SQL选择关系数是否大于X？

Question

I have 2 tables

" orders " has 1-to-N relation with " articles "

SELECT `orders`.* FROM `orders` INNER JOIN `articles` ON 
`articles`.`orders_id` = `orders`.`id` where ????

How can I make a sub-count on articles and get only orders with 4 or more articles?

Thanks you for help!

Answer 1

You can do this as:

SELECT o.*
FROM orders o INNER JOIN
     articles a
     ON a.orders_id = o.id
GROUP BY o.id
HAVING COUNT(*) >= 4; 

This is even a legitimate use of SELECT * with GROUP BY , because o.id is (presumably) the primary key on orders .

Another method is:

select o.*
from orders o
where (select count(*)
       from articles a
       where a.order_id = o.id
      ) >= 4;

This has the advantage that it can use an index on articles(order_id) , so it can have better performance.

Answer 2

Use aggregation with a filter on aggregated result by using HAVING clause

SELECT `o`.*
FROM `orders`  o
INNER JOIN `articles`  a
ON `a`.`orders_id` = `o`.`id`
GROUP BY o.id
HAVING COUNT(DISTINCT a.id) >=4