SQL Server join and insert data - sql-server

Question

My name is Thorsten and I'm new to SQL Server. Now I am facing a problem after setting a join... I joined two tables, and it worked so far, but I don't have enough knowledge to proceed.
Here is table1:
| Item | validDate | Price |
| ---- | --------- | ----- |
| A | 01.01.2017 | 100 |
| A | 31.03.2017 | 100 |
| A | 01.04.2017 | 120 |
| A | 31.07.2017 | 120 |
Now I want to create a table that includes a dataset for the gap in table1:
| Item | validDate | Price |
| ---- | --------- | ----- |
| A | 01.01.2017 | 100 |
| A | 28.02.2017 | 100 |
| A | 31.03.2017 | 120 |
... and so on.
My idea was to set a join from table1 to a date table, were every month end is included. But I have to insert the gap as well by creating a new dataset. With what code I'll be able to solve this issue?
As mentioned - I'm a beginner, so I hope I was able to describe my problem.
Thanks in advance for help!

Answer 1

Try to make use of below Query :
DECLARE #Table TABLE (Item VARCHAR(2), validDate DATE, Price INT)
INSERT INTO #Table VALUES
('A','2017-01-01',100),
('A','2017-03-31',100),
('A','2017-04-01',120),
('A','2017-07-31',120)
SELECT Item,DATEADD(DD,-1,validDate) AS Date,Price FROM #Table
OUTPUT
Item Date Price
A 2016-12-31 100
A 2017-03-30 100
A 2017-03-31 120
A 2017-07-30 120

Answer 2

I'm not sure if I understand you correctly but to fill the 'gaps' in your table you need to insert last days of each month. Here's the script that will do that for you. Since I'm fairly new in SQL too, this might not be the best solution, but worked for me. Please not that it will not insert records with dates that already exist in your validDate column:
declare #dateVar date = '2017-01-01' -- script will start calculating last day of the month from this date. DON'T modify the day value
declare #yearVar int = '2017' -- script insert months until the end of the year in this variable
declare #endDates table
(
item nvarchar(1),
endOfMonthDate date
)
while datepart(year, #dateVar) = #yearVar
begin
insert into #endDates
(item, endOfMonthDate)
values (
'A',
dateadd(day, -1, dateadd(month, 1, #dateVar))
)
set #dateVar = dateadd(month, 1, #dateVar)
end
insert into dbo.table1
(Item, validDate)
(
select item, endOfMonthDate
from #endDates
where endOfMonthDate not in (
select validDate
from table1)
)
Now, updating the records with the correct prices will be a little tricky. First we set up the Price for the last days of each month based on the price from the beginning of the month.
update dbo.table1
set table1.Price = t2.Price
from table1
left join table1 as t2
on month(table1.validDate) = MONTH(t2.validDate)
And then, we update the rest records that don't have prices with values from previous months:
declare #loopVar int = 0
declare #nullsNumb int = (select sum(case when table1.Price is null then 1 else 0 end) from table1) --calculates number of nulls in the Price column
while #loopVar < #nullsNumb --not so great solution that inserts previous month's price to every record that doesn't have any price at this moment
begin
update dbo.table1
set table1.Price = t2.Price
from table1
left join table1 as t2
on month(table1.validDate) = MONTH(t2.validDate) + 1
where table1.Price is null
set #loopVar = #loopVar + 1
end
Here's how the data in table1 look now when ordered by validDate:
Item validDate Price
A 2017-01-01 100
A 2017-01-31 100
A 2017-02-28 100
A 2017-03-31 100
A 2017-04-01 120
A 2017-04-30 120
A 2017-05-31 120
A 2017-06-30 120
A 2017-07-31 120
A 2017-08-31 120
A 2017-09-30 120
A 2017-10-31 120
A 2017-11-30 120
A 2017-12-31 120
Let me know if I was able to help.