Chaining JSON_EXTRACT with CAST or STR_TO_DATE fails

问题

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!

回答1:

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");

来源：https://stackoverflow.com/questions/39742801/chaining-json-extract-with-cast-or-str-to-date-fails

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