数据库SQL语句学习笔记（2）-检索数据库
首先设置好此连载笔记的演示所用的表
CREATE database sells; #创建名叫sells的数据库 USE sells; #告诉mysql，我要开始使用这个数据库了 CREATE TABLE Customers #创建Customers表 ( cust_id char(10) NOT NULL , cust_name char(50) NOT NULL , cust_address char(50) NULL , cust_city char(50) NULL , cust_state char(5) NULL , cust_zip char(10) NULL , cust_country char(50) NULL , cust_contact char(50) NULL , cust_email char(255) NULL ); CREATE TABLE OrderItems #创建OrderItems表 ( order_num int NOT NULL , order_item int NOT NULL , prod_id char(10) NOT NULL , quantity int NOT NULL , item_price decimal(8,2) NOT NULL ); CREATE TABLE Orders #创建Orders表 ( order_num int NOT NULL , order_date datetime NOT NULL , cust_id char(10) NOT NULL ); CREATE TABLE Products #创建表Product ( prod_id char(10) NOT NULL , vend_id char(10) NOT NULL , prod_name char(255) NOT NULL , prod_price decimal(8,2) NOT NULL , prod_desc text NULL ); CREATE TABLE Vendors #创建表Vendors ( vend_id char(10) NOT NULL , vend_name char(50) NOT NULL , vend_address char(50) NULL , vend_city char(50) NULL , vend_state char(5) NULL , vend_zip char(10) NULL , vend_country char(50) NULL ); #设置主键 ALTER TABLE Customers ADD PRIMARY KEY (cust_id); ALTER TABLE OrderItems ADD PRIMARY KEY (order_num, order_item); ALTER TABLE Orders ADD PRIMARY KEY (order_num); ALTER TABLE Products ADD PRIMARY KEY (prod_id); ALTER TABLE Vendors ADD PRIMARY KEY (vend_id); #设置外键 ALTER TABLE OrderItems ADD CONSTRAINT FK_OrderItems_Orders FOREIGN KEY (order_num) REFERENCES Orders (order_num); ALTER TABLE OrderItems ADD CONSTRAINT FK_OrderItems_Products FOREIGN KEY (prod_id) REFERENCES Products (prod_id); ALTER TABLE Orders ADD CONSTRAINT FK_Orders_Customers FOREIGN KEY (cust_id) REFERENCES Customers (cust_id); ALTER TABLE Products ADD CONSTRAINT FK_Products_Vendors FOREIGN KEY (vend_id) REFERENCES Vendors (vend_id); #插入记录 INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact, cust_email) VALUES('1000000001', 'Village Toys', '200 Maple Lane', 'Detroit', 'MI', '44444', 'USA', 'John Smith', 'sales@villagetoys.com'); INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact) VALUES('1000000002', 'Kids Place', '333 South Lake Drive', 'Columbus', 'OH', '43333', 'USA', 'Michelle Green'); INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact, cust_email) VALUES('1000000003', 'Fun4All', '1 Sunny Place', 'Muncie', 'IN', '42222', 'USA', 'Jim Jones', 'jjones@fun4all.com'); INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact, cust_email) VALUES('1000000004', 'Fun4All', '829 Riverside Drive', 'Phoenix', 'AZ', '88888', 'USA', 'Denise L. Stephens', 'dstephens@fun4all.com'); INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact) VALUES('1000000005', 'The Toy Store', '4545 53rd Street', 'Chicago', 'IL', '54545', 'USA', 'Kim Howard'); INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country) VALUES('BRS01','Bears R Us','123 Main Street','Bear Town','MI','44444', 'USA'); INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country) VALUES('BRE02','Bear Emporium','500 Park Street','Anytown','OH','44333', 'USA'); INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country) VALUES('DLL01','Doll House Inc.','555 High Street','Dollsville','CA','99999', 'USA'); INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country) VALUES('FRB01','Furball Inc.','1000 5th Avenue','New York','NY','11111', 'USA'); INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country) VALUES('FNG01','Fun and Games','42 Galaxy Road','London', NULL,'N16 6PS', 'England'); INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country) VALUES('JTS01','Jouets et ours','1 Rue Amusement','Paris', NULL,'45678', 'France'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('BR01', 'BRS01', '8 inch teddy bear', 5.99, '8 inch teddy bear, comes with cap and jacket'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('BR02', 'BRS01', '12 inch teddy bear', 8.99, '12 inch teddy bear, comes with cap and jacket'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('BR03', 'BRS01', '18 inch teddy bear', 11.99, '18 inch teddy bear, comes with cap and jacket'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('BNBG01', 'DLL01', 'Fish bean bag toy', 3.49, 'Fish bean bag toy, complete with bean bag worms with which to feed it'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('BNBG02', 'DLL01', 'Bird bean bag toy', 3.49, 'Bird bean bag toy, eggs are not included'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('BNBG03', 'DLL01', 'Rabbit bean bag toy', 3.49, 'Rabbit bean bag toy, comes with bean bag carrots'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('RGAN01', 'DLL01', 'Raggedy Ann', 4.99, '18 inch Raggedy Ann doll'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('RYL01', 'FNG01', 'King doll', 9.49, '12 inch king doll with royal garments and crown'); INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc) VALUES('RYL02', 'FNG01', 'Queen doll', 9.49, '12 inch queen doll with royal garments and crown'); INSERT INTO Orders(order_num, order_date, cust_id) VALUES(20005, '2012-05-01', '1000000001'); INSERT INTO Orders(order_num, order_date, cust_id) VALUES(20006, '2012-01-12', '1000000003'); INSERT INTO Orders(order_num, order_date, cust_id) VALUES(20007, '2012-01-30', '1000000004'); INSERT INTO Orders(order_num, order_date, cust_id) VALUES(20008, '2012-02-03', '1000000005'); INSERT INTO Orders(order_num, order_date, cust_id) VALUES(20009, '2012-02-08', '1000000001'); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20005, 1, 'BR01', 100, 5.49); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20005, 2, 'BR03', 100, 10.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20006, 1, 'BR01', 20, 5.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20006, 2, 'BR02', 10, 8.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20006, 3, 'BR03', 10, 11.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20007, 1, 'BR03', 50, 11.49); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20007, 2, 'BNBG01', 100, 2.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20007, 3, 'BNBG02', 100, 2.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20007, 4, 'BNBG03', 100, 2.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20007, 5, 'RGAN01', 50, 4.49); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20008, 1, 'RGAN01', 5, 4.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20008, 2, 'BR03', 5, 11.99); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20008, 3, 'BNBG01', 10, 3.49); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20008, 4, 'BNBG02', 10, 3.49); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20008, 5, 'BNBG03', 10, 3.49); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20009, 1, 'BNBG01', 250, 2.49); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20009, 2, 'BNBG02', 250, 2.49); INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price) VALUES(20009, 3, 'BNBG03', 250, 2.49);
1. SELECT语句
关键字（keyword）作为SQL组成部分的保留字，不可以作为表或者列的名字。
使用SELECT语句，要清楚需要选择什么？从哪里选择的问题。
2. 检索单个列
SELECT prod_name FROM Products;
结果：
分析：从Product表中检索一个名为prod_name的列。列名写在关键字SELECT之后，FROM关键字表明从哪个表中检索。这样返回的数据没有特定的顺序。
提示：（1）多条SQL语句必须以；分隔
（2）SQL语句不区分大小写，约定俗成的是关键字大写，表名和列名小写
（3）以分号结尾，看到分号就知道这一条语句结束了
3.检索多个列
SELECT关键字后给出多个列名，列名之间以逗号分隔，最后一个列名后不可以加逗号
SELECT prod_id,prod_name,prod_price FROM Products;
结果： 
4.检索所有列
通配符星号（*）可以检索所有列
SELECT * FROM Products;
5. 检索不同的值
SELESTC语句返回所有匹配的行，但是有很多值重复出现，此时就要用到检索不同的值
SELECT DISTINCT vend_id FROM Products;
注意：DISTINCT关键字作用于所有选择的列
6.限制结果
SELECT prod_name FROM Products LIMIT 5;
结果：
LIMIT 5指示MYSQL返回不超过5行数据
LIMIT 5 OFFSET 5从第5行起，检索5个数,注意第一行是0
简写为LIMIT 5,5 第一个数是OFFSET,第二个是LIMIT
7.使用注释
--嵌在行内
# 行内注释，在一行开始处使用#，这一整行都将作为注释
/* */开始结束中间都作为注释
