例句：
SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);
当中，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。
而其实它有可能在子查询内部再嵌套子查询。
子查询必须出如今圆括号之间。
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\',\'server/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 IIserver\',\'server/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\',\'server/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server 2583i14\',\'server/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);
-- 求全部电脑产品的平均价格,而且保留两位小数，AVG,MAX,MIN、COUNT、SUM为聚合函数
SELECT ROUND(AVG(goods_price),2) AS avg_price FROM tdb_goods;
-- 查询全部价格大于平均价格的商品，而且按价格降序排序
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，而且按价格降序排列
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表的全部记录，而且按"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来创建数据表而且同一时候写入记录
-- 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语句改动数据表结构
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数据表写入随意记录
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\');
-- 查询全部商品的具体信息(通过内连接实现)例如以下如：
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;
-- 查询全部商品的具体信息(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;
-- 查询全部商品的具体信息(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);
-- 查找全部分类及其父类
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;
-- 查找全部分类及其子类
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;
-- 查找全部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加入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);
-- 查找反复记录
SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING count(goods_name) >= 2;
-- 删除反复记录
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;