从 SQL 查询 Oracle 获取记录范围

Question

I want to use this SQL query to get only the records between 8 and 10:

select *
from(
SELECT a.*,rownum rn 
FROM ACTIVESESSIONSLOG  a
ORDER BY USERID ASC)
WHERE rn  >= 8 and rn <= 10

When I implement this SQL query into pagination I get every time 1 row on the second page no matter how many rows I have configured to be displayed into the pages. Is this SQL query valid?

This is the table structure:

-- TABLE ACTIVESESSIONSLOG

CREATE TABLE ACTIVESESSIONSLOG(
  ASESSIONID VARCHAR2(30 ) NOT NULL,
  USERID VARCHAR2(30 ),
  ACTIVITYSTART TIMESTAMP(6),
  ACTIVITYEND TIMESTAMP(6),
  ACTIVITY CLOB
)
/

Best wishes

Answer 1

rownum is applied before the ORDER BY so your query is almost certainly not doing what you expect. Your query is essentially asking for an arbitrary 3 rows and the ORDER BY isn't doing anything useful.

You could use the analytic function row_number instead, ie

SELECT *
  FROM (SELECT a.*,
               row_number() over (order by userid asc) rn
          FROM activeSessionsLog a)
 WHERE rn BETWEEN 8 AND 10

which will page through the results

SQL> ed
Wrote file afiedt.buf

  1  select empno, ename, job
  2    from (select e.*,
  3                 row_number() over (order by empno) rn
  4            from emp e)
  5*  where rn between 1 and 3
SQL> /

     EMPNO ENAME      JOB
---------- ---------- ---------
      7369 SMITH      CLERK
      7499 ALLEN      SALESMAN
      7521 WARD       SALESMAN

SQL> ed
Wrote file afiedt.buf

  1  select empno, ename, job
  2    from (select e.*,
  3                 row_number() over (order by empno) rn
  4            from emp e)
  5*  where rn between 4 and 8
SQL> /

     EMPNO ENAME      JOB
---------- ---------- ---------
      7566 JONES      MANAGER
      7654 MARTIN     SALESMAN
      7698 BLAKE      MANAGER
      7782 CLARK      MANAGER
      7788 SCOTT      ANALYST

SQL> ed
Wrote file afiedt.buf

  1  select empno, ename, job
  2    from (select e.*,
  3                 row_number() over (order by empno) rn
  4            from emp e)
  5*  where rn between 9 and 11
SQL> /

     EMPNO ENAME      JOB
---------- ---------- ---------
      7839 KING       PRESIDENT
      7844 TURNER     SALESMAN
      7876 ADAMS      CLERK

It may be more efficient, however, to do something like this where Oracle can use the inner rownum <= 10 predicate to know that it can stop sorting the data once it has identified the first 10 rows.

SELECT c.*
  FROM (SELECT b.*, rownum rn
          FROM (SELECT a.*
                  FROM activeSessionsLog a
                 ORDER BY userid asc) b
         WHERE rownum <= 10) c
 WHERE rn >= 8

Answer 2

Since Oracle 12c you can now use offset/fetch clause.

You can now write queries like:

SELECT *
FROM user_objects
ORDER object_name
OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY

Please review this new feature and also consider the improvements in the execution plan.