SQL获取玛丽亚数据库中的每个类别数据

Question

I need to fetch 4 random values from each category. What should be the correct sql syntax for maria db. I have attached one image of table structure.

Please click here to check the structure

Should i write some procedure or i can do it with basic sql syntax?

Answer 1

You can do that with a SQL statement if you only have a few rows:

SELECT id, question, ... FROM x1 ORDER BY rand() LIMIT 1

This works fine if you have only a few rows - as soon as you have thousands of rows the overhead for sorting the rows becomes important, you have to sort all rows for getting only one row.

A trickier but better solution would be:

SELECT id, question from x1  JOIN (SELECT CEIL(RAND() * (SELECT(MAX(id)) FROM x1)) AS id) as id using(id);

Running EXPLAIN on both SELECTS will show you the difference...

If you need random value for different categories combine the selects via union and add a where clause

Answer 2

http://mysql.rjweb.org/doc.php/groupwise_max#top_n_in_each_group

But then ORDER BY category, RAND() . (Your category is the blog's province .)

Notice how it uses @variables to do the counting.

If you have MariaDB 10.2, then use one of its Windowing functions.

Answer 3

SELECT column FROM table WHERE category_id = XXX 
ORDER BY RAND() 
LIMIT 4

do it for all categories