子查询与连接
Posted 旷野足迹
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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。
子查询可以包含多个关键字或条件：
如DISTINCT、GROUP BY、ORDER BY、LIMIT、等。
子查询的外层查询可以是：SELECT ,INSERT,UPDATE,SET或DO.
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。子查询可以返回标量、一行、一列或子查询。
优化子查询
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-- 创建数据表：
root@127.0.0.1 t2>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_salesoff BOOLEAN NOT NULL DEFAULT 1 -> ); Query OK, 0 rows affected (0.20 sec)
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写入数据：
root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'R510VC 15.6英寸笔记本\',\'笔记本\',\'华硕\',\'3399\',DEFAULT,DEFAULT); ERROR 1366 (HY000): Incorrect string value: \'\\xB4\\xE7\\xB1\\xCA\\xBC\\xC7...\' for column \'goods_name\' at row 1 root@127.0.0.1 t2>SET tdb_goods GBK; ERROR 1193 (HY000): Unknown system variable \'tdb_goods\' root@127.0.0.1 t2>SET NAMES GBK; Query OK, 0 rows affected (0.00 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'R510VC 15.6英寸笔记本\',\'笔记本\',\'华硕\',\'3399\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.15 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'Y400N 14.0英寸笔记本电脑\',\'笔记本\',\'联想\',\'4899\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'G150TH 15.6英寸游戏本\',\'游戏本\',\'雷神\',\'8499\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>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); ERROR 1054 (42S22): Unknown column \'is_saleoff\' in \'field list\' root@127.0.0.1 t2> INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'X550CC 15.6英寸笔记本\',\'笔记本\',\'华硕\',\'2799\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'X240(20ALA0EYCD) 12.5英寸超极本\',\'超级本\',\'联想\',\'4999\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'U330P 13.3英寸超极本\',\'超级本\',\'联想\',\'4299\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'SVP13226SCB 13.3英寸触控超极本\',\'超级本\',\'索尼\',\'7999\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'iPad mini MD531CH/A 7.9英寸平板电脑\',\'平板电脑\',\'苹果\',\'1998\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) 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); Query OK, 1 row affected (0.16 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) 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); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'IdeaCentre C340 20英寸一体电脑 \',\'台式机\',\'联想\',\'3499\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'Vostro 3800-R1206 台式电脑\',\'台式机\',\'戴尔\',\'2899\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.16 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'iMac ME086CH/A 21.5英寸一体电脑\',\'台式机\',\'苹果\',\'9188\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) 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); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'Z220SFF F4F06PA工作站\',\'服务器/工作站\',\'惠普\',\'4288\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'PowerEdge T110 II服务器\',\'服务器/工作站\',\'戴尔\',\'5388\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'Mac Pro MD878CH/A 专业级台式电脑\',\'服务器/工作站\',\'苹果\',\'28888\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\' HMZ-T3W 头戴显示设备\',\'笔记本配件\',\'索尼\',\'6999\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'商务双肩背包\',\'笔记本配件\',\'索尼\',\'99\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'X3250 M4机架式服务器 2583i14\',\'服务器/工作站\',\'IBM\',\'6888\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.16 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\' HMZ-T3W 头戴显示设备\',\'笔记本配件\',\'索尼\',\'6999\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.14 sec) root@127.0.0.1 t2>INSERT tdb_goods (goods_name,goods_cate,brand_name,goods_price,is_show,is_salesoff) VALUES(\'商务双肩背包\',\'笔记本配件\',\'索尼\',\'99\',DEFAULT,DEFAULT); Query OK, 1 row affected (0.13 sec)
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由比较运算符引发的子查询：
-- 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;
ANY 和 SOME 两个修饰符是等价的，它们两个修饰符提示之返回一个值：
注意： 1. 子查询只能返回一行记录，不然会报错。 2. 子查询还可以包含子查询。 关键字 ANY SOME ALL >、>= 最小值 最小值 最大值 <、<= 最大值 最大值 最小值 = 任意值 任意值 <>、！= 任意值
=ANY 等价于 IN !=ALL或<>ANY等价于NOT IN 使用[NOT]EXIST引发的子查询 （不常用）
使用[NOT]EXIST引发的子查询 （不常用）如果子查询返回了任何的结果，我们就返回true，否则的话我们就返回了false。
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使用INSERT.....SELECT插入记录：
root@127.0.0.1 t2>CREATE TABLE IF NOT EXISTS tdb_goods_cates( -> cate_id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT, -> cate_name VARCHAR(40) NOT NULL); Query OK, 0 rows affected (0.20 sec)
将分组结果写入到tdb_goods_cates数据表
INSERT [INTO] tbl_name [(col_name,...)] SELECT ...
root@127.0.0.1 t2>INSERT tdb_goods_cates (cate_name) SELECT goods_cate FROM tdb_goods GROUP BY goods_cate; Query OK, 7 rows affected (0.14 sec) Records: 7 Duplicates: 0 Warnings: 0 root@127.0.0.1 t2>SELECT * FROM tdb_goods_cates; +---------+---------------+ | cate_id | cate_name | +---------+---------------+ | 1 | 台式机 | | 2 | 平板电脑 | | 3 | 服务器/工作站 | | 4 | 游戏本 | | 5 | 笔记本 | | 6 | 笔记本配件 | | 7 | 超级本 | +---------+---------------+ 7 rows in set (0.00 sec
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多表更新：
UPDATE table_references SET col_name1={expr1 DEFAULT} [,col_name2={expr2 DEFAULT}]... [WHERE where_condition]
通过tdb_goods_cates数据表来更新tdb_goods表
root@127.0.0.1 t2>UPDATE tdb_goods INNER JOIN tdb_goods_cates ON goods_cate = cate_name -> SET goods_cate = cate_id ; Query OK, 22 rows affected (0.14 sec) Rows matched: 22 Changed: 22 Warnings: 0 root@127.0.0.1 t2>SELECT * FROM tdb_goods\\G; *************************** 1. row *************************** goods_id: 1 goods_name: R510VC 15.6英寸笔记本 goods_cate: 5 brand_name: 华硕 goods_price: 3399.000 is_show: 1 is_salesoff: 1
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多表更新一步到位：
建表、查询、写入三合一： 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;
root@127.0.0.1 t2>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; Query OK, 9 rows affected (0.22 sec) Records: 9 Duplicates: 0 Warnings: 0 root@127.0.0.1 t2>SELECT * FROM tdb_goods_brands; +----------+------------+ | brand_id | brand_name | +----------+------------+ | 1 | IBM | | 2 | 华硕 | | 3 | 宏碁 | | 4 | 惠普 | | 5 | 戴尔 | | 6 | 索尼 | | 7 | 联想 | | 8 | 苹果 | | 9 | 雷神 | +----------+------------+ 9 rows in set (0.00 sec)
我们还可以使用UPDATE来进行关联：
root@127.0.0.1 t2>UPDATE tdb_goods INNER JOIN tdb_goods_brands ON tdb_goods.brand_name = tdb_goods_brands.brand_name -> SET tdb_goods.brand_name = tdb_goods_brands.brand_id; Query OK, 22 rows affected (0.15 sec) Rows matched: 22 Changed: 22 Warnings: 0
root@127.0.0.子查询与连接