The LAG and LEAD analytic functions were introduced in 8.1.6 to give access to multiple rows within a table, without the need for a self-join. If you are new to analytic functions you should probably read this introduction to analytic functions first.
Related articles.
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;
Both LAG and LEAD functions have the same usage, as shown below. The analytic clause elements are described in more detail here.
LAG
{ ( value_expr [, offset [, default]]) [ { RESPECT | IGNORE } NULLS ]
| ( value_expr [ { RESPECT | IGNORE } NULLS ] [, offset [, default]] )
}
OVER ([ query_partition_clause ] order_by_clause)
LEAD
{ ( value_expr [, offset [, default]] ) [ { RESPECT | IGNORE } NULLS ]
| ( value_expr [ { RESPECT | IGNORE } NULLS ] [, offset [, default]] )
}
OVER ([ query_partition_clause ] order_by_clause)
value_expr - Can be a column or a built-in function, except for other analytic functions.offset - The number of rows preceeding/following the current row, from which the data is to be retrieved. The default value is 1.default - The value returned if the offset is outside the scope of the window. The default value is NULL.Looking at the EMP table, we query the data in salary (SAL) order.
SELECT empno,
ename,
job,
sal
FROM emp
ORDER BY sal;
EMPNO ENAME JOB SAL
---------- ---------- --------- ----------
7369 SMITH CLERK 800
7900 JAMES CLERK 950
7876 ADAMS CLERK 1100
7521 WARD SALESMAN 1250
7654 MARTIN SALESMAN 1250
7934 MILLER CLERK 1300
7844 TURNER SALESMAN 1500
7499 ALLEN SALESMAN 1600
7782 CLARK MANAGER 2450
7698 BLAKE MANAGER 2850
7566 JONES MANAGER 2975
7788 SCOTT ANALYST 3000
7902 FORD ANALYST 3000
7839 KING PRESIDENT 5000
SQL>
The LAG function is used to access data from a previous row. The following query returns the salary from the previous row to calculate the difference between the salary of the current row and that of the previous row. Notice that the ORDER BY of the LAG function is used to order the data by salary.
SELECT empno,
ename,
job,
sal,
LAG(sal, 1, 0) OVER (ORDER BY sal) AS sal_prev,
sal - LAG(sal, 1, 0) OVER (ORDER BY sal) AS sal_diff
FROM emp;
EMPNO ENAME JOB SAL SAL_PREV SAL_DIFF
---------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 800 0 800
7900 JAMES CLERK 950 800 150
7876 ADAMS CLERK 1100 950 150
7521 WARD SALESMAN 1250 1100 150
7654 MARTIN SALESMAN 1250 1250 0
7934 MILLER CLERK 1300 1250 50
7844 TURNER SALESMAN 1500 1300 200
7499 ALLEN SALESMAN 1600 1500 100
7782 CLARK MANAGER 2450 1600 850
7698 BLAKE MANAGER 2850 2450 400
7566 JONES MANAGER 2975 2850 125
7788 SCOTT ANALYST 3000 2975 25
7902 FORD ANALYST 3000 3000 0
7839 KING PRESIDENT 5000 3000 2000
SQL>
If the LAG would span a partition boundary, the default value is returned. In the following example we partition by department, so the SAL_PREV column has a default value of "0" for the first row in each department.
SELECT deptno,
empno,
ename,
job,
sal,
LAG(sal, 1, 0) OVER (PARTITION BY deptno ORDER BY sal) AS sal_prev
FROM emp;
DEPTNO EMPNO ENAME JOB SAL SAL_PREV
---------- ---------- ---------- --------- ---------- ----------
10 7934 MILLER CLERK 1300 0
10 7782 CLARK MANAGER 2450 1300
10 7839 KING PRESIDENT 5000 2450
20 7369 SMITH CLERK 800 0
20 7876 ADAMS CLERK 1100 800
20 7566 JONES MANAGER 2975 1100
20 7788 SCOTT ANALYST 3000 2975
20 7902 FORD ANALYST 3000 3000
30 7900 JAMES CLERK 950 0
30 7654 MARTIN SALESMAN 1250 950
30 7521 WARD SALESMAN 1250 1250
30 7844 TURNER SALESMAN 1500 1250
30 7499 ALLEN SALESMAN 1600 1500
30 7698 BLAKE MANAGER 2850 1600
SQL>
The LEAD function is used to return data from rows further down the result set. The following query returns the salary from the next row to calculate the difference between the salary of the current row and the following row.
SELECT empno,
ename,
job,
sal,
LEAD(sal, 1, 0) OVER (ORDER BY sal) AS sal_next,
LEAD(sal, 1, 0) OVER (ORDER BY sal) - sal AS sal_diff
FROM emp;
EMPNO ENAME JOB SAL SAL_NEXT SAL_DIFF
---------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 800 950 150
7900 JAMES CLERK 950 1100 150
7876 ADAMS CLERK 1100 1250 150
7521 WARD SALESMAN 1250 1250 0
7654 MARTIN SALESMAN 1250 1300 50
7934 MILLER CLERK 1300 1500 200
7844 TURNER SALESMAN 1500 1600 100
7499 ALLEN SALESMAN 1600 2450 850
7782 CLARK MANAGER 2450 2850 400
7698 BLAKE MANAGER 2850 2975 125
7566 JONES MANAGER 2975 3000 25
7788 SCOTT ANALYST 3000 3000 0
7902 FORD ANALYST 3000 5000 2000
7839 KING PRESIDENT 5000 0 -5000
SQL>
If the LEAD would span a partition boundary, the default value is returned. In the following example we partition by department, so the SAL_NEXT column has a default value of "0" for the last row in each department.
SELECT deptno,
empno,
ename,
job,
sal,
LEAD(sal, 1, 0) OVER (PARTITION BY deptno ORDER BY sal) AS sal_next
FROM emp;
DEPTNO EMPNO ENAME JOB SAL SAL_NEXT
---------- ---------- ---------- --------- ---------- ----------
10 7934 MILLER CLERK 1300 2450
10 7782 CLARK MANAGER 2450 5000
10 7839 KING PRESIDENT 5000 0
20 7369 SMITH CLERK 800 1100
20 7876 ADAMS CLERK 1100 2975
20 7566 JONES MANAGER 2975 3000
20 7788 SCOTT ANALYST 3000 3000
20 7902 FORD ANALYST 3000 0
30 7900 JAMES CLERK 950 1250
30 7654 MARTIN SALESMAN 1250 1250
30 7521 WARD SALESMAN 1250 1500
30 7844 TURNER SALESMAN 1500 1600
30 7499 ALLEN SALESMAN 1600 2850
30 7698 BLAKE MANAGER 2850 0
SQL>
The "*" indicates the function supports the full analytic syntax, including the windowing clause.
For more information see:
Hope this helps. Regards Tim...
Back to normal view: https://oracle-base.com/articles/misc/lag-lead-analytic-functions