lag lead 学习

lag lead 语法如下：

 




Lag和Lead函数可以在一次查询中取出同一字段的前N行的数据和后N行的值

lag(exp_str,offset,defval) over()

exp_str 是要做对比的字段

offset 是exp_str字段的偏移量 比如说 offset 为3 则 拿exp_str的第一行和第四行对比，第二行和第五行，依次类推，offset的默认值为1！

defval是当该函数无值可用的情况下返回的值。Lead函数的用法类似。

--lag(exp_str)，如下：

select ename,job,sal ,lag(sal) over(order by sal) last_sal from emp;

	ENAME	JOB	SAL	LAST_SAL
1	SMITH	CLERK	800
2	JAMES	CLERK	950	800
3	ADAMS	CLERK	1100	950
4	WARD	SALESMAN	1250	1100
5	MARTIN	SALESMAN	1250	1250
6	CLARK	MANAGER	1300	1250
7	MILLER	CLERK	1300	1300
8	TURNER	SALESMAN	1500	1300
9	ALLEN	SALESMAN	1600	1500
10	BLAKE	MANAGER	2850	1600
11	JONES	MANAGER	2975	2850
12	SCOTT	ANALYST	3000	2975
13	FORD	ANALYST	3000	3000
14	KING	PRESIDENT	5000	3000

--lead如同
select ename,job,sal ,lead(sal) over(order by sal) last_sal from emp;

	ENAME	JOB	SAL	LAST_SAL
1	SMITH	CLERK	800	950
2	JAMES	CLERK	950	1100
3	ADAMS	CLERK	1100	1250
4	WARD	SALESMAN	1250	1250
5	MARTIN	SALESMAN	1250	1300
6	CLARK	MANAGER	1300	1300
7	MILLER	CLERK	1300	1500
8	TURNER	SALESMAN	1500	1600
9	ALLEN	SALESMAN	1600	2850
10	BLAKE	MANAGER	2850	2975
11	JONES	MANAGER	2975	3000
12	SCOTT	ANALYST	3000	3000
13	FORD	ANALYST	3000	5000
14	KING	PRESIDENT	5000

--lad(sal,1,100)  over(order by sal)  sal向前便宜1，null值会被100填充
select ename,job,sal ,lag(sal,1,100) over(order by sal) last_sal from emp;

	ENAME	JOB	SAL	LAST_SAL
1	SMITH	CLERK	800	100
2	JAMES	CLERK	950	800
3	ADAMS	CLERK	1100	950
4	WARD	SALESMAN	1250	1100
5	MARTIN	SALESMAN	1250	1250
6	CLARK	MANAGER	1300	1250
7	MILLER	CLERK	1300	1300
8	TURNER	SALESMAN	1500	1300
9	ALLEN	SALESMAN	1600	1500
10	BLAKE	MANAGER	2850	1600
11	JONES	MANAGER	2975	2850
12	SCOTT	ANALYST	3000	2975
13	FORD	ANALYST	3000	3000
14	KING	PRESIDENT	5000	3000

--lead(sal,1,100)  over(order by sal)   向后偏移1

select ename,job,sal ,lead(sal,1,100) over(order by sal) last_sal from emp;

	ENAME	JOB	SAL	LAST_SAL
1	SMITH	CLERK	800	950
2	JAMES	CLERK	950	1100
3	ADAMS	CLERK	1100	1250
4	WARD	SALESMAN	1250	1250
5	MARTIN	SALESMAN	1250	1300
6	CLARK	MANAGER	1300	1300
7	MILLER	CLERK	1300	1500
8	TURNER	SALESMAN	1500	1600
9	ALLEN	SALESMAN	1600	2850
10	BLAKE	MANAGER	2850	2975
11	JONES	MANAGER	2975	3000
12	SCOTT	ANALYST	3000	3000
13	FORD	ANALYST	3000	5000
14	KING	PRESIDENT	5000	100

--偏移2如下

select ename,job,sal ,lag(sal,2) over(order by sal) last_sal from emp;

	ENAME	JOB	SAL	LAST_SAL
1	SMITH	CLERK	800
2	JAMES	CLERK	950
3	ADAMS	CLERK	1100	800
4	WARD	SALESMAN	1250	950
5	MARTIN	SALESMAN	1250	1100
6	CLARK	MANAGER	1300	1250
7	MILLER	CLERK	1300	1250
8	TURNER	SALESMAN	1500	1300
9	ALLEN	SALESMAN	1600	1300
10	BLAKE	MANAGER	2850	1500
11	JONES	MANAGER	2975	1600
12	SCOTT	ANALYST	3000	2850
13	FORD	ANALYST	3000	2975
14	KING	PRESIDENT	5000	3000