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LAG and LEAD Analytic Functions

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.

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;

Introduction

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)

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>

LAG

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>

LEAD

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>

Quick Links

The "*" indicates the function supports the full analytic syntax, including the windowing clause.

AVG * CLUSTER_DETAILS CLUSTER_DISTANCE CLUSTER_ID CLUSTER_PROBABILITY
CLUSTER_SET CORR * COUNT * COVAR_POP * COVAR_SAMP *
CUME_DIST DENSE_RANK FEATURE_DETAILS FEATURE_ID FEATURE_SET
FEATURE_VALUE FIRST FIRST_VALUE * LAG LAST
LAST_VALUE * LEAD LISTAGG MATCH_RECOGNIZE MAX *
MEDIAN MIN * NTH_VALUE * NTILE PERCENT_RANK
PERCENTILE_CONT PERCENTILE_DISC PREDICTION PREDICTION_COST PREDICTION_DETAILS
PREDICTION_PROBABILITY PREDICTION_SET RANK RATIO_TO_REPORT REGR_ (Linear Regression) Functions *
ROW_NUMBER STDDEV * STDDEV_POP * STDDEV_SAMP * SUM *
VAR_POP * VAR_SAMP * VARIANCE * String Aggregation Top-N Queries

For more information see:

Hope this helps. Regards Tim...

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