如何在每次执行SQL查询时选择随机唯一记录

Question

I have a table "masterurls" it has morethan 1 million records. I want to fetch random records each time the query executed. It should not have any of the records that were fetched in previous executions. I'm already have this query:

SELECT m.url FROM masterurls ORDER BY RAND() LIMIT 200

The problem is the above query returns only first 200 hundred records and randomizes it each time.

Answer 1

How are you going to know if the url is already accessed before. My best suggestion would be setting a flag to know this in the table. Add a field like view in the table which will accept two values 1 or 0, 1 for already accessed and 0 for not accessed. Then you could use

SELECT m.url FROM masterurls m WHERE view='1' ORDER BY RAND() LIMIT 200;

Answer 2

Since you can pass a seed parameter to the RAND() function, you can "paginate" the random results by generating a seed before the first page.

Sample code: For the first page (varies by language):

int seed = Math.abs(new Random().nextInt());

SQL query:

SELECT url FROM masterurls ORDER BY RAND({seed}) LIMIT 200;

Store the seed somewhere (for web-based applications you can use a url parameter or session). For the next pages:

SELECT url FROM masterurls ORDER BY RAND({seed}) LIMIT 200 * {pageNumber}, 200;

Note: Sorting by RAND() is a heavy operation, you might be better off storing a indexed column with the Hash Code of the url, then using a module-based or other random functions.