1.子查询是指在另一个查询语句中的SELECT子句。

例句：

SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);

其中，SELECT * FROM t1 ...称为Outer Query[外查询](或者Outer Statement),

SELECT column1 FROM t2 称为Sub Query[子查询]。

所以，我们说子查询是嵌套在外查询内部。而事实上它有可能在子查询内部再嵌套子查询。

子查询必须出现在圆括号之间。

行级子查询

SELECT * FROM t1 WHERE (col1,col2) = (SELECT col3, col4 FROM t2 WHERE id = 10);

SELECT * FROM t1 WHERE ROW(col1,col2) = (SELECT col3, col4 FROM t2 WHERE id = 10);

行级子查询的返回结果最多为一行。

优化子查询

-- 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

);

-- 写入记录

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','',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);

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

SELECT ROUND(AVG(goods_price),2) AS avg_price 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 > 5845.10 ORDER BY goods_price DESC;

-- 使用子查询来实现

SELECT goods_id,goods_name,goods_price FROM tdb_goods

WHERE goods_price > (SELECT ROUND(AVG(goods_price),2) AS avg_price 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 = ANY(SELECT goods_price FROM tdb_goods WHERE goods_cate = '超级本')

ORDER BY goods_price DESC;

-- = ANY 或 = SOME 等价于 IN

SELECT goods_id,goods_name,goods_price FROM tdb_goods

WHERE goods_price IN (SELECT goods_price FROM tdb_goods WHERE goods_cate = '超级本')

ORDER BY goods_price DESC;

-- 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)

);

-- 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

-- SELECT brand_name FROM tdb_goods GROUP BY brand_name;

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;

-- 通过tdb_goods_brands数据表来更新tdb_goods数据表(错误)

UPDATE tdb_goods INNER JOIN tdb_goods_brands ON brand_name = brand_name

SET brand_name = brand_id;

-- Column 'brand_name' in field list is ambigous

-- 正确

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');

-- 查询所有商品的详细信息(通过内连接实现)

SELECT goods_id,goods_name,cate_name,brand_name,goods_price FROM tdb_goods AS g

INNER JOIN tdb_goods_cates AS c ON g.cate_id = c.cate_id

INNER JOIN tdb_goods_brands AS b ON g.brand_id = b.brand_id\G;

-- 查询所有商品的详细信息(通过左外连接实现)

SELECT goods_id,goods_name,cate_name,brand_name,goods_price FROM tdb_goods AS g

LEFT JOIN tdb_goods_cates AS c ON g.cate_id = c.cate_id

LEFT JOIN tdb_goods_brands AS b ON g.brand_id = b.brand_id\G;

-- 查询所有商品的详细信息(通过右外连接实现)

SELECT goods_id,goods_name,cate_name,brand_name,goods_price FROM tdb_goods AS g

RIGHT JOIN tdb_goods_cates AS c ON g.cate_id = c.cate_id

RIGHT JOIN tdb_goods_brands AS b ON g.brand_id = b.brand_id\G;

-- 无限分类的数据表设计

CREATE TABLE tdb_goods_types(

type_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,

type_name VARCHAR(20) NOT NULL,

parent_id SMALLINT UNSIGNED NOT NULL DEFAULT 0

);

INSERT tdb_goods_types(type_name,parent_id) VALUES('家用电器',DEFAULT);

INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑、办公',DEFAULT);

INSERT tdb_goods_types(type_name,parent_id) VALUES('大家电',1);

INSERT tdb_goods_types(type_name,parent_id) VALUES('生活电器',1);

INSERT tdb_goods_types(type_name,parent_id) VALUES('平板电视',3);

INSERT tdb_goods_types(type_name,parent_id) VALUES('空调',3);

INSERT tdb_goods_types(type_name,parent_id) VALUES('电风扇',4);

INSERT tdb_goods_types(type_name,parent_id) VALUES('饮水机',4);

INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑整机',2);

INSERT tdb_goods_types(type_name,parent_id) VALUES('电脑配件',2);

INSERT tdb_goods_types(type_name,parent_id) VALUES('笔记本',9);

INSERT tdb_goods_types(type_name,parent_id) VALUES('超级本',9);

INSERT tdb_goods_types(type_name,parent_id) VALUES('游戏本',9);

INSERT tdb_goods_types(type_name,parent_id) VALUES('CPU',10);

INSERT tdb_goods_types(type_name,parent_id) VALUES('主机',10);

-- 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;