MySQL笔记 连接

创建数据表的同时给数据表添加记录：

CREATE TABLE tdb_goods_brands
     (
    brand_id INT PRIMARY KEY AUTO_INCREMENT,
     brand_name CHAR(40) NOT NULL
     )

     SELECT brand_name FROM tdb_goods GROUP BY brand_name;

-- 查找所有分类及其父类


  SELECT s.type_id,s.type_name,p.type_name FROM tdb_goods_types AS s LEFT JOIN tdb_goods_types AS  p ON s.parent_id = p.type_id;
   
-- 查找所有分类及其子类


  SELECT p.type_id,p.type_name,s.type_name FROM tdb_goods_types AS p LEFT JOIN tdb_goods_types AS  s ON s.parent_id = p.type_id;


-- 查找所有分类及其子类的数目


  SELECT p.type_id,p.type_name,count(s.type_name) AS children_count FROM tdb_goods_types AS p LEFT JOIN tdb_goods_types AS s ON s.parent_id = p.type_id GROUP BY p.type_name ORDER BY p.type_id;


-- 为tdb_goods_types添加child_count字段


  ALTER TABLE tdb_goods_types ADD child_count MEDIUMINT UNSIGNED NOT NULL DEFAULT 0;


-- 将刚才查询到的子类数量更新到tdb_goods_types数据表


  UPDATE tdb_goods_types AS t1 INNER JOIN ( SELECT p.type_id,p.type_name,count(s.type_name) AS children_count FROM tdb_goods_types AS p 


                                            LEFT JOIN tdb_goods_types AS s ON s.parent_id = p.type_id 
                             
                                            GROUP BY p.type_name 


                                            ORDER BY p.type_id ) AS t2 


  ON  t1.type_id = t2.type_id 


  SET t1.child_count = t2.children_count;




-- 复制编号为12,20的两条记录


  SELECT * FROM tdb_goods WHERE goods_id IN (19,20);




-- INSERT ... SELECT实现复制


  INSERT tdb_goods(goods_name,cate_id,brand_id) SELECT goods_name,cate_id,brand_id FROM tdb_goods WHERE goods_id IN (19,20);


-- 查找重复记录


  SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name) >= 2;


-- 删除重复记录


  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;