使用asp.net c＃从mysql数据库中选择1个随机数

Question

How can i select 1 random number from database thats ID is autoincrement field.I have 25,000[ID] records in database.Each every time select one new random ID. How can get it.

Answer 1

You can use the following query to generate a random number between 0 and COUNT -1 of your table rows:

SELECT ROUND((RAND() * (SELECT COUNT(*) FROM mytable)-1)) 

Then use this number as an offset in the LIMIT clause of your query to get a random row from your table.

Using a prepared statement , you can do:

SET @offset = (SELECT ROUND((RAND() * (SELECT COUNT(*) FROM mytable)-1)));

PREPARE STMT FROM 'SELECT * FROM mytable LIMIT ?, 1';
EXECUTE STMT USING @offset;

Note that use of the prepared statement is mandatory since as documented in the manual :

LIMIT takes one or two numeric arguments, which must both be non-negative integer constants ( except when using prepared statements ).

Demo here

Answer 2

Use following sql(slow)

'SELECT * FROM tablename ORDER BY RAND() LIMIT 1'

or you can use(fast)

SELECT * FROM myTable, (SELECT FLOOR(MAX(myTable.id) * RAND()) AS randId FROM myTable) AS someRandId WHERE myTable.id = someRandId.randId 

The inner SELECT gives you a random id in the right range. The outer SELECT looks for the right row in the table.