SQL Select: Get the previous Date in column - sql

Question

I have a table in my SQL server with some dates. Now I would like to create a Select which gives me a column with all dates then a second column with the previous dates of the first column and a third column with the previous dates of the previous date column(c2).
For Exempel:
c1(orginal) c2(prevoius of c1) c3(previous of c2)
2017-10-15 00:00:00 2017-04-15 00:00:00 2016-10-15 00:00:00
2017-04-15 00:00:00 2016-10-15 00:00:00 2016-04-15 00:00:00
2016-10-15 00:00:00 2016-04-15 00:00:00 2015-10-15 00:00:00
2016-04-15 00:00:00 2015-10-15 00:00:00 null
2015-10-15 00:00:00 null null
Example with colors:
Is it possible to make a SELECT where the first row would be the first date from column 1, the second from column 1 and the third from column 1. The second row would be the second date from column1, the third from column 1 and the forth from column 1.
My current query
SELECT DISTINCT(BFSSTudStichdatum) AS C1, BFSSTudStichdatum AS C2,
BFSSTudStichdatum AS C3 FROM BFSStudierende
ORDER BY C1 DESC
result:

Answer 1

Because you need to get a distinct list of your dates first, you will need to split your query into a common table expression and then use lag to get your c2 and c3 values:
declare #t table(c1 datetime);
insert into #t values ('2017-10-15 00:00:00'),('2017-04-15 00:00:00'),('2016-10-15 00:00:00'),('2016-04-15 00:00:00'),('2015-10-15 00:00:00')
,('2017-10-15 00:00:00'),('2017-04-15 00:00:00'),('2016-10-15 00:00:00'),('2016-04-15 00:00:00'),('2015-10-15 00:00:00');
with c as
(
select distinct c1
from #t
)
select c1
,lag(c1, 1) over (order by c1) as c2
,lag(c1, 2) over (order by c1) as c3
from c
order by c1 desc;
Output:
+-------------------------+-------------------------+-------------------------+
| c1 | c2 | c3 |
+-------------------------+-------------------------+-------------------------+
| 2017-10-15 00:00:00.000 | 2017-04-15 00:00:00.000 | 2016-10-15 00:00:00.000 |
| 2017-04-15 00:00:00.000 | 2016-10-15 00:00:00.000 | 2016-04-15 00:00:00.000 |
| 2016-10-15 00:00:00.000 | 2016-04-15 00:00:00.000 | 2015-10-15 00:00:00.000 |
| 2016-04-15 00:00:00.000 | 2015-10-15 00:00:00.000 | NULL |
| 2015-10-15 00:00:00.000 | NULL | NULL |
+-------------------------+-------------------------+-------------------------+

Answer 2

Are you looking for lag()?
select col1,
lag(col1, 1) over (order by col1) as col1_prev,
lag(col1, 2) over (order by col1) as col1_prev2
from t;

Answer 3

For SQL Server 2008 and later:
WITH DataSource AS
(
SELECT DISTINCT *
,DENSE_RANK() OVER (ORDER BY c1) rowID
FROM #t
)
SELECT DS1.[c1]
,DS2.[c1]
,DS3.[c1]
FROM DataSource DS1
LEFT JOIN DataSource DS2
ON DS1.[rowID] = DS2.[rowID] + 1
LEFT JOIN DataSource DS3
ON DS1.[rowID] = DS3.[rowID] + 2;

Answer 4

For SQL Server 2008 and later:
Hope you did want an auto column generation with lagging one value behind from the past columns first value. Try the following snippet.
Created a dynamic query with respect to the number of columns in the dataset.
create table BFSStudierende
(
BFSSTudStichdatum datetime
)
insert into BFSStudierende
Select getdate()
union
Select dateadd(day,1,getdate())
union
Select dateadd(day,2,getdate())
union
Select dateadd(day,3,getdate())
union
Select dateadd(day,4,getdate())
Declare #count int=(Select count(BFSSTudStichdatum ) from BFSStudierende)
Declare #query nvarchar(max)='with BFSStudierendeCte as (Select *,row_number() over(order by BFSSTudStichdatum)rn from BFSStudierende) Select *from BFSStudierendeCte as BFSStudierendeCte1'
Declare #i int=2 ;
Declare #j int ;
while(#i<=#count)
begin
Set #j=#i-1
Set #query=#query+' left outer join BFSStudierendeCte as BFSStudierendeCte'+cast(#i as varchar(5)) +' on BFSStudierendeCte1.rn+'+cast(#j as varchar(5))+'=BFSStudierendeCte'+cast(#i as varchar(5))+'.rn';
set #i+=1;
End
print #query
Execute(#query)
Note: Duplicate date will not be removed from the results. If you require duplicate to be removed. Please change the following line in the above snippet.
Declare #count int=(Select count(distinct BFSSTudStichdatum ) from BFSStudierende)
Declare #query nvarchar(max)='with BFSStudierendeCte as (Select *,row_number() over(order by BFSSTudStichdatum)rn from(Select distinct BFSSTudStichdatum from BFSStudierende)l ) Select *from BFSStudierendeCte as BFSStudierendeCte1'