基本用法和类型：
SELECT function(列) FROM 表
在 SQL 中，基本的函数类型和种类有若干种。函数的基本类型是：Aggregate 函数 和 Scalar 函数
Aggregate 函数的操作面向一系列的值，并返回一个单一的值。Scalar 函数的操作面向某个单一的值，并返回基于输入值的一个单一的值。
Aggregate函数
1、AVG函数 返回平均值
SELECT AVG(OrderPrice) AS OrderAverage FROM Orders
SELECT Customer FROM Orders
WHERE OrderPrice>(SELECT AVG(OrderPrice) FROM Orders) 本质就是筛选price大于平均值的
2、COUNT 返回匹配指定条件的行数
SELECT COUNT(DISTINCT column_name) FROM table_name
SELECT COUNT(Customer) AS CustomerNilsen FROM Orders WHERE Customer='Carter' 客户Carter的订单数
3、FIRST / LAST / MAX / MIN
SELECT FIRST(column_name) FROM table_name
SELECT LAST(OrderPrice) AS LastOrderPrice FROM Orders
SELECT MAX(OrderPrice) AS LargestOrderPrice FROM Orders 同理MIN
4、SUM / GROUP BY
SELECT SUM(OrderPrice) AS OrderTotal FROM Orders
合计函数 (比如 SUM) 常常需要添加 GROUP BY 语句。
SELECT Customer,SUM(OrderPrice) FROM Orders
GROUP BY Customer 希望查找每个客户的总金额（总订单）需要对客户进行分组
5、HAVING WHERE 无法与合计函数一起使用
查找订单总金额少于 2000 的客户：
SELECT Customer,SUM(OrderPrice) FROM Orders
GROUP BY Customer
HAVING SUM(OrderPrice)<2000
查找客户 "Bush" 或 "Adams" 拥有超过 1500 的订单总金额：
SELECT Customer,SUM(OrderPrice) FROM Orders
WHERE Customer='Bush' OR Customer='Adams'
GROUP BY Customer
HAVING SUM(OrderPrice)>1500
Scalar函数
1、UCASE LCASE
UCASE 函数把字段的值转换为大写。LCASE 函数把字段的值转换为小写。
2、MID 从文本字段中提取字符
SELECT MID(column_name,start[,length]) FROM table_name
SELECT MID(City,1,3) as SmallCity FROM Persons
"City" 列中每一行都提取前 3 个字符
3、LEN 返回值的长度
4、FORMAT 格式化字段显示
SELECT FORMAT(column_name,format) FROM table_name
SELECT ProductName, UnitPrice, FORMAT(Now(),'YYYY-MM-DD') as PerDate FROM Products
5、ROUND 把数值字段舍入为指定的小数位数
SELECT ROUND(column_name,decimals) FROM table_name
demicals：规定要返回的小数位数
TO BE CONT'D.