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February 5, 2015

2/05/2015 10:33:00 PM


They are also both referred to as pseudo-columns. That is, they are not "real" columns that will show up when you DESC a table. They don't actually exist anywhere in the database. But they're available for you to use.

In fact, ROWNUM only exists for a row once it is retrieved from a query. It represents the sequential order in which Oracle has retrieved the row. Therefore it will always exist, be at least 1, and be unique (among the rows returned by the query). Obviously it will change from query-to-query. Let's look at a quick example:
scott@Robert> SELECT ROWNUM, ENAME, SAL

  2  FROM EMP;



    ROWNUM ENAME             SAL

---------- ---------- ----------

         1 SMITH             800

         2 ALLEN            1600

         3 WARD             1250

         4 JONES            2975

         5 MARTIN           1250

         6 BLAKE            2850

         7 CLARK            2450

         8 SCOTT            3000

         9 VOLLMAN          5000

        10 TURNER           1500

        11 ADAMS            1100

        12 JAMES             950

        13 FORD             3000

        14 MILLER           1300


Ok so let's say we want the 5 highest paid employees. Should be easy:
scott@Robert> SELECT ROWNUM, ENAME, SAL

  2  FROM EMP

  3  WHERE ROWNUM < 6

  4  ORDER BY SAL DESC;



    ROWNUM ENAME             SAL

---------- ---------- ----------

         4 JONES            2975

         2 ALLEN            1600

         3 WARD             1250

         5 MARTIN           1250

         1 SMITH             800


Whoops! Turns out ROWNUM is assigned before results are ordered, not after. Knowing that, we can write it like this:
scott@Robert> SELECT ENAME, SAL

  2  FROM (SELECT ENAME, SAL FROM EMP ORDER BY SAL DESC) E

  3  WHERE ROWNUM < 6;



ENAME             SAL

---------- ----------

VOLLMAN          5000

SCOTT            3000

FORD             3000

JONES            2975

BLAKE            2850


What about ROWID? ROWID actually represents the physical location of the record/row in the database. That being the case, it is (according to Oracle documentation) the fastest way to retrieve a particular row. Faster than an index, even.

Can you use ROWID to differentiate between duplicate rows?
Yes, you can. Since it actually represents the physical location of a row, no two rows within the same table will have the same ROWID. Notice the caveat I added: within the same table. If you're using clustering, two records from different tables could theoretically share the same ROWID.

Do ROWIDs change?
Yes, especially with index organized or partitioned tables. Because ROWIDs represent the physical location of a record/row, the ROWID will change every time the record is physically moved.

Can you use ROWID as a primary key?
No, that's not advisable. While the ROWID will be unique, you would ideally want to use a primary key that doesn't change.

How do you use ROWID to figure out what was the last record that was processed?
Using DBMS_SQL.LAST_ROW_ID to get the ROWID of the last row processed.

Can I use rowid as a primary key

SQL> create table t1(c1 number);

Table created.

SQL> alter table t1 add constraint pk_t1
2 primary key (rowid);
primary key (rowid)
*
ERROR at line 2:
ORA-00904: "ROWID": invalid identifier


Not only not advisable, not doable. You won't be able to reference it in a foreign key constraint either.