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Simple Random Samples from a (My)Sql database
Hi!
Let say that I have a table of users. I want a query that shuffle the result and only show 5 users. How do I do that in MySQL query without using any php?
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Ways to avoid performance issues associated with CROSS JOINs: Use another JOIN (INNER/LEFT/RIGHT) with 2 ON conditions. Use the GROUP BY clause to pre-aggregate data.
To avoid a Cartesian product, you must specify how the tables should be combined. Typically, you want to pair rows based on matching values in one or more key columns of each table.
You can use rand(), but the performance is terrible
select * from users order by rand() limit 5; <-- slow
I would suggest, store list of all user id into an serialize array and cache into a disk-file. (periodically update)
So, you can un-serialize it back using PHP, and use PHP array_rand to pick 5 random users.
To fetch the full information, you can do
select * from users where user_id in(...); <-- very fast
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