;with cte as
(
    select time from a
    union
    select time from b
    union
    select time from c
)

select t.time,a.data as A_data,b.data as B_data,c.data as C_data
from cte t left join a on t.time = a.time
           left join b on t.time = b.time
           left join c on t.time = c.time

------解决方案--------------------

SQL code

if object_id('[a]') is not null drop table [a]
go
create table [a]([Time] varchar(5),[Data] int)
insert [a]
select '08:59',12 union all
select '09:00',13 union all
select '10:01',15
go
if object_id('[b]') is not null drop table [b]
go
create table [b]([Time] varchar(5),[Data] int)
insert [b]
select '07:59',1 union all
select '09:00',2 union all
select '11:00',3
go
if object_id('[c]') is not null drop table [c]
go
create table [c]([Time] varchar(5),[Data] int)
insert [c]
select '08:59',23 union all
select '11:00',25 union all
select '12:00',26
go

select isnull(isnull(a.time,b.time),c.time) as time,
a.data,b.data,c.data
from a
full join b on a.time=b.time
full join c on a.time=c.time or b.time=c.time

/**
time  data        data        data
----- ----------- ----------- -----------
08:59 12          NULL        23
09:00 13          2           NULL
10:01 15          NULL        NULL
07:59 NULL        1           NULL
11:00 NULL        3           25
12:00 NULL        NULL        26

(6 行受影响)
**/