Get employee with on-off-on weekend work pattern - sql

Question

I have an employee table which has columns like
employee_ID, punch_in_date, punch_out_date.
Now, what I need is to find those employees who have worked on-off-on weekend pattern.
It is like if an employee has worked in week1 then he/she should not have worked in week2 and must have worked in Week3.
Week1, week2, and week3 are the consecutive weekend days.
I tried using the lag function of sql.
SELECT employee_id,
punch_in_date,
Lag(punch_in_date) OVER(partition BY employee_id ORDER BY employee_id) AS week_lag,
Datediff(day,Lag(punch_in_date) OVER(partition BY employee_id ORDER BY employee_id) ,punch_in_date) AS days
FROM employee
WHERE Datediff(day,Lag(punch_in_date) OVER(partition BY employee_id ORDER BY employee_id) ,punch_in_date)>= 14
AND datediff(day, punch_in_date, 'Today's date') <= 90 /*This means the data must falls under 3 months duration*/;
But I am getting an error like
SQL Error [4108] [S0001]: Windowed functions can only appear in the
SELECT or ORDER BY clauses.
How can I get the required result?
sample data:
employee_ID |punch_in_date |punch_out_date |
------------|--------------|---------------|
2 |2015-12-05 |2015-12-05 |
2 |2015-12-12 |2015-12-12 |
2 |2015-12-19 |2015-12-19 |
2 |2016-01-02 |2016-01-02 |
2 |2016-01-23 |2016-01-24 |
2 |2016-01-24 |2016-01-25 |
2 |2016-01-30 |2016-01-30 |
2 |2016-02-06 |2016-02-06 |
2 |2016-02-06 |2016-02-06 |
2 |2016-02-06 |2016-02-07 |
2 |2016-02-13 |2016-02-14 |
2 |2016-02-27 |2016-02-28 |
2 |2016-03-12 |2016-03-13 |

Answer 1

I suspect you want:
select employee_id, punch_in_date, week_lag
datediff(day, week_lag, punch_in_date) AS days
from (select e.*,
lag(punch_in_date) over (partition by employee_id order by employee_id) as week_lag
from employee e
) e
where week_lag >= 14 and
datediff(day, punch_in_date, getdate()) <= 90 ;
When using window functions, be very careful about where filtering. The filters are applied before the window function, so you might miss some rows that you want.

Answer 2

As the error message states; Windowed function are only allowed in select and order by.
What you can do is to use your query in a subquery
Select Employee_id,punch_in_date, week_lag,[days] FROM(
SELECT employee_id,
punch_in_date,
Lag(punch_in_date) OVER(partition BY employee_id ORDER BY employee_id)
AS week_lag,
Datediff(day,Lag(punch_in_date) OVER(partition BY employee_id ORDER BY
employee_id) ,punch_in_date) AS [days]
FROM employee
where punch_in_date >= dateadd(day,-90,getdate())
) q
WHERE [days]>= 14