Query array column in BigQuery by condition - google-cloud-platform

Question

I have a table in Bigquery with this format:
+------------+-----------------+------------+-----------------+---------------------------------+
| event_date | event_timestamp | event_name | event_params.key| event_params.value.string_value |
+------------+-----------------+------------+-----------------+---------------------------------+
| 20201110 | 2929929292 | my_event | previous_page | /some-page |
+------------+-----------------+------------+-----------------+---------------------------------+
| | layer | /some-page/layer |
| +-----------------+---------------------------------+
| | session_id | 99292 |
| +-----------------+---------------------------------+
| | user._id | 2929292 |
+------------+-----------------+------------+-----------------+---------------------------------+
| 20201110 | 2882829292 | my_event | previous_page | /some-page |
+------------+-----------------+------------+-----------------+---------------------------------+
| | layer | /some-page/layer |
| +-----------------+---------------------------------+
| | session_id | 29292 |
| +-----------------+---------------------------------+
| | user_id | 229292 |
+-------------------------------------------+-----------------+---------------------------------+
I want to perform a query to get all rows where event_params.value.string_value contains the regex /layer.
I have tried this:
SELECT
"event_params.value.string_value",
FROM `my_project.my_dataset.my_events_20210110`,
UNNEST(event_params) AS event_param
WHERE event_param.key = 'layer' AND
REGEXP_CONTAINS(event_param.value.string_value, r'/layer')
LIMIT 100
But I'm getting this output:
+---------------------------------+
| event_params.value.string_value |
+---------------------------------+
| event_params.value.string_value |
+---------------------------------+
| event_params.value.string_value |
+---------------------------------+
| event_params.value.string_value |
+---------------------------------+
| event_params.value.string_value |
+---------------------------------+
Some ideas of what I'm doing wrong?

Answer 1

You are selecting a string - you should select a column.
The other problem is that you're cross joining the table with its arrays - effectively bloating up the table.
Your solution is to use a subquery in the WHERE clause:
SELECT
* -- Not sure what you actually need from the table ...
FROM `my_project.my_dataset.my_events_20210110`
WHERE
-- COUNT(*)>0 means "if you find more than zero" then return TRUE
(SELECT COUNT(*)>0 FROM UNNEST(event_params) AS event_param
WHERE event_param.key = 'layer' AND
REGEXP_CONTAINS(event_param.value.string_value, r'/layer')
)
LIMIT 100
If you actually want the values from the array your quick solution is removing the quotes:
SELECT
event_params.value.string_value
FROM `my_project.my_dataset.my_events_20210110`,
UNNEST(event_params) AS event_param
WHERE event_param.key = 'layer' AND
REGEXP_CONTAINS(event_param.value.string_value, r'/layer')
LIMIT 100