department we may use something like.
select empno,
deptno,
sal,
min(sal) keep (dense_rank first order by sal) over (partition by deptno) as lowest
from emp
order by deptno, sal;
EMPNO DEPTNO SAL LOWEST
---------- ---------- ---------- ----------
7934 10 1300 1300
7782 10 2450 1300
7839 10 5000 1300
7369 20 800 800
7876 20 1100 800
7566 20 2975 800
7788 20 3000 800
7902 20 3000 800
7900 30 950 950
7654 30 1250 950
7521 30 1250 950
7844 30 1500 950
7499 30 1600 950
7698 30 2850 950
SQL>The MIN function is almost irrelevant here as it's FIRST and KEEP that are picking the row whose value will be used. We can demonstrate this by using MAX for the low value.
select empno,
deptno,
sal,
max(sal) keep (dense_rank first order by sal) over (partition by deptno) as lowest
from emp
order by deptno, sal;
EMPNO DEPTNO SAL LOWEST
---------- ---------- ---------- ----------
7934 10 1300 1300
7782 10 2450 1300
7839 10 5000 1300
7369 20 800 800
7876 20 1100 800
7566 20 2975 800
7788 20 3000 800
7902 20 3000 800
7900 30 950 950
7654 30 1250 950
7521 30 1250 950
7844 30 1500 950
7499 30 1600 950
7698 30 2850 950
SQL>We get the same result.
We could also achieve the same result using FIRST_VALUE or MIN as basic analytic functions. In practice I don't use FIRST very often.
LAST Analytic Function
The LAST analytic function can be used to return the last value from an ordered sequence. Say we want to display the salary of each employee, along with the highest within their department we may use something like.
select empno,
deptno,
sal,
max(sal) keep (dense_rank last order by sal) over (partition by deptno) as highest
from emp
order by deptno, sal;
EMPNO DEPTNO SAL HIGHEST
---------- ---------- ---------- ----------
7934 10 1300 5000
7782 10 2450 5000
7839 10 5000 5000
7369 20 800 3000
7876 20 1100 3000
7566 20 2975 3000
7788 20 3000 3000
7902 20 3000 3000
7900 30 950 2850
7654 30 1250 2850
7521 30 1250 2850
7844 30 1500 2850
7499 30 1600 2850
7698 30 2850 2850
SQL>The MAX function is almost irrelevant here as it's LAST and KEEP that are picking the row whose value will be used. We can demonstrate this by using MIN for the high value.
select empno,
deptno,
sal,
min(sal) keep (dense_rank last order by sal) over (partition by deptno) as highest
from emp
order by deptno, sal;
EMPNO DEPTNO SAL HIGHEST
---------- ---------- ---------- ----------
7934 10 1300 5000
7782 10 2450 5000
7839 10 5000 5000
7369 20 800 3000
7876 20 1100 3000
7566 20 2975 3000
7788 20 3000 3000
7902 20 3000 3000
7900 30 950 2850
7654 30 1250 2850
7521 30 1250 2850
7844 30 1500 2850
7499 30 1600 2850
7698 30 2850 2850
SQL>We get the same result.
We could also achieve the same result using LAST_VALUE or MAX as basic analytic functions. In practice I don't use LAST very often.
Quick Links
The "*" indicates the function supports the full analytic syntax, including the windowing clause.
For more information see:
- Analytic Functions : All Articles
- Analytic Functions
- FIRST_VALUE and LAST_VALUE Analytic Functions
- FIRST
- LAST
Hope this helps. Regards Tim...
我阅读了这篇文章，这篇文章呢，作者自己也说Most of the time I find myself using FIRST_VALUE and LAST_VALUE Analytic Functions in preference to FIRST and LAST. Pick which feels best for your use case.
但是，好处有以下：
- 写法固定，避免了讨厌的order by,如果某个人不小心写了order by之后，发现结果为什么不对，那是因为window clause的default action是range between unbouned preceding and current row，这样只扫描了当前行及其之前的所有行，但是经我测试下来，发现如果你在query partition clause之后写order by，会报错:ORA-30487: ORDER BY not allowed here。code如下
select empno, deptno, sal, min(sal) keep (dense_rank first order by sal) over (partition by deptno order by sal) as lowest from emp order by deptno, sal; - dense_rank能不能改？答案是不能，改了会报错：ORA-02000: missing DENSE_RANK keyword。
- 文章中用到的aggregate function - min or max 能不能修改?答案是可以的，因为last和first已然指定，那么用任意一个function都是不搭嘎的。