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:
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.
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.
