Access Values from Previous or Following Rows in Oracle

You want to use Oracle aggregate function XMLAGG for string aggregation.?

Solution:

You would like to include calculations based on preceding and following rows in the result set.

Oracle supports the LAG and LEAD analytical functions to provide access to multiple rows in a table, utilizing preceding or following logic and you won’t need to resort to joining the source data to itself. To demonstrate the usage we will use the students data.

The LAG function can be used to see which student/s joining followed another, and also to calculate the elapsed time between joining.

SQL: Identify the student joining information

Example

SELECT first_name,
      last_name,
      join_date,
      lag(join_date, 1, '01-JAN-2001') OVER (ORDER BY join_date)             AS prev_join_data,
      join_date - lag(join_date, 1, '01-JAN-2001') OVER (ORDER BY join_date) AS join_date_diff
FROM students
ORDER BY join_date;

Output

|first_name | last_name | join_date | prev_join_data | joining_date_diff
| ---       | ---        | ---       |  ---          |  --- |
WILLIAMS   |ROBERT      |13/JAN/01      |01/JAN/01      |12
WILSON      |THOMAS      |16/AUG/02      |13/JAN/01      |580
RODRIGUEZ   |JOSEPH      |17/AUG/0      2|16/AUG/02      |1
SMITH      |JAMES      |17/JUN/03      |17/AUG/02      |304
MILLER      |DAVID      |25/JUN/05      |17/JUN/03      |739
JOHNSON      |JOHN      |21/SEP/05      |25/JUN/05      |88
MARTINEZ      |CHRISTOPHER|28/SEP/05      |21/SEP/05   |7
ANDERSON      |DANIEL      |30/SEP/05      |28/SEP/05   |2
BROWN      |MICHAEL      |03/JAN/06      |30/SEP/05   |95
DAVIS      |RICHARD      |05/FEB/06      |03/JAN/06   |33
TAYLOR      |PAUL      |07/MAR/06      |05/FEB/06   |30
GARCIA      |CHARLES      |07/FEB/07   |07/MAR/06   |337
JONES      |WILLIAM      |21/MAY/07   |07/FEB/07   |103
THOMAS      |MARK      |07/DEC/07   |21/MAY/07      |200

The LAG and LEAD functions are similar to most other analytical functions in that they operate once the non analytic portion of the query is complete. Oracle performs a second pass over the intermediate result set to apply any analytical predicates.

The LEAD function works the same way as LAG function, but instead tracks following rows rather than preceding ones.

To demonstrate, We will show joins along with the join_date of the next student, and a similar elapsed time window between their joindates.

Example

SELECT first_name  ,
      last_name  ,
      join_date   ,
      lead(join_date, 1, sysdate) OVER (ORDER BY join_date)              AS next_join_date   ,
      lead(join_date, 1, sysdate) OVER (ORDER BY join_date) -  join_date  AS days_between_join
FROM students
ORDER BY join_date

Output

Data Preparation: Data used for the problem is shown below.

Example

DROP TABLE students;
COMMIT;

CREATE TABLE students
    ( student_id     NUMBER(6)
    , first_name     VARCHAR2(20)
    , last_name      VARCHAR2(25)
    , email          VARCHAR2(40)
    , phone_number   VARCHAR2(20)
    , join_date      DATE
    , class_id       VARCHAR2(20)  
    , fees           NUMBER(8,2)
    , professor_id   NUMBER(6)
    , department_id  NUMBER(4)
    ) ;

Example

CREATE UNIQUE INDEX stu_id_pk ON students (student_id) ;
INSERT INTO students VALUES (100,'SMITH','JAMES','[email protected]','111.111.1245',TO_DATE('17-06-2003','DD-MM-YYYY'),'INS_CHAIRMAN',24000,NULL,NULL);
INSERT INTO students VALUES (101,'JOHNSON','JOHN','[email protected]','111.111.1246',TO_DATE('21-09-2005','DD-MM-YYYY'),'INS_VP',17000,100,90);
INSERT INTO students VALUES (102,'WILLIAMS','ROBERT','[email protected]','111.111.1247',TO_DATE('13-01-2001','DD-MM-YYYY'),'INS_VP',17000,100,90);
INSERT INTO students VALUES (103,'BROWN','MICHAEL','[email protected]','111.111.1248',TO_DATE('03-01-2006','DD-MM-YYYY'),'INS_STAFF',9000,102,60);
INSERT INTO students VALUES (104,'JONES','WILLIAM','[email protected]','111.111.1249',TO_DATE('21-05-2007','DD-MM-YYYY'),'INS_STAFF',6000,103,60);
INSERT INTO students VALUES (105,'MILLER','DAVID','[email protected]','111.111.1250',TO_DATE('25-06-2005','DD-MM-YYYY'),'INS_STAFF',4800,103,60);
INSERT INTO students VALUES (106,'DAVIS','RICHARD','[email protected]','111.111.1251',TO_DATE('05-02-2006','DD-MM-YYYY'),'INS_STAFF',4800,103,60);
INSERT INTO students VALUES (107,'GARCIA','CHARLES','[email protected]','111.111.1252',TO_DATE('07-02-2007','DD-MM-YYYY'),'INS_STAFF',4200,103,60);
INSERT INTO students VALUES (108,'RODRIGUEZ','JOSEPH','[email protected]','111.111.1253',TO_DATE('17-08-2002','DD-MM-YYYY'),'CL_PHY',12008,101,100);
INSERT INTO students VALUES (109,'WILSON','THOMAS','[email protected]','111.111.1254',TO_DATE('16-08-2002','DD-MM-YYYY'),'CL_MATH',9000,108,100);
INSERT INTO students VALUES (110,'MARTINEZ','CHRISTOPHER','[email protected]','111.111.1255',TO_DATE('28-09-2005','DD-MM-YYYY'),'CL_MATH',8200,108,100);
INSERT INTO students VALUES (111,'ANDERSON','DANIEL','[email protected]','111.111.1256',TO_DATE('30-09-2005','DD-MM-YYYY'),'CL_MATH',7700,108,100);
INSERT INTO students VALUES (112,'TAYLOR','PAUL','[email protected]','111.111.1257',TO_DATE('07-03-2006','DD-MM-YYYY'),'CL_MATH',7800,108,100);
INSERT INTO students VALUES (113,'THOMAS','MARK','[email protected]','111.111.1258',TO_DATE('07-12-2007','DD-MM-YYYY'),'CL_MATH',6900,108,100);

COMMIT;