关于SQLServer2005的学习笔记——生日问题

生日问题是个看似简单逻辑上却又比较复杂的小问题

主要的逻辑难点第一个是关于闰月尾天的计算，第二个是判断本年度生日是否已过

本文给出了三种解决办法，
第一种是最常用的解决办法，即常用的 SQL 语法，不过看起来比较复杂，适合环境为 SQLServer2000 以上
第二种是采用函数的方法，把对日期的逻辑处理放到函数中，调用起来会简约一下，适合环境为 SQLServer2000 以上
第三种是采用 CTE 的方法，用 CTE 来封装判断逻辑，适合环境为 SQLServer2005 以上
-- 创建表和数据
CREATE TABLE employees
(
name VARCHAR(50),
birthday DATETIME
)
INSERT INTO employees VALUES('WBQ','1948-12-08');
INSERT INTO employees VALUES('CZH','1952-02-19');
INSERT INTO employees VALUES('LB','1963-08-30');
INSERT INTO employees VALUES('YLL','1937-09-19');
INSERT INTO employees VALUES('YGQ','1955-03-04');
INSERT INTO employees VALUES('CHH','1963-07-02');
INSERT INTO employees VALUES('SWG','1960-05-29');
INSERT INTO employees VALUES('HW','1958-01-01');
INSERT INTO employees VALUES('YY','1972-02-29');
INSERT INTO employees VALUES('LM','1999-02-08');
INSERT INTO employees VALUES('ZY','1972-06-09');
INSERT INTO employees VALUES('WZH','1999-02-28');
SELECT name,birthday FROM employees

-- 普通的 SQL 实现
SELECT
name,
birthday,
GETDATE() getdate,
CONVERT(VARCHAR(10),GETDATE(),120) todayVarchar,
CONVERT(DATETIME,CONVERT(VARCHAR(10),GETDATE(),120)) todayDateTime,
DATEDIFF(YY,birthday,GETDATE()) DateBetween,
DATEADD(YY,DATEDIFF(YY,birthday,GETDATE()),birthday) ThisBirthday,
DATEADD(YY,DATEDIFF(YY,birthday,GETDATE())+1,birthday) NextBirthday,
CASE WHEN CONVERT(DATETIME,CONVERT(VARCHAR(10),GETDATE(),120))>DATEADD(YY,DATEDIFF(YY,birthday,GETDATE()),birthday)
THEN DATEADD(YY,DATEDIFF(YY,birthday,GETDATE())+1,birthday)
ELSE DATEADD(YY,DATEDIFF(YY,birthday,GETDATE()),birthday)
END newBirthdayStandard,
CASE WHEN CONVERT(DATETIME,CONVERT(VARCHAR(10),GETDATE(),120))>DATEADD(YY,DATEDIFF(YY,birthday,GETDATE()),birthday)
THEN DATEADD(YY,DATEDIFF(YY,birthday,GETDATE())+1,birthday)+
CASE WHEN DAY(birthday)=29 AND DAY(DATEADD(YY,DATEDIFF(YY,birthday,GETDATE())+1,birthday))=28 THEN 1 ELSE 0 END
ELSE DATEADD(YY,DATEDIFF(YY,birthday,GETDATE()),birthday) +
CASE WHEN DAY(birthday)=29 AND DAY(DATEADD(YY,DATEDIFF(YY,birthday,GETDATE()),birthday))=28 THEN 1 ELSE 0 END
END newBirthdayForeign
FROM employees

-- 使用函数来实现
CREATE FUNCTION GetBirthday(@birthday DATETIME,@flag INT)
RETURNS DATETIME
AS
BEGIN
DECLARE @BirthdayRet DATETIME,@BirthdayThis DATETIME,@BirthdayNext DATETIME,@today DATETIME,@dateBetween INT
SET @today=CONVERT(DATETIME,CONVERT(VARCHAR(10),GETDATE(),120))
SET @dateBetween=DATEDIFF(YY,@birthday,GETDATE())
SET @BirthdayThis=DATEADD(YY,@DateBetween,@birthday)
SET @BirthdayNext=DATEADD(YY,@DateBetween+1,@birthday)
IF @flag=1 --2 月 29 日的生日计算为 2 月 28 日
BEGIN
IF @today>@BirthdayThis
SET @BirthdayRet=@BirthdayNext
ELSE
SET @BirthdayRet=@BirthdayThis
END
ELSE --2 月 29 日的生日计算为 3 月 1 日
BEGIN
IF @today>@BirthdayThis
IF DAY(@birthday)=29 AND DAY(@BirthdayNext)=28
SET @BirthdayRet=@BirthdayNext+1
ELSE
SET @BirthdayRet=@BirthdayNext
ELSE
IF DAY(@birthday)=29 AND DAY(@BirthdayThis)=28
SET @BirthdayRet=@BirthdayThis+1
ELSE
SET @BirthdayRet=@BirthdayThis END
RETURN @BirthdayRet
END

SELECT name,birthday,dbo.GetBirthday(birthday,0),dbo.GetBirthday(birthday,1) FROM employees

-- 通过 CTE 来实现
WITH DateBetween AS
(
SELECT
name,
birthday,
GETDATE() getdate,
CONVERT(VARCHAR(10),GETDATE(),120) todayVarchar,
CONVERT(DATETIME,CONVERT(VARCHAR(10),GETDATE(),120)) todayDateTime,
DATEDIFF(YY,birthday,GETDATE()) DateBetween
FROM employees
),
DateBirthdayThisAndNext AS
(
SELECT
name,birthday,getdate,todaydatetime,datebetween,
DATEADD(YY,DateBetween,birthday) AS DateCur,
DATEADD(YY,DateBetween+1,birthday) AS DateNext
FROM DateBetween
),
DateBirthdayThisAndNextForeign AS
(
SELECT
name,birthday,todaydatetime,
DateCur,DateNext,
DateCur+CASE WHEN DAY(birthday)=29 AND DAY(DateCur)=28 THEN 1 ELSE 0 END AS DateCurForeign,
DateNext+CASE WHEN DAY(birthday)=29 AND DAY(DateNext)=28 THEN 1 ELSE 0 END AS DateNextForeign
FROM DateBirthdayThisAndNext
),
DateBirthday AS
(
SELECT
name,birthday,
CASE WHEN DateCurForeign>=todaydatetime THEN DateCurForeign ELSE DateNextForeign END AS birthDayForeign,
CASE WHEN DateCur>=todaydatetime THEN DateCur ELSE DateNext END AS birthDayStandard
FROM DateBirthdayThisAndNextForeign
)
SELECT name,birthday,birthDayForeign,birthDayStandard FROM DateBirthday