Chaining JSON_EXTRACT with CAST or STR_TO_DATE fails

Question

I'm trying to extract a datetime from a JSONFIELD "data" in MySQL.

If I do a simple JSON_EXTRACT however, the return field type is a JSON.

mysql> select JSON_EXTRACT(data, "$.new_time") from analytics limit 10;
+----------------------------------+
| JSON_EXTRACT(data, "$.new_time") |
+----------------------------------+
| NULL                             |
| "2016-09-30T04:00:00+00:00"      |
| "2016-09-29T05:30:00+00:00"      |
| NULL                             |
| "2016-10-01T05:30:00+00:00"      |
| "2016-09-27T23:00:00+00:00"      |
| NULL                             |
| "2016-09-23T01:30:00+00:00"      |
| "2016-09-23T04:00:00+00:00"      |
| "2016-09-27T01:30:00+00:00"      |
+----------------------------------+

I want to convert this to a MySQL DATETIME. But if I chain JSON_EXTRACT and STR_TO_DATETIME, I get all NULL values:

mysql> select STR_TO_DATE(JSON_EXTRACT(data, "$.new_time") ,"%Y-%m-%d") from analytics_calendaranalytics limit 10;
+-----------------------------------------------------------+
| STR_TO_DATE(JSON_EXTRACT(data, "$.new_time") ,"%Y-%m-%d") |
+-----------------------------------------------------------+
| NULL                                                      |
| NULL                                                      |
| NULL                                                      |
| NULL                                                      |
| NULL                                                      |
| NULL                                                      |
| NULL                                                      |
| NULL                                                      |
| NULL                                                      |
| NULL                                                      |
+-----------------------------------------------------------+

Likewise, a CAST as DATETIME also fails:

mysql> select CAST(JSON_EXTRACT(data, "$.new_time") as DATETIME) from analytics_calendaranalytics limit 10;
+----------------------------------------------------+
| CAST(JSON_EXTRACT(data, "$.new_time") as DATETIME) |
+----------------------------------------------------+
| NULL                                               |
| NULL                                               |
| NULL                                               |
| NULL                                               |
| NULL                                               |
| NULL                                               |
| NULL                                               |
| NULL                                               |
| NULL                                               |
| NULL                                               |
+----------------------------------------------------+

Both these commands work when I start with the string value:

mysql> select CAST("2016-09-30T04:00:00+00:00" as DATETIME);
+-----------------------------------------------+
| CAST("2016-09-30T04:00:00+00:00" as DATETIME) |
+-----------------------------------------------+
| 2016-09-30 04:00:00                           |
+-----------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> select STR_TO_DATE("2016-09-30T04:00:00+00:00", "%Y-%m-%d");
+------------------------------------------------------+
| STR_TO_DATE("2016-09-30T04:00:00+00:00", "%Y-%m-%d") |
+------------------------------------------------------+
| 2016-09-30                                           |
+------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

Would appreciate any help in resolving this!

score 38 · Accepted Answer · answered Sep 28 '16 at 09:15

38

You have to use JSON_UNQUOTE

select CAST( JSON_UNQUOTE( JSON_EXTRACT(data, "$.new_time")) as DATETIME) from analytics_calendaranalytics limit 10;

Would work. I am saying would because you haven't provided sample data. I tried as follows:

select @js := JSON_OBJECT('new_time',"2016-09-30T04:00:00+00:00"  );

select CAST(JSON_UNQUOTE(JSON_EXTRACT(@js,'$.new_time')) as DATETIME);

The following query also works

 select STR_TO_DATE(JSON_UNQUOTE(JSON_EXTRACT(@js,'$.new_time')) ,"%Y-%m-%d");

answered Sep 28 '16 at 09:15

e4c5

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Perfect! Thanks @e4c5 – rtindru Sep 28 '16 at 15:18
1

Glad to have been of help – e4c5 Sep 28 '16 at 22:33
This fixed my issue too, I was simply trying to update a tables column based on a json field and it kept throwing the CAST error. Make sure you unquote if you are trying to put into an INT field. Thanks – WebTim Sep 25 '18 at 09:34
JSON_UNQUOTE Really helps MySql to CAST things. @e4c5 Thanks – stopanko Dec 05 '22 at 22:28
This helped me doing a query on a date contained in json data – Kerkness Jun 29 '23 at 12:46

score 2 · Answer 2 · answered Apr 21 '21 at 15:09

More READABLE way to do this is using ->>;

SELECT CAST(data->>'$.new_time' AS DATETIME) FROM analytics limit 10;

Example:

mysql> CREATE TABLE analytics (data json not null);
Query OK, 0 rows affected (0.03 sec)

mysql> INSERT INTO analytics (data) VALUES ('{"new_time": "2021-04-21T04:00:00+00:00"}');
Query OK, 1 row affected (0.01 sec)

mysql> SELECT CAST(data->>'$.new_time' AS DATETIME) FROM analytics limit 10;
+---------------------------------------+
| CAST(data->>'$.new_time' AS DATETIME) |
+---------------------------------------+
| 2021-04-21 04:00:00                   |
+---------------------------------------+
1 row in set, 1 warning (0.00 sec)

Chaining JSON_EXTRACT with CAST or STR_TO_DATE fails

2 Answers2

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