MySQL的樹木

Question

工作，我得到這個表：

CREATE TABLE 'category' (
    'id' INT(11) NOT NULL AUTO_INCREMENT, 
    'parent_category_id' INT(11) NULL DEFAULT NULL, 
    'name' VARCHAR(100) NOT NULL, 
    PRIMARY KEY ('id'), 
    INDEX 'parent_category_id' ('parent_category_id'), 
    CONSTRAINT 'category_ibfk_1' FOREIGN KEY ('parent_category_id') REFERENCES 'category' ('id') 
) COLLATE='utf8_general_ci' ENGINE=InnoDB; 

如何選擇具有小於3個小類（沒有深度）類別， 以及如何選擇哪些沒有子元素的類別。 謝謝！

Answer 1

不到3：

select parent.* 
from category parent left outer join 
    category child 
    on parent.id = child.parent_category_id 
group by parent.id 
having count(child.id) < 3 

對於無類：

select parent.* 
from category parent left outer join 
    category child 
    on parent.id = child.parent_category_id 
where child.id is null