How to select a random sample of records from MySQL?
===

Consider the following table:

    tweets
    -----------------------
    id  tweet         class
    -----------------------
    1   Foo bar baz!  2
    2   Lorem ipsum   2
    3   Foobar lorem  3
    4   Activi set    1
    5   Baz baz bar?  3
    7   Dolor mez foo 3
    8   Samet set bar 1

In reality, the table has 600,000 records, but as indicated in the example table, some `id`s have been deleted (so the highest `id` > 600,000).  `class` can be either `1`, `2` or `3`. 

I need a **random selection** 1200 tweets, 400 of each `class`. Is this possible to do with one query?
