mysql 10-12

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#第10章 创建计算字段 
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use test;
select * from vendors;
####拼接字段 concat把多个字符串拼接起来 ，串与串之间用逗号分隔 
select concat(vend_name,' (',vend_country, ')') from vendors
order by vend_name;#四部分，vend_name,(,vend_country,)
#RTrim去掉右侧的空格
select concat(rtrim(vend_name),' (',rtrim(vend_country), ')') from vendors
order by vend_name;
#别名 as
select concat(rtrim(vend_name),' (',rtrim(vend_country), ')') as vend_title from vendors
order by vend_name;
####执行算术计算 + _ * /
select prod_id ,quantity,item_price,quantity*item_price as expanded_price from orderitems
where order_num=20005;


###############################
#第十一章 使用数据处理函数 
###############################

##文本处理函数 
#left length locate lower ltrim right rtrim soundex substring upper
select vend_name,Upper(vend_name) as vend_name_upcase from vendors
order by vend_name;

select cust_name,cust_contact from customers 
where cust_contact ='Y. Lie';
select cust_name,cust_contact from customers 
where soundex(cust_contact) =soundex('Y. Lie');#模糊语音识别 

##日期和时间处理函数 日期格式yyyy-mm-dd
select cust_id,order_num from orders
where Date(order_date)='2005-09-01';#指示数据库只对应日期的部分 

select cust_id,order_num from orders
where date(order_date) between '2005-09-01' and '2005-09-30';#寻找一个时间段 

select cust_id,order_num from orders
where year(order_date)=2005 and month(order_date)=9;#另一种实现方法 

##数值处理函数 
#abs cos exp mod pi rand sin sqrt tan


##########################
#第十二章 汇总数据
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##聚集函数 avg count max min sum
select avg(prod_price) as avg_price from products
where vend_id=1003;#多个列平均 ，avg要使用多次 且自动忽略null

#count(*)对表中行计数 
#count(columns)对这个列有数值的行计数 
select count(*) as num_cust from customers;#返回所有客户数 
select count(cust_email) as num_cust from customers;#返回有email的客户数

#max  min sum(返回指定列值的和） 
select max(prod_price) as max_price from products;
select min(prod_price) as max_price from products;
select * from orderitems;
select sum(quantity) as item_ordered from orderitems
where order_num=20005;
select sum(quantity*item_price) as total_price from orderitems
where order_num=20005;

##聚集不同的值 
select avg(distinct prod_price) as avg_price from products
where vend_id=1003;

##组合聚集函数 
select count(*) as num_items,
       min(prod_price) as price_min,
       avg(prod_price) as price_avg from products;