例1：查询某个文章及其对应的评论（单个详情）
SELECT id, name, (SELECT comment_id, title, content FROM B WHERE user_id = A.id ORDER BY create_time DESC LIMIT 1) FROM A;
例2：查询分类表中，每种分类各包含多少商品（汇总）
SELECT category_id, (SELECT count(goods_id) FROM Goods WHERE category_id = A.category_id) FROM category;