SQL函数

SQL Aggregate函数

计算从列中返回的值，返回一个单一值。

-- AVG()：平均值
SELECT AVG(column_name) AS new_name FROM table_name;
-- COUNT()：计数
SELECT COUNT(column_name) AS new_name FROM table_name;
-- FIRST()：第一个
SELECT column_name FROM table_name ORDER BY column_name ASC LIMIT 1;
-- LAST()：最后一个
SELECT column_name FROM table_name ORDER BY column_name DESC LIMIT 1;
-- MAX()：最大
SELECT MAX(column_name) AS new_name FROM table_name;
-- MIN()：最小
SELECT MIN(column_name) AS new_name FROM table_name;
-- SUM()：和
SELECT SUM(column_name) AS new_name FROM table_name;

GROUP BY语句

用了GROUP BY以后，聚合函数在每个分组上进行。

HAVING语句

WHERE中不能有聚合函数，因此要用HAVING语句。

SQL Scalar函数

-- UCASE()：大写
SELECT UCASE(column_name) FROM table_name;
-- LCASE()：小写 
SELECT LCASE(column_name) FROM table_name;
-- MID()：截取子串
MID(column_name,start[,length])
-- LENGTH：字段值长度
SELECT LENGTH(column_name) FROM table_name;
-- ROUND()：四舍五入
ROUND(column_name)四舍五入为整数
ROUND(column_name[,保留位数])
-- NOW()：现在的时间