Demo, Sql Server - rextester

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--Sql Server 2014 Express Edition
--Batches are separated by 'go'

create table dbo.foo (
employee_key bigint,
effective_start_date date,
effective_end_date date,
account_name varchar(100));

insert into foo (employee_key,
effective_start_date,
effective_end_date,
account_name)
values (1
    ,'2017-01-01'
    ,'2017-01-31'
    ,'Google')
,(1
    ,'2017-02-01'
    ,'2017-02-28'
    ,'Apple')
,(1
    ,'2017-03-01'
    ,'2017-03-31'
    ,'Google')
,(1
    ,'2017-04-01'
    ,'9999-12-31'
    ,'Google')

;With Cte as (
        Select *, Bucket = sum(accn) over(partition by employee_key order by effective_start_date) 
        from (
        Select *,case when account_name <> lag(account_name) over(partition by employee_key order by effective_start_date) 
            then 1 else 0 end as AccN
            from foo ) A
    )
    Select top (1) with ties employee_key, Min(effective_start_date) over(partition by Bucket) AS [effective_start_date],       
                Max(effective_end_date) over(partition by Bucket) AS [effective_end_date], account_name
    from cte
    order by row_number() over(partition by bucket order by effective_start_date)

View schema
Execution time: 0,02 sec, rows selected: 3, rows affected: 4, absolute service time: 0,16 sec