--> 测试数据：#
if object_id('tempdb.dbo.#') is not null drop table #
create table #(src varchar(21))
insert into #
select 'http://www.abc.com/' union all
select 'http://www.a.cn/1.htm' union all
select 'http://e.s.d.gov/1/2/' union all
select 'http://f.w.d.gov/' union all
select 'http://e.c.a.com/' union all
select null union all
select '空'

select *, left( src, charindex('/', src, charindex('//',src)+2) )host from # where src like '%://%'
/*
src                   host
--------------------- ---------------------
http://www.abc.com/   http://www.abc.com/
http://www.a.cn/1.htm http://www.a.cn/
http://e.s.d.gov/1/2/ http://e.s.d.gov/
http://f.w.d.gov/     http://f.w.d.gov/
http://e.c.a.com/     http://e.c.a.com/
*/

------解决方案--------------------
SQL code
declare @url nvarchar(30);
declare @index int;
set @url='http://baidu.com/hello/world/a.html';

select SUBSTRING(@url,0,(CharIndex('/',replace(@url,'http://',''))+7))


------------------------------
http://baidu.com

(1 行受影响)