set @rn = 0;

with tasks as (
    select distinct Start_Date, End_Date
    from Projects
    order by Start_Date
)

select
Start_Date, End_Date,
case 
when lag(End_Date, 1) over(Order by Start_Date) != Start_Date then @rn:=@rn+1
else @rn
end as cnt
from tasks

In the above query @rn := @rn+1 is executed even when the when condition is false.

I encountered this issue which trying to solve a problem from HackeRank.

If someone can explain why @run := @rn+1 is executed even when the condition is false it would be great.

Here is the output that the above query is generating for the specified problem :

2015-10-01 2015-10-02 0

2015-10-02 2015-10-03 1

2015-10-03 2015-10-04 2

2015-10-04 2015-10-05 3

2015-10-11 2015-10-12 24

2015-10-12 2015-10-13 5

2015-10-15 2015-10-16 24

2015-10-17 2015-10-18 24

2015-10-19 2015-10-20 24

2015-10-21 2015-10-22 24

2015-10-25 2015-10-26 24

...