该函数在MYSQL下编写，5.1版本测试通过。
函数需要传入三个参数，类型(用于区别应返回时差年、时差月...时差秒)，起始时间(用于计算)，结束时间(用于计算)
1.[代码]DateBetween函数
DELIMITER $$
CREATE FUNCTION DateBetween
(AType INT, ADateStart DATETIME, ADateEnd DATETIME)
RETURNS BIGINT
BEGIN
DECLARE AResult BIGINT DEFAULT -1;
/*入参AType INT类型，结果类型 1:年 2:月 3:日 4:小时 5:分钟 6:秒钟 (必须在1-6内，否则结果返回-1)
*入参ADateStart DATETIME类型，起始时间 (必须是一个正确的日期参数)
*入参ADateEnd DATETIME类型，结束时间 (必须是一个正确的日期参数)
*返回值 BIGINT类型，返回起始之间与结束时间的指定时差 (成功返回计算出的时差，失败返回-1)
*/
IF ADateStart != '' AND ADateEnd != '' AND AType >=1 AND AType <= 6 THEN
CASE AType
WHEN 1 THEN
SELECT TIMESTAMPDIFF(YEAR, ADateStart, ADateEnd) INTO AResult;
WHEN 2 THEN
SELECT TIMESTAMPDIFF(MONTH, ADateStart, ADateEnd) INTO AResult;
WHEN 3 THEN
SELECT TIMESTAMPDIFF(DAY, ADateStart, ADateEnd) INTO AResult;
WHEN 4 THEN
SELECT TIMESTAMPDIFF(HOUR, ADateStart, ADateEnd) INTO AResult;
WHEN 5 THEN
SELECT TIMESTAMPDIFF(MINUTE, ADateStart, ADateEnd) INTO AResult;
WHEN 6 THEN
SELECT TIMESTAMPDIFF(SECOND, ADateStart, ADateEnd) INTO AResult;
END CASE;
END IF;
RETURN(AResult);
END$$
2.[代码]测试用例
SELECT DateBetween(0, '-01-01 00:00:00', '-01-01 10:00:00');
SELECT DateBetween(1, '2001-01-01', '-01-01');
SELECT DateBetween(2, '-01-01', '-12-01');
SELECT DateBetween(3, '-01-01', '-01-31');
SELECT DateBetween(4, '-01-01 00:00:00', '-01-01 10:00:00');
SELECT DateBetween(5, '-01-01 00:00:00', '-01-01 10:00:00');
SELECT DateBetween(6, '-01-01 00:00:00', '-01-01 10:00:00');
SELECT DateBetween(7, '-01-01 00:00:00', '-01-01 10:00:00');
