mysql子查询案例

源SQL如下：

创建数据表

CREATE TABLE IF NOT EXISTS tdb_goods(
    goods_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    goods_name VARCHAR(150) NOT NULL,
    goods_cate VARCHAR(40) NOT NULL,
    brand_name VARCHAR(40) NOT NULL,
    goods_price DECIMAL(15,3) UNSIGNED NOT NULL DEFAULT 0,
    is_show BOOLEAN NOT NULL DEFAULT 1,
    is_saleoff BOOLEAN NOT NULL DEFAULT 0
);

字段对应注释 商品id、商品名称、商品种类、品牌名称、商品价格、是否上架（默认1上架）、是否售出（默认0未售出）

写入数据表

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('R510VC 15.6英寸笔记本','笔记本','华硕','3399',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Y400N 14.0英寸笔记本电脑','笔记本','联想','4899',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('G150TH 15.6英寸游戏本','游戏本','雷神','8499',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X550CC 15.6英寸笔记本','笔记本','华硕','2799',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X240(20ALA0EYCD) 12.5英寸超极本','超级本','联想','4999',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('U330P 13.3英寸超极本','超级本','联想','4299',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('SVP13226SCB 13.3英寸触控超极本','超级本','索尼','7999',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iPad mini MD531CH/A 7.9英寸平板电脑','平板电脑','苹果','1998',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iPad Air MD788CH/A 9.7英寸平板电脑 （16G WiFi版）','平板电脑','苹果','3388',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' iPad mini ME279CH/A 配备 Retina 显示屏 7.9英寸平板电脑 （16G WiFi版）','平板电脑','苹果','2788',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('IdeaCentre C340 20英寸一体电脑 ','台式机','联想','3499',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Vostro 3800-R1206 台式电脑','台式机','戴尔','2899',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('iMac ME086CH/A 21.5英寸一体电脑','台式机','苹果','9188',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('AT7-7414LP 台式电脑 （i5-3450四核 4G 500G 2G独显 DVD 键鼠 Linux ）','台式机','宏碁','3699',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Z220SFF F4F06PA工作站','服务器/工作站','惠普','4288',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('PowerEdge T110 II服务器','服务器/工作站','戴尔','5388',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('Mac Pro MD878CH/A 专业级台式电脑','服务器/工作站','苹果','28888',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' HMZ-T3W 头戴显示设备','笔记本配件','索尼','6999',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('商务双肩背包','笔记本配件','索尼','99',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('X3250 M4机架式服务器 2583i14','服务器/工作站','IBM','6888',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('玄龙精英版 笔记本散热器','笔记本配件','九州风神','5000',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES(' HMZ-T3W 头戴显示设备','笔记本配件','索尼','6999',DEFAULT,DEFAULT);

INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_saleoff) VALUES('商务双肩背包','笔记本配件','索尼','99',DEFAULT,DEFAULT);

我们SELECT一下

求所有电脑的平均价格，并且保留两位小数，AVG,MAX,MIN、COUNT、SUM为聚合函数

SELECT ROUND(AVG(goods_price),2) FROM tdb_goods;

查询所有价格大于平均价格的商品，并且按价格降序排序

SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price > 5608.70 ORDER BY goods_price DESC;

换成用子查询语句

ELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price > (SELECT ROUND(AVG(goods_price),2) FROM tdb_goods) ORDER BY goods_price DESC;

查询类型为“超记本”的商品价格

SELECT goods_price FROM tdb_goods WHERE goods_cate = '超级本';

查询价格大于或等于"超级本"价格的商品，并且按价格降序排列

SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price >= (SELECT goods_price FROM tdb_goods WHERE goods_cate = '超级本');

这里出现了错误ERROR 1242 (21000): Subquery returns more than 1 row，我们需要在这里使用ANY关键字

创建商品分类表

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

查询tdb_goods表的所有记录，并且按"类别"分组

SELECT goods_cate FROM tdb_goods GROUP BY goods_cate;

将分组结果写入到tdb_goods_cates数据表

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

通过tdb_goods_cates数据表来更新tdb_goods表

UPDATE tdb_goods INNER JOIN tdb_goods_cates ON goods_cate = cate_name SET goods_cate = cate_id;

通过CREATE...SELECT来创建数据表并且同时写入记录

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;

过tdb_goods_brands数据表来更新tdb_goods数据表

UPDATE tdb_goods INNER JOIN tdb_goods_brands ON brand_name = brand_name SET brand_name = brand_id;

上面的命令会报错，原因是两个brand_name意义不明
ERROR 1052 (23000): Column 'brand_name' in field list is ambiguous

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;

查看tdb_goods的数据表结构

DESC tdb_goods;

通过ALTER TABLE语句修改数据表结构

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

分别在tdb_goods_cates和tdb_goods_brands表插入记录

INSERT tdb_goods_cates(cate_name) VALUES('路由器'),('交换机'),('网卡');
INSERT tdb_goods_brands(brand_name) VALUES('海尔'),('清华同方'),('神舟');

在tdb_goods数据表写入任意记录

INSERT tdb_goods(goods_name,cate_id,brand_id,goods_price) VALUES(' LaserJet Pro P1606dn 黑白激光打印机','12','4','1849');