十四、简单函数

SELECT AVG(column_name) FROM table_n
SELECT Customer FROM Orders WHERE OrderPrice>(SELECT AVG(OrderPrice) FROM Orders) #不是where orderprice>AVG(prderprice)!!!!!


SELECT COUNT(column_name) FROM table_name  #NULL不计入
SELECT COUNT(*) FROM table_name
SELECT COUNT(DISTINCT column_name) FROM table_name

SELECT FIRST(column_name) FROM table_name
SELECT LAST(column_name) FROM table_name

SELECT MAX(column_name) FROM table_name
SELECT MIN(column_name) FROM table_name

SELECT SUM(column_name) FROM table_name

SELECT UCASE(column_name) FROM table_name#返回大写
SELECT UPPER(column_name) FROM table_name  #返回大写
SELECT LCASE(column_name) FROM table_name #返回小写
SELECT LOWER(column_name) FROM table_name#返回小写

MID() 函数

MID 函数用于从文本字段中提取字符。

SELECT MID(column_name,start[,length]) FROM table_name

LEN() 函数

LEN 函数返回文本字段中值的长度。

SELECT LEN(column_name) FROM table_name

ROUND() 函数

ROUND 函数用于把数值字段舍入为指定的小数位数。

SELECT ROUND(column_name,decimals) FROM table_name

NOW() 函数

NOW 函数返回当前的日期和时间。

FORMAT() 函数

FORMAT 函数用于对字段的显示进行格式化。

SELECT FORMAT(column_name,format) FROM table_name

时间格式转换用DATE_FORMAT()

SELECT SNAME,DATE_FORMAT(SBIRTHDAY,'%d %b %y') AS sdate
FROM student

具体日期格式见https://www.w3school.com.cn/sql/func_date_format.asp