SQL 获取连续编号中断号的最小值

问题描述；返回最小的缺失数,要求一定是正数.
IF OBJECT_ID('dbo.T1') IS NOT NULL
DROP TABLE dbo.T1;
GO
CREATE TABLE dbo.T1
(
keycol  INT         NOT NULL PRIMARY KEY CHECK(keycol > 0),
datacol VARCHAR(10) NOT NULL
);
INSERT INTO dbo.T1(keycol, datacol) VALUES(3, 'a');
INSERT INTO dbo.T1(keycol, datacol) VALUES(4, 'b');
INSERT INTO dbo.T1(keycol, datacol) VALUES(6, 'c');
INSERT INTO dbo.T1(keycol, datacol) VALUES(7, 'd');

--方法1：case when ....end
select
case when not exists(select * from T1 where keycol=1) then 1
else (select MIN(keycol) from T1 a  where not exists(select * from T1 where keycol=a.keycol+1))+1 end
--方法2;COALESCE(a,b)函数--具体翻MSDN吧
select coalesce(MIN(keycol+1),1)
from T1 a
where not exists(select * from T1 where keycol=a.keycol+1)
and exists(select * from T1 where keycol=1)--这个EXISTS如果为NULL where 条件为假，那么MIN(KEYCOL+1)得到一个NULL,那么函数取第二个参数1
--方法3：临时表(这里的临时表产生方法我在后面会说，当然我之前第一次的学习笔记也有，有兴趣的可以去翻翻)
select top 1 N
from T1 right join  NUM on T1.keycol=NUM.N--这里的NUN表是一个从1-1000000的表
where   N<=(select MAX(keycol) from T1 ) and keycol is null
--方法4：表之间的OUTER JOIN
SELECT case when not exists(select * from T1 where keycol=1) then 1
else (select MIN(A.keycol + 1)
FROM dbo.T1 AS A
LEFT OUTER JOIN dbo.T1 AS B
ON B.keycol = A.keycol + 1
WHERE B.keycol IS NULL)end