DB2的 LIMIT 等价物

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() (since v5r4) 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;

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

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.

@elcool's solution is a smart idea, but you need to know total number of rows (which can even change while you are executing the query!). So I propose a modified version, which unfortunately needs 3 subqueries instead of 2:

select * from (
select * from (
select * from MYLIB.MYTABLE
order by MYID asc
fetch first {last} rows only
) I
order by MYID desc
fetch first {length} rows only
) II
order by MYID asc

where {last} should be replaced with row number of the last record I need and {length} should be replaced with the number of rows I need, calculated as last row - first row + 1.

E.g. if I want rows from 10 to 25 (totally 16 rows), {last} will be 25 and {length} will be 25-10+1=16.

There are 2 solutions to paginate efficiently on a DB2 table :

1 - the technique using the function row_number() and the clause OVER which has been presented on another post ("SELECT row_number() OVER ( ORDER BY ... )"). On some big tables, I noticed sometimes a degradation of performances.

2 - the technique using a scrollable cursor. The implementation depends of the language used. That technique seems more robust on big tables.

I presented the 2 techniques implemented in PHP during a seminar next year. The slide is available on this link : http://gregphplab.com/serendipity/uploads/slides/DB2_PHP_Best_practices.pdf

Sorry but this document is only in french.

Here's the solution I came up with:

select FIELD from TABLE where FIELD > LASTVAL order by FIELD fetch first N rows only;

By initializing LASTVAL to 0 (or '' for a text field), then setting it to the last value in the most recent set of records, this will step through the table in chunks of N records.

Support for OFFSET and LIMIT was recently added to DB2 for i 7.1 and 7.2. You need the following DB PTF group levels to get this support:

• SF99702 level 9 for IBM i 7.2
• SF99701 level 38 for IBM i 7.1

See here for more information: OFFSET and LIMIT documentation, DB2 for i Enhancement Wiki

Theres these available options:-

DB2 has several strategies to cope with this problem.
You can use the "scrollable cursor" in feature.
In this case you can open a cursor and, instead of re-issuing a query you can FETCH forward and backward.
This works great if your application can hold state since it doesn't require DB2 to rerun the query every time.
You can use the ROW_NUMBER() OLAP function to number rows and then return the subset you want.
This is ANSI SQL
You can use the ROWNUM pseudo columns which does the same as ROW_NUMBER() but is suitable if you have Oracle skills.
You can use LIMIT and OFFSET if you are more leaning to a mySQL or PostgreSQL dialect.

Try this

SELECT * FROM
(
SELECT T.*, ROW_NUMBER() OVER() R FROM TABLE T
)
WHERE R BETWEEN 10000 AND 20000

The LIMIT clause allows you to limit the number of rows returned by the query. The LIMIT clause is an extension of the SELECT statement that has the following syntax:

SELECT select_list
FROM table_name
ORDER BY sort_expression
LIMIT n [OFFSET m];

In this syntax:

• n is the number of rows to be returned.
• m is the number of rows to skip before returning the n rows.

Another shorter version of LIMIT clause is as follows:

LIMIT m, n;

This syntax means skipping m rows and returning the next n rows from the result set.

A table may store rows in an unspecified order. If you don’t use the ORDER BY clause with the LIMIT clause, the returned rows are also unspecified. Therefore, it is a good practice to always use the ORDER BY clause with the LIMIT clause.

See Db2 LIMIT for more details.