Find the next non-NULL row in a series with SQL
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Find the next non-NULL row in a series with SQL
I’ve stumbled across this fun SQL question on Reddit, recently. The question was looking at a time series of data points where some events happened. For each event, we have the start time and the end time:
The desired output of the query should be this additional count column:
So, the rule is simple. Whenever an event starts, we would like to know how many consecutive entries it takes until the event stops again. We can visually see how that makes sense:
Some observations and assumptions about the problem at hand:
- No two events will ever overlap
- The time series does not progress monotonously, i.e. even if most data points are 1h apart, there can be larger or smaller gaps between data points
- There are, however, no two identical timestamps in the series
How can we solve this problem?
Create the data set, first
We’re going to be using PostgreSQL for this example, but it will work with any database that supports window functions, which are most databases these days.
In PostgreSQL, we can use the
VALUES() clause to generate data in memory easily. For the sake of simplicity, we’re not going to use timestamps, but integer representations of the timestamps. I’ve included the same out-of-the-ordinary gap between 4 and 6:
values (1, 1, null), (2, null, null), (3, null, null), (4, null, 1), (6, null, null), (7, null, null), (8, 1, null), (9, null, 1)
If we run this statement (yes, this is a standalone statement in PostgreSQL!), then the database will simply echo back the values we’ve sent it:
How to deal with non-monotonously growing series
The fact that
column1 is not growing monotonously means that we cannot use it / trust it as a means to calculate the length of an event. We need to calculate an additional column that has a guaranteed monotonously growing set of integers in it. The
ROW_NUMBER() window function is perfect for that.
Consider this SQL statement:
with d(a, b, c) as ( values (1, 1, null), (2, null, null), (3, null, null), (4, null, 1), (6, null, null), (7, null, null), (8, 1, null), (9, null, 1) ), t as ( select row_number() over (order by a) as rn, a, b, c from d ) select * from t;
rn column is a row number calculated for each row based on the ordering of a. For simplicity, I’ve aliased:
a = timestamp
b = start
c = end
The result of this query is:
Nothing fancy yet.
Now, how to use this rn column to find the length of an event?
Visually, we can get the idea quickly, seeing that an event’s length can be calculated using the formula
RN2 - RN1 + 1:
We have two events:
- 4 – 1 + 1 = 4
- 8 – 7 + 1 = 2
So, all we have to do is for each starting point of an event at RN1, find the corresponding RN2, and run the arithmetic. This is quite a bit of syntax, but it isn’t so hard, so bear with me while I explain:
with d(a, b, c) as ( values (1, 1, null), (2, null, null), (3, null, null), (4, null, 1), (6, null, null), (7, null, null), (8, 1, null), (9, null, 1) ), t as ( select row_number() over (order by a) as rn, a, b, c from d ) -- Interesting bit here: select a, b, c, case when b is not null then min(case when c is not null then rn end) over (order by rn rows between 1 following and unbounded following) - rn + 1 end cnt from t;
Let’s look at this new
cnt column, step by step. First, the easy part:
The CASE expression
There’s a case expression that goes like this:
All this does is check if
b is not null and if this is true, then calculate something. Remember,
b = start, so we’re putting a calculated value in the row where an event started. That was the requirement.
The new window function
So, what do we calculate there?
A window function that finds the minimum value over a window of data. That minimum value is RN2, the next row number value where the event ends. So, what do we put in the
min() function to get that value?
Another case expression. When
c is not null, we know the event has ended (remember,
c = end). And if the event has ended, we want to find that row’s
rn value. So that would be the minimum value of that case expression for all the rows after the row that started the event. Visually:
Now, we only need to specify that
OVER() clause to form a window of all rows that followthe current row.
The window is ordered by
rn and it starts 1 row after the current row (
1 following) and ends in infinity (
The only thing left to do now is do the arithmetic:
This is a verbose way of calculating
RN2 - RN1 + 1, and we’re doing that only in those columns that start an event. The result of the complete query above is now: