Getting random record from database with group by - php

Question

Hello i have a question on picking random entries from a database. I have 4 tables, products, bids and autobids, and users.
Products
-------
id 20,21,22,23,24(prime_key)
price...........
etc...........
users
-------
id(prim_key)
name user1,user2,user3
etc
bids
-------
product_id
user_id
created
autobids
--------
user_id
product_id
Now a multiple users can have an autobid on an product. So for the next bidder I want to select a random user from the autobid table
example of the query in language:
for each product in the autobid table I want a random user, which is not the last bidder.
On product 20 has user1,user2,user3 an autobidding.
On product 21 has user1,user2,user3 an autobidding
Then I want a resultset that looks for example like this
20 – user2
21 – user3
Just a random user. I tried miximg the GOUP BY (product_id) and making it RAND(), but I just can't get the right values from it. Now I am getting a random user, but all the values that go with it don't match.
Can someone please help me construct this query, I am using php and mysql

Answer 1

The first part of the solution is concerned with identifying the latest bid for each product: these eventually wind up in temporary table "latest_bid".
Then, we assign randon rank values to each autobid for each product - excluding the latest bid for each product. We then choose the highest rank value for each product, and then output the user_id and product_id of the autobids with those highest rank values.
create temporary table lastbids (product_id int not null,
created datetime not null,
primary key( product_id, created ) );
insert into lastbids
select product_id, max(created)
from bids
group by product_id;
create temporary table latest_bid ( user_id int not null,
product_id int not null,
primary key( user_id, product_id) );
insert into latest_bid
select product_id, user_id
from bids b
join lastbids lb on lb.product_id = b.product_id and lb.created = b.created;
create temporary table rank ( user_id int not null,
product_id int not null,
rank float not null,
primary key( product_id, rank ));
# "ignore" duplicates - it should not matter
# left join on latest_bid to exclude latest_bid for each product
insert ignore into rank
select user_id, product_id, rand()
from autobids a
left join latest_bid lb on a.user_id = lb.user_id and a.product_id = lb.product_id
where lb.user_id is null;
create temporary table choice
as select product_id,max(rank) choice
from rank group by product_id;
select user_id, res.product_id from rank res
join choice on res.product_id = choice.product_id and res.rank = choice.choice;

Answer 2

You can use the LIMIT statement in conjunction with server-side PREPARE.
Here is an example that selects a random row from the table mysql.help_category:
select #choice:= (rand() * count(*)) from mysql.help_category;
prepare rand_msg from 'select * from mysql.help_category limit ?,1';
execute rand_msg using #choice;
deallocate prepare rand_msg;
This will need refining to prevent #choice becoming zero, but the general idea works.
Alternatively, your application can construct the count itself by running the first select, and constructing the second select with a hard-coded limit value:
select count(*) from mysql.help_category;
# application then calculates limit value and constructs the select statement:
select * from mysql.help_category limit 5,1;