Equivalent of LIMIT for DB2

Question

How do you do LIMIT in DB2 for iSeries?

I have a table with more than 50,000 records and I want to return records 0 to 10,000, and records 10,000 to 20,000.

I know in SQL you write LIMIT 0,10000 at the end of the query for 0 to 10,000 and LIMIT 10000,10000 at the end of the query for 10000 to 20,000

So, how is this done in DB2? Whats the code and syntax? (full query example is appreciated)

Answer 1

Using FETCH FIRST [n] ROWS ONLY:

http://publib.boulder.ibm.com/infocenter/dzichelp/v2r2/index.jsp?topic=/com.ibm.db29.doc.perf/db2z_fetchfirstnrows.htm

SELECT LASTNAME, FIRSTNAME, EMPNO, SALARY
  FROM EMP
  ORDER BY SALARY DESC
  FETCH FIRST 20 ROWS ONLY;

To get ranges, you'd have to use ROW_NUMBER() and use that within the WHERE clause: (stolen from here: http://www.justskins.com/forums/db2-select-how-to-123209.html)

SELECT code, name, address
FROM ( SELECT row_number() OVER ( ORDER BY code ) AS rid,
code, name, address
FROM contacts
WHERE name LIKE '%Bob%' ) AS t
WHERE t.rid BETWEEN 20 AND 25;

Answer 2

Developed this method:

You NEED a table that has an unique value that can be ordered.

If you want rows 10,000 to 25,000 and your Table has 40,000 rows, first you need to get the starting point and total rows:

int start = 40000 - 10000;

int total = 25000 - 10000;

And then pass these by code to the query:

SELECT * FROM 
(SELECT * FROM schema.mytable 
ORDER BY userId DESC fetch first {start} rows only ) AS mini 
ORDER BY mini.userId ASC fetch first {total} rows only

Answer 3

You should also consider the OPTIMIZE FOR n ROWS clause. More details on all of this in the DB2 LUW documentation in the Guidelines for restricting SELECT statements topic:

• The OPTIMIZE FOR clause declares the intent to retrieve only a subset of the result or to give priority to retrieving only the first few rows. The optimizer can then choose access plans that minimize the response time for retrieving the first few rows.

Answer 4

ROW_NUMBER() was only implemented in iSeries DB2 V5R4. For previous versions try using RRN() which is similar.