WITH ordered_events AS (
 
  SELECT
    *,
 
    LAG(event_timestamp) OVER (
      PARTITION BY user_pseudo_id
      ORDER BY event_timestamp
    ) AS previous_timestamp,
 
    LAG(source) OVER (
      PARTITION BY user_pseudo_id
      ORDER BY event_timestamp
    ) AS previous_source
 
  FROM base_events
 
)

This query is preparing your GA4 event data for session reconstruction.

The goal is:

“For every event, look at the event immediately before it for the same user.”

That allows you to detect:

• inactivity gaps
• traffic source changes
• session boundaries
• journey continuity

You start with raw GA4 events like this:

But session stitching requires knowing:

“What happened BEFORE this event?”

That’s what LAG() does.

What This Query Is Doing

WITH ordered_events AS (

This creates a temporary table called:

ordered_events

Think of it like:

“A cleaned and sequenced event stream.”

Step 1: Select All Existing Fields

SELECT  *,

This keeps all original columns from base_events.

So if base_events has:

• user_pseudo_id
• event_name
• event_timestamp
• source
• medium

they all remain.

Step 2: Get Previous Event Timestamp

LAG(event_timestamp) OVER (
  PARTITION BY user_pseudo_id  
  ORDER BY event_timestamp
) AS previous_timestamp

This is the important part.

What LAG() Means

LAG() says:

“Look one row backward.”

So for each event:

• get the previous event’s timestamp
• for the SAME user
• ordered chronologically
• Exmple

Suppose user A has:

After LAG():

Now you can calculate:

11:00 - 10:05 = 55 minutes

That might indicate:

“New session started.”

Why PARTITION BY user_pseudo_id Matters

PARTITION BY user_pseudo_id

This resets the calculation for each user.

Without it:

• events from different users would mix together
• session logic would break

So this says:

“Only compare events within the same user.”

Why ORDER BY event_timestamp Matters

ORDER BY event_timestamp

This ensures events are processed chronologically.

Otherwise:

• previous events would be random
• inactivity calculations become meaningless

Step 3: Get Previous Traffic Source

LAG(source) OVER (  
     PARTITION BY user_pseudo_id  
     ORDER BY event_timestamp
)
AS previous_source

Now we do the same thing — but for traffic source.Example

Before:

After:

Now you can detect:

“Did acquisition source change?”

If:

source != previous_source

you may decide:

“Start a new session.”

Final Output Structure

After this query, your data becomes:

This gives you the foundation for:

• inactivity analysis
• session stitching
• attribution continuity
• journey reconstruction

Why This Matters in GA4 BigQuery

GA4 exports:

• raw events
• NOT reconstructed sessions

So to rebuild sessions:

1. order events
2. compare against previous behavior
3. detect boundaries
4. assign session IDs

This query handles:

Step #2: behavioral comparison.

The Mental Model

Think of LAG() as:

“Attach the previous row’s value onto the current row.”

That’s it.

For analytics engineering, this becomes extremely powerful for:

• sessionization
• funnel progression
• churn analysis
• attribution modeling
• pathing analysis