MySQL学习笔记（18）——多表更新、连接

前置工作

新建一张物品类别表：

 CREATE TABLE IF NOT EXISTS tdb_goods_cates
     (
     cate_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
     cate_name VARCHAR(40) NOT NULL
     );

从tdb_goods表中查出所有类别：

 SELECT goods_cate FROM tdb_goods GROUP BY goods_cate;

使用INSERT tb_name SELECT …添加记录：

INSERT tdb_goods_cates(cate_name) SELECT goods_cate FROM tdb_goods GROUP BY goods_cate;

于是，tdb_goods_cates数据如下:

---

多表更新

参照另外的表来对本表的更新。
UPDATE table_references
SET col_name1={expr1 | DEFAULT}
[, col_name2={expr2 | DEFAULT}] …
[WHERE where_condition]

连接

MySQL在SELECT语句、多表更新、多表删除语句中支持JOIN操作。
连接类型:

• inner join,内连接
  在mysql，join,cross join和inner join 是等价的。
• left [outer] join,左外连接
• right [outer] join,右外连接

语法结构:
table_references {[INNER | CROSS] JOIN | {LEFT | RIGHT} [OUTER] JOIN} table_reference ON conditional_expr

demo:
UPDATE tdb_goods INNER JOIN tdb_goods_cates ON goods_cate=cate_name SET goods_cate=cate_id;
参照tdb_goods_cates表的cate_name来修改tdb_goods的goods_cate。

---

CREATE … SELECT

创建数据表通知书将查询结果写入到数据表
CREATE TABLE [IF NOT EXISTS] tb_name
[(create_definition,…)]
select_statement

demo:
目标：把tdb_goods的brand_name参照tdb_goods_brands表更新成ID形式
tdb_goods_brands表，创建:

 CREATE TABLE tdb_goods_brands
     (
     brand_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
     brand_name VARCHAR(40) NOT NULL
     )
     SELECT brand_name FROM tdb_goods GROUP BY brand_name;

1. SELECT * FROM tddb_goods_brands;
2. UPDATE tdb_goods AS g INNER JOIN tdb_goods_brands AS b ON g.brand_name=b.brand_name SET g.brand_name=b.brand_id;
   

PS:记得修改列的数据类型来减少字节数的使用。

ALTER TABLE tdb_goods
     CHANGE goods_cate cate_id SMALLINT UNSIGNED NOT NULL,
     CHANGE brand_name brand_id SMALLINT UNSIGNED NOT NULL;