Getting the row before a row with a certain value in SQL

Question

I have a table like below where user actions are stored with a timestamp. My goal is to identify the action that happened before a specific action (named reference_action) and count the number of those actions to see which actions happens before the specific actions and how they are distributed.

I am aware of window functions like LAG() where I can get the row before a certain row but can't figure out how to include a constraint like WHERE action_name = "reference_action".

The query engine is Presto and the tables are Hive tables but I'm mostly interested in the general SQL approach, therefore that shouldn't matter much.

session	action_name	timestamp
1	"some_action"	1970-01-01 00:01:00
1	"some_action"	1970-01-01 00:02:00
1	"some_action"	1970-01-01 00:03:00
1	"desired_action1"	1970-01-01 00:04:00
1	"reference_action"	1970-01-01 00:05:00
1	"some_action"	1970-01-01 00:06:00
1	"some_action"	1970-01-01 00:07:00
2	"some_action"	1970-01-01 01:23:00
2	"some_action"	1970-01-01 02:34:00
2	"desired_action1"	1970-01-01 03:45:00
2	"reference_action"	1970-01-01 04:56:00
2	"some_action"	1970-01-01 05:58:00
3	"some_action"	1970-01-01 01:23:00
3	"some_action"	1970-01-01 02:34:00
3	"desired_action2"	1970-01-01 03:45:00
3	"reference_action"	1970-01-01 04:56:00
3	"some_action"	1970-01-01 05:58:00

The result should look like:

action	count
"desired_action1"	2
"desired_action2"	1

There are two rows where "desired_action1" is directly followed by a row with "reference_action", when ordered by timestamp, hence the count being 2. The same logic applies for why the count is 1 for "desired_action2".

The goal is to know what a user did before he made a purchase (purchase = reference_action). To understand what he did before, I want to look up the action that happened before a purchase. Therefore I need to know the action_name in the row before a reference_action. desired_actions have to be counted, reference_actions are just the rows after the actions I want to count and used to determine which values should be counted.

Answer 1

The steps to do this are actually pretty simple in any SQL variant that supports LAG:

• Pull out the previous action_name using LAG, in a derived table or CTE.
• Filter on the outside to only rows which contain 'reference_action', these rows will also have the previous action from the first step. You want to do this on the outside, otherwise you won't get the correct LAG values.
• Group by the previous action, and count.

SELECT
    prevAction AS action,
    COUNT(*) AS [count]
FROM (
    SELECT *,
        LAG(action_name) OVER (ORDER BY timestamp) AS prevAction -- you may want a PARTITION also
    FROM YourTable
) AS t
WHERE action_name = 'reference_action'
GROUP BY prevAction;