The ROWNUM pseudo-column v16

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ROWNUM is a pseudo-column that's assigned an incremental, unique integer value for each row based on the order the rows were retrieved from a query. Therefore, the first row retrieved has ROWNUM of 1, the second row has ROWNUM of 2, and so on.

Limiting the number or rows returned

You can use this feature to limit the number of rows retrieved by a query:

SELECT empno, ename, job FROM emp WHERE ROWNUM < 5;
Output
 empno | ename |   job
-------+-------+----------
 7369  | SMITH | CLERK
 7499  | ALLEN | SALESMAN
 7521  | WARD  | SALESMAN
 7566  | JONES | MANAGER
(4 rows)

The ROWNUM value is assigned to each row before any sorting of the result set takes place. Thus, the result set is returned in the order given by the ORDER BY clause. However, the ROWNUM values might not be in ascending order:

SELECT ROWNUM, empno, ename, job FROM emp WHERE ROWNUM < 5 ORDER BY ename;
Output
 rownum | empno | ename |   job
--------+-------+-------+----------
      2 |  7499 | ALLEN | SALESMAN
      4 |  7566 | JONES | MANAGER
      1 |  7369 | SMITH | CLERK
      3 |  7521 | WARD  | SALESMAN
(4 rows)

Adding a sequence number to rows in a table

The following example shows how you can add a sequence number to every row in the jobhist table. First add a column named seqno to the table. Then set seqno to ROWNUM in the UPDATE command.

ALTER TABLE jobhist ADD seqno NUMBER(3);
UPDATE jobhist SET seqno = ROWNUM;

The following SELECT command shows the new seqno values:

SELECT seqno, empno, TO_CHAR(startdate,'DD-MON-YY') AS start, job FROM
jobhist;
Output
seqno  | empno |   start   |    job
-------+-------+-----------+-----------
     1 |  7369 | 17-DEC-80 | CLERK
     2 |  7499 | 20-FEB-81 | SALESMAN
     3 |  7521 | 22-FEB-81 | SALESMAN
     4 |  7566 | 02-APR-81 | MANAGER
     5 |  7654 | 28-SEP-81 | SALESMAN
     6 |  7698 | 01-MAY-81 | MANAGER
     7 |  7782 | 09-JUN-81 | MANAGER
     8 |  7788 | 19-APR-87 | CLERK
     9 |  7788 | 13-APR-88 | CLERK
    10 |  7788 | 05-MAY-90 | ANALYST
    11 |  7839 | 17-NOV-81 | PRESIDENT
    12 |  7844 | 08-SEP-81 | SALESMAN
    13 |  7876 | 23-MAY-87 | CLERK
    14 |  7900 | 03-DEC-81 | CLERK
    15 |  7900 | 15-JAN-83 | CLERK
    16 |  7902 | 03-DEC-81 | ANALYST
    17 |  7934 | 23-JAN-82 | CLERK
(17 rows)
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