ROWNUM Functions

ROWNUM, INST_NUM

ROWNUM
INST_NUM()

The ROWNUM function returns the number representing the order of the records that will be generated by the query result. The first result record is assigned 1, and the second result record is assigned 2.

Return type:INT

ROWNUM and INST_NUM() can be used in the SELECT statement, and GROUPBY_NUM() can be used in the SELECT statement with GROUP BY clauses. The ROWNUM function can be used to limit the number of result records of the query in several ways. For example, it can be used to search only the first 10 records or to return even or odd number records.

The ROWNUM function has a result value as an integer, and can be used wherever an expression is valid such as the SELECT or WHERE clause. However, it is not allowed to compare the result of the ROWNUM function with the attribute or the correlated subquery.

Remark

  • The ROWNUM function specified in the WHERE clause works the same as the INST_NUM() function. Whereas INST_NUM() is a scalar function, GROUPBY_NUM() is a kind of an aggregate function. In a SELECT statement with a GROUP BY clause, GROUPBY_NUM() must be used instead of INST_NUM().
  • The ROWNUM function belongs to each SELECT statement. That is, if a ROWNUM function is used in a subquery, it returns the sequence of the subquery result while it is being executed. Internally, the result of the ROWNUM function is generated right before the searched record is written to the query result set. At this moment, the counter value that generates the serial number of the result set records increases.
  • If an ORDER BY clause is included in the SELECT statement, the value of the ROWNUM function specified in the WHERE clause is generated before sorting for the ORDER BY clause. If a GROUP BY clause is included in the SELECT statement, the value of the GROUPBY_NUM() function specified in the HAVING clause is calculated after the query results are grouped. After the sorting process is completed using the ORDER BY clause, you need to use the ORDERBY_NUM() function in the ORDER BY clause in order to get a sequence of the result records.
  • The ROWNUM function can also be used in SQL statements such as INSERT, DELETE and UPDATE in addition to the SELECT statement. For example, as in the query INSERT INTO table_name SELECT ... FROM ... WHERE ..., you can search for part of the row from one table and then insert it into another by using the ROWNUM function in the WHERE clause.

The following example shows how to retrieve country names ranked first to fourth based on the number of gold (gold) medals in the 1988 Olympics in the demodb database.

--Limiting 4 rows using ROWNUM in the WHERE condition
SELECT  * FROM
(SELECT nation_code FROM participant WHERE host_year = 1988
     ORDER BY gold DESC) AS T
WHERE ROWNUM <5;

  nation_code
======================
  'URS'
  'GDR'
  'USA'
  'KOR'

--Limiting 4 rows using FOR ORDERBY_NUM()
SELECT ROWNUM, nation_code FROM participant WHERE host_year = 1988
ORDER BY gold DESC
FOR ORDERBY_NUM() < 5;

       rownum  nation_code
===================================
          156  'URS'
          155  'GDR'
          154  'USA'
          153  'KOR'

--Unexpected results : ROWNUM operated before ORDER BY
SELECT ROWNUM, nation_code FROM participant
WHERE host_year = 1988 AND ROWNUM < 5
ORDER BY gold DESC;

       rownum  nation_code
===================================
            1  'ZIM'
            2  'ZAM'
            3  'ZAI'
            4  'YMD'

GROUPBY_NUM

GROUPBY_NUM()

The GROUPBY_NUM() function is used with the ROWNUM or INST_NUM() function to limit the number of result rows. The difference is that the GROUPBY_NUM() function is combined after the GROUP BY … HAVING clause to give order to a result that has been already sorted. In addition, while the INST_NUM() function is a scalar function, the GROUPBY_NUM() function is kind of an aggregate function.

Return type:INT

That is, when retrieving only some of the result rows by using ROWNUM in a condition clause of the SELECT statement that includes the GROUP BY clause, ROWNUM is applied first and then group sorting by GROUP BY is performed. On the other hand, when retrieving only some of the result rows by using the GROUPBY_NUM() function, ROWNUM is applied to the result of group sorting by GROUP BY.

The following example shows how to retrieve the fastest record in the previous five Olympic Games from the history table in the  demodb database.

--Group-ordering first and then limiting rows using GROUPBY_NUM()
SELECT host_year, MIN(score) FROM history
GROUP BY host_year HAVING GROUPBY_NUM() BETWEEN 1 AND 5;

    host_year  min(score)
===================================
         1968  '8.9'
         1980  '01:53.0'
         1984  '13:06.0'
         1988  '01:58.0'
         1992  '02:07.0'

--Limiting rows first and then Group-ordering using ROWNUM
SELECT host_year, MIN(score) FROM history
WHERE ROWNUM BETWEEN 1 AND 5 GROUP BY host_year;

    host_year  min(score)
===================================
         2000  '03:41.0'
         2004  '01:45.0'

ORDERBY_NUM

ORDERBY_NUM()

The ORDERBY_NUM() function is used with the ROWNUM() or INST_NUM() function to limit the number of result rows. The difference is that the ORDERBY_NUM() function is combined after the ORDER BY clause to give order to a result that has been already sorted. That is, when retrieving only some of the result rows by using ROWNUM in a condition clause of the SELECT statement that includes the ORDER BY clause, ROWNUM is applied first and then group sorting by ORDER BY is performed. On the other hand, when retrieving only some of the result rows by using the ORDER_NUM() function, ROWNUM is applied to the result of sorting by ORDER BY.

Return type:INT

The following example shows how to retrieve athlete names ranked 3rd to 5th and their records in the history table in the demodb database.

--Ordering first and then limiting rows using FOR ORDERBY_NUM()
SELECT athlete, score FROM history
ORDER BY score FOR ORDERBY_NUM() BETWEEN 3 AND 5;

  athlete               score
============================================
  'Luo Xuejuan'         '01:07.0'
  'Rodal Vebjorn'       '01:43.0'
  'Thorpe Ian'          '01:45.0'

--Limiting rows first and then Ordering using ROWNUM
SELECT athlete, score FROM history
WHERE ROWNUM BETWEEN 3 AND 5 ORDER BY score;

  athlete               score
============================================
  'Thorpe Ian'          '01:45.0'
  'Thorpe Ian'          '03:41.0'
  'Hackett Grant'       '14:43.0'

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