How to fetch data from oracle in batches from java in equal batches

问题

I have a table -

emp_record

which has 40,000 records And I want to fetch the records from java code and below is the requirement -

At a time only 1000 records should be returned
In the next hit next 1000 records
continue till all the records exhaust
The SQL query should not be nested, like

 select *
 from(
 SELECT a.*,rownum rn 
 FROM distributor  a)
 WHERE rn  >= 3000 and rn < 4000;

Any sort of help is much appreciated.

回答1:

This sounds very artifical as 40.000 records is nothing. I would just read them in one query and perhaps hand them out in batches. In that case you can use statement.setFetchSize(1000) to make the JDBC driver fetch 1000 records at a time and position that as solving the requirement.

Alternatively if you are on Oracle 12 you can use:

select * from distributor
order by something_unique
offset x rows fetch next 1000 rows only

where x is the starting position. It does the same thing as the rownum construction, but with a much nicer syntax.

回答2:

You could use a query that uses ROWNUM by first ordering the rows by a primary key/unique key. Idea from the query here : Best practice for pagination in Oracle?

SELECT * 
  FROM (SELECT A.*, rownum rn
          FROM (SELECT *
                  FROM emp_record
                 ORDER BY pri_key_col) A
         WHERE rownum <= :n * 1000)
 WHERE rn > (:n - 1) * 1000;

pri_key_col : Primary key/ unique key column of your table.
n : number indicating the batch. If n = 1, it returns first 1000 records ordered that way, n=2 gives next 1000 and so on.

来源：https://stackoverflow.com/questions/50562167/how-to-fetch-data-from-oracle-in-batches-from-java-in-equal-batches

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