Google analytics API: different sum of sessions with 'ga:eventLabel' function and without it

Question

I added 'ga:eventLabel' function to my script and sum of sessions decreased from 2238 to 994. Why?

I expect the same result from both script

dim=['ga:eventLabel', 'ga:source','ga:sourceMedium']
met=['ga:sessions', 'ga:users']
start_date='2019-07-01'
end_date='2019-07-03'
transaction_type='Goal'
goal_number=''
refresh_token=token
condition=''
data_2=google_analytics_reporting_api_data_extraction(viewID,dim,met,start_date,end_date,refresh_token,transaction_type,goal_number,condition)

viewID='*********'
dim=['ga:source','ga:sourceMedium']
met=['ga:sessions', 'ga:users']
start_date='2019-07-01'
end_date='2019-07-03'
transaction_type='Goal'
goal_number=''
refresh_token=token
condition=''
data=google_analytics_reporting_api_data_extraction(viewID,dim,met,start_date,end_date, refresh_token,transaction_type, goal_number,condition)```

Here are the results:

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Answer 1

The two queries have two different meanings, and won't give you the same result, unless you have a data set, where all sessions have at least one event type hit associated with them.

The first query says: count all my users and sessions for the given date range, breaking it down by event label, source, source/medium and date. So in this case, you implicitly filter for any known event labels, where (not set) is an empty, but existing label for a recorded event. Sessions without any events are excluded.

The second query says: count all my users and sessions for the given date range, breaking it down by source, source/medium and date (regardless, if they had any events).

You can verify this behavior, if you create these custom reports in Google Analytics web UI. It is similar to querying custom dimensions: if no value was set for a given custom dimension, those records are excluded.