MYSQL（五）

连接：

链接，左右外连接，左右内链接

无限极分类表：

CREATE TABLE tdb_goods_type(

type_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,

type_name VARCHAR(20) NOT NULL,

parent_id SMALLINT UNSIGNED NOT NULL DEFAULT 0);

自身链接：

想象右边有个子表：

SELECT s.type_id,s.stype_name,p.type_name FROM tdb_goods_types AS s INNER JOINtdb_goods_types ON s.parent_id = p.type_id

多表删除：

DELETE t1 FROM tdb_goods AS t1 left join (SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name  HAVING count(goods_name>=2) AS t2 ON t1.goods_name = t2.goods_name WHERE t1.goods_id>t2.goods_id;