一、查询
1.order by XX DESC 按照XX降序

2.distinct 去重

3.order by XX,XXX DESC

如果XX条件一样，按照XXX降序

4.通配符查询

XX like "%toy%"

二.函数
1.substring 子串 substring(XX,1,2)

` concat拼接 concat(A,B)

upper大写 upper(A)`

计算某列的和Sum(A)

计算某列的行数Count(A)

having必须和group by一起使用

三.连接
（1）group by 需要包含select所有出现的字段：

#返回顾客名称和相关订单号以及每个订单的总价

select cust_name,Orders.order_num,sum(quantity*item_price) OrderTotal from Customers join Orders
on Customers.cust_id=Orders.cust_id
join OrderItems on Orders.order_num=OrderItems.order_num
group by cust_name,Orders.order_num 
order by cust_name,Orders.order_num

(2)left join,right join,join区别

(3)union用法：拼接两个表，字段要一样

select prod_id,quantity from OrderItems where quantity = 100

union 

select prod_id,quantity from OrderItems where prod_id like "BN%"

 order by prod_id;