mysql - mysql 与树一起工作

Question

我得到了这张桌子：

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;

如何选择具有少于 3 个子类别（无深度）的类别，以及如何选择没有子元素的类别。谢谢！

Answer 1

少于三个：

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

对于没有类别：

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