1. 创建计算字段 拼接字段
	select concat(vend_name, '(', vend_country, ')') as name from vendors order by vend_name;
	
	删除数据右侧或左侧多余的空格
	select concat(RTrim(vend_name), '(', RTrim(vend_country), ')') as name from vendors order by vend_name;
	
2. 执行算数计算
	select prod_id, quantity, item_price, 
    -> quantity*item_price as expanded_price from orderitems where order_num = 20005;
	+---------+----------+------------+----------------+
	| prod_id | quantity | item_price | expanded_price |
	+---------+----------+------------+----------------+
	| ANV01   |       10 | 5.99       | 59.90          |
	| ANV02   |        3 | 9.99       | 29.97          |
	| TNT2    |        5 | 10         | 50.00          |
	| FB      |        1 | 10         | 10.00          |
	+---------+----------+------------+----------------+
	
	select now(); select trim('ada');
	
3. 文本处理函数
	select vend_name, upper(vend_name) as vend_name_upcase from vendors order by vend_name;
	+----------------+------------------+
	| vend_name      | vend_name_upcase |
	+----------------+------------------+
	| ACME           | ACME             |
	| Anvils R Us    | ANVILS R US      |
	| Furball Inc.   | FURBALL INC.     |
	| Jet Set        | JET SET          |
	| Jouets Et Ours | JOUETS ET OURS   |
	| LT Supplies    | LT SUPPLIES      |
	+----------------+------------------+
	
	Soundex 的使用，发音的匹配
	 select cust_name, cust_contact from customers where soundex(cust_contact) = soundex('Y Lie');
	+-------------+--------------+
	| cust_name   | cust_contact |
	+-------------+--------------+
	| Coyote Inc. | Y Lee        |
	+-------------+--------------+
	
4. 日期和时间处理函数
	AddDate()		增加一个日期
	AddTime()		增加一个时间
	CurDate()		返回当前日期
	CurTime()		返回当前时间
	Date()			返回日期时间的日期部分
	DateFiff()		计算两个日期之差
	Date_Add()		高度灵活的日期运算函数
	Date_Format()	返回一个格式化的日期或时间串
	Day()			返回一个日期的天数部分
	DayOfWeek()		对于一个日期，返回对应的星期
	Hour()			返回时间的小时部分
	Minute()		返回一个时间的分钟部分
	Month()			返回一个日期的月份部分
	Now()			返回当前日期和时间
	Second()		返回一个时间的秒部分
	Time()			返回一个日期时间的时间部分
	Year()			返回一个日期的年份部分
	
	select cust_id, order_num from orders where order_date = '2005-09-01';
	Datetime 类型只匹配日期 2005-09-01 11:30:05
	select cust_id, order_num from orders where Date(order_date) = '2005-09-01';
	
	检索2005年9月份所有订单
	select cust_id, order_num from orders
    -> where Date(order_date) between '2005-09-01' and '2005-09-30';
	
	select cust_id, order_num from orders
    -> where Year(order_date) = 2005 and Month(order_date) = 9;
	
	 select cust_id, order_num from orders
    -> where Date(order_date) >= '2005-09-01' and Date(order_date) <= '2005-09-30';
	+---------+-----------+
	| cust_id | order_num |
	+---------+-----------+
	|   10001 |     20005 |
	|   10003 |     20006 |
	|   10004 |     20007 |
	+---------+-----------+
	
5. 聚集函数 运行在行组上，计算和返回单个值的函数
	AVG()		返回某列的平均值
	COUNT()		返回某列的行数
	MAX()		返回某列的最大值
	MIN()		返回某列的最小值
	SUM()		返回某列之和
	
	AVG() 忽略NULL 值的行
	select AVG(prod_price) as avg_price from products;
	+-----------+
	| avg_price |
	+-----------+
	| 16.133571 |
	+-----------+
	
	COUNT()
	COUNT(*) 对表中行的数目进行计数，不管表列中包含的是空（NULL），还是非空值。
	COUNT(column)对表中特定劣种具有值的进行计数，忽略NULL 值。
	 select count(cust_email) as num_cust from customers;
	+----------+
	| num_cust |
	+----------+
	|        3 |
	+----------+
	
	MAX() MIN()
	select max(prod_price) as max_price, prod_name from products;
	+-----------+--------------+
	| max_price | prod_name    |
	+-----------+--------------+
	| 55        | .5 ton anvil |
	+-----------+--------------+
	
	SUM() 
	select sum(quantity) as items_ordered from orderitems where order_num = 20005;
	+---------------+
	| items_ordered |
	+---------------+
	| 19            |
	+---------------+
	
	SUM 也可以用来合计计算值
	select sum(quantity*item_price) as total_price from orderitems where order_num = 20005;
	+-------------+
	| total_price |
	+-------------+
	| 149.87      |
	+-------------+
	
	聚集不同值 DISTINCT
	select AVG(distinct prod_price) as avg_price from products where vend_id = 1003;
	+-----------+
	| avg_p