如何从Oracle中的上一行或下一行访问值？
您想使用Oracle聚合函数XMLAGG进行字符串聚合。
解：
您想在结果集中基于前一行和后一行进行计算。
Oracle支持LAG和LEAD分析功能，以利用前面或后面的逻辑提供对表中多行的访问，您无需诉诸于将源数据与其自身相连。为了演示用法，我们将使用学生数据。
LAG功能可用于查看哪个学生加入了另一个学生，还可以计算两次加入之间的经过时间。
SQL：识别学生的加入信息
示例
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;输出结果
|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
LAG和LEAD函数与大多数其他分析函数相似，它们在查询的非分析部分完成后便会运行。Oracle对中间结果集进行第二次传递以应用任何分析谓词。
LEAD函数的工作方式与LAG函数相同，但是它跟踪的是后面的行而不是前面的行。
为了演示，我们将显示连接以及下一个学生的join_date以及他们的joindate之间类似的经过时间窗口。
示例
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输出结果
数据准备：用于该问题的数据如下所示。
示例
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)
) ;示例
CREATE UNIQUE INDEX stu_id_pk ON students (student_id) ;
INSERT INTO students VALUES (100,'SMITH','JAMES','SMITH.JAMES@notreal.com','111.111.1245',TO_DATE('17-06-2003','DD-MM-YYYY'),'INS_CHAIRMAN',24000,NULL,NULL);
INSERT INTO students VALUES (101,'JOHNSON','JOHN','JOHNSON.JOHN@notreal.com','111.111.1246',TO_DATE('21-09-2005','DD-MM-YYYY'),'INS_VP',17000,100,90);
INSERT INTO students VALUES (102,'WILLIAMS','ROBERT','WILLIAMS.ROBERT@notreal.com','111.111.1247',TO_DATE('13-01-2001','DD-MM-YYYY'),'INS_VP',17000,100,90);
INSERT INTO students VALUES (103,'BROWN','MICHAEL','BROWN.MICHAEL@notreal.com','111.111.1248',TO_DATE('03-01-2006','DD-MM-YYYY'),'INS_STAFF',9000,102,60);
INSERT INTO students VALUES (104,'JONES','WILLIAM','JONES.WILLIAM@notreal.com','111.111.1249',TO_DATE('21-05-2007','DD-MM-YYYY'),'INS_STAFF',6000,103,60);
INSERT INTO students VALUES (105,'MILLER','DAVID','MILLER.DAVID@notreal.com','111.111.1250',TO_DATE('25-06-2005','DD-MM-YYYY'),'INS_STAFF',4800,103,60);
INSERT INTO students VALUES (106,'DAVIS','RICHARD','DAVIS.RICHARD@notreal.com','111.111.1251',TO_DATE('05-02-2006','DD-MM-YYYY'),'INS_STAFF',4800,103,60);
INSERT INTO students VALUES (107,'GARCIA','CHARLES','GARCIA.CHARLES@notreal.com','111.111.1252',TO_DATE('07-02-2007','DD-MM-YYYY'),'INS_STAFF',4200,103,60);
INSERT INTO students VALUES (108,'RODRIGUEZ','JOSEPH','RODRIGUEZ.JOSEPH@notreal.com','111.111.1253',TO_DATE('17-08-2002','DD-MM-YYYY'),'CL_PHY',12008,101,100);
INSERT INTO students VALUES (109,'WILSON','THOMAS','WILSON.THOMAS@notreal.com','111.111.1254',TO_DATE('16-08-2002','DD-MM-YYYY'),'CL_MATH',9000,108,100);
INSERT INTO students VALUES (110,'MARTINEZ','CHRISTOPHER','MARTINEZ.CHRISTOPHER@notreal.com','111.111.1255',TO_DATE('28-09-2005','DD-MM-YYYY'),'CL_MATH',8200,108,100);
INSERT INTO students VALUES (111,'ANDERSON','DANIEL','ANDERSON.DANIEL@notreal.com','111.111.1256',TO_DATE('30-09-2005','DD-MM-YYYY'),'CL_MATH',7700,108,100);
INSERT INTO students VALUES (112,'TAYLOR','PAUL','TAYLOR.PAUL@notreal.com','111.111.1257',TO_DATE('07-03-2006','DD-MM-YYYY'),'CL_MATH',7800,108,100);
INSERT INTO students VALUES (113,'THOMAS','MARK','THOMAS.MARK@notreal.com','111.111.1258',TO_DATE('07-12-2007','DD-MM-YYYY'),'CL_MATH',6900,108,100);
COMMIT;