MySql012——检索数据：创建计算字段（拼接字段、使用别名、执行算术计算）

准备工作1：在study库中创建表vendors，并插入数据

说明：vendors表包含供应商名和位置信息。

use `study`;

CREATE TABLE vendors
(
  vend_id      int      NOT NULL AUTO_INCREMENT,
  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 ,
  PRIMARY KEY (vend_id)
) ENGINE=InnoDB;

INSERT INTO vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES(1001,'Anvils R Us','123 Main Street','Southfield','MI','48075', 'USA');
INSERT INTO vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES(1002,'LT Supplies','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(1003,'ACME','555 High Street','Los Angeles','CA','90046', 'USA');
INSERT INTO vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES(1004,'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(1005,'Jet Set','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(1006,'Jouets Et Ours','1 Rue Amusement','Paris', NULL,'45678', 'France');

一、拼接字段

为了说明如何使用计算字段，举一个创建由两列组成的标题的简单例子：

vendors表包含供应商名和位置信息。假如需要在供应商的名字中按照name(location)这样的格式列出供应商的位置。而表中数据存储在两个列vend_name和vend_ country中。此外，需要用括号将vend_country括起来，这些东西都没有明确存储在数据库表中。我们来看看怎样编写返回供应商名和位置的SELECT语句。

解决办法是把两个列拼接起来。在MySQL的SELECT语句中，可使用Concat()函数来拼接两个列。

SELECT 
    CONCAT(vend_name, '(', vend_country, ')')
FROM
    study.vendors
ORDER BY vend_name;

二、使用别名

从上面的结果可以看出，结果是查出来了，但是列名却不是我们想要的简短见名知意效果；

为了解决这个问题，SQL支持列别名。别名（alias）是一个字段或值的替换名。别名用AS关键字赋予。例如上面语句可以修改成：

SELECT 
    CONCAT(vend_name, '(', vend_country, ')') AS `vend_message`
FROM
    study.vendors
ORDER BY vend_name;

其中，AS后面的名称是我们自定义的，你也可以根据需要换成其他的别名，SQL执行结果如下：

准备工作2：在study库中创建表orderitems，并插入数据

说明：orders表包含收到的所有订单

use `study`;

CREATE TABLE orderitems
(
  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 ,
  PRIMARY KEY (order_num, order_item)
) ENGINE=InnoDB;

INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20005, 1, 'ANV01', 10, 5.99);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20005, 2, 'ANV02', 3, 9.99);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20005, 3, 'TNT2', 5, 10);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20005, 4, 'FB', 1, 10);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20006, 1, 'JP2000', 1, 55);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20007, 1, 'TNT2', 100, 10);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20008, 1, 'FC', 50, 2.50);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20009, 1, 'FB', 1, 10);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20009, 2, 'OL1', 1, 8.99);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20009, 3, 'SLING', 1, 4.49);
INSERT INTO orderitems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20009, 4, 'ANV03', 1, 14.99);

三、执行算术计算

首先，我们查询订单号20005中的所有物品，结果如下：

SELECT 
   order_num, prod_id, quantity, item_price
FROM
    orderitems
WHERE
    order_num = 20005;

这样我们就查询到订单号20005中的所有物品信息，其中包括订单号、产品ID、数量、单价

3.1、MySQL算术操作符

  操作符			           说明
--------------------------------------
     + 					    加
     - 						减
     * 						乘
     / 					    除

但是，我还想知道每一种产品总共花了多少钱，怎么做呢？这样我们就需要使用数量乘以单价。

SELECT 
    order_num,
    prod_id,
    quantity,
    item_price,
    item_price * item_price AS expanded_price -- 做运算同时取别名
FROM
    orderitems
WHERE
    order_num = 20005;