Google Big Query check if json key exists

Question

I have a Google Big Query Table column, which contains a json string. Over time, new keys were introduced to this json string.

time              col_b     
--------------------------------------------------
timestamp3        {"key1": "value", "key2": "value"}
timestamp2        {"key1": "value"}
timestamp1        {"key1": "value"}

How can I extract key2 and in case it doesn't exist fill with NaN? I thought about something like:

SELECT 
    JSON_EXTRACT(col_b, "$.key2) AS key2
FROM db;

Doing this will throw the error, so it thinks it should be a double instead of a string cause of the null values.

Bad double type: value

How can I check in advance whether key2 is existing and if not fill with null?

Answer 1

See below example

#standardSQL
WITH `project.dataset.table` AS (
  SELECT 'timestamp3' time, '{"key1": "value", "key2": "value"}' col_b UNION ALL
  SELECT 'timestamp2', '{"key1": "value"}' UNION ALL
  SELECT 'timestamp1', '{"key1": "value"}' 
)
SELECT *, IFNULL(JSON_EXTRACT_SCALAR(col_b, '$.key2'), 'NaN') AS key2
FROM `project.dataset.table`

with output

Row time        col_b                               key2     
1   timestamp3  {"key1": "value", "key2": "value"}  value    
2   timestamp2  {"key1": "value"}                   NaN  
3   timestamp1  {"key1": "value"}                   NaN  

Answer 2

Had similar problem, but was looking for a key that could be nullable and was interested if the key was present, despite it being a null. I found example above to be incorrect in my specific case, here's my example below - hope someone will find it useful.

#standardSQL
CREATE TEMPORARY FUNCTION
  jsonHasKey(libs STRING, key STRING)
  RETURNS BOOL
  LANGUAGE js AS '''
      try {
        x = JSON.parse(libs);        
        return x.hasOwnProperty(key);
      } catch (e) {
        return false;
      }
    ''';

WITH `project.dataset.table` AS (
  SELECT 'timestamp3' time, '{"key1": "value", "key2": "value"}' col_b UNION ALL
  SELECT 'timestamp2', '{"key1": "value"}' UNION ALL
  SELECT 'timestamp1', '{"key1": "value"}' UNION ALL
  SELECT 'timestamp0', '{"key1": "value", "key2": null}'
)

SELECT *, IFNULL(JSON_EXTRACT_SCALAR(col_b, '$.key2'), 'NaN') AS key2, jsonHasKey(col_b, 'key2') AS is_present
FROM `project.dataset.table`

which produces the following results:

time        col_b                                           key2    is_present
timestamp3  "{""key1"": ""value"", ""key2"": ""value""}"    value   true
timestamp2  "{""key1"": ""value""}"                         NaN     false
timestamp1  "{""key1"": ""value""}"                         NaN     false
timestamp0  "{""key1"": ""value"", ""key2"": null}"         NaN     true