Friday, January 6, 2017

Using MySQL to Output JSON

MySQL has had a JSON data type since version 5.7 was released way back in '15. But did you know you could produce JSON output from non-JSON columns? It is very simple and saves a lot of time over trying to format it in your application.

World Database

We will be using the good old World database that MySQL has used for years in documentation, examples, and in the classroom. Starting with a simple query we will build up to something more complex.

SELECT Name, District, Population FROM City;

This will output the data from the table in a tabular format.

'Kabul', 'Kabol', '1780000'
'Qandahar', 'Qandahar', '237500'

Array or Object?

We have two options for composing JSON data: JSON_ARRAY and JSON_OBJECT.

Of the two, you will find JSON_ARRAY the least fussy. It will JSON-ize your data very easily. It takes a list of values or an empty list and returns a JSON array.

We add this function to our example query and it becomes SELECT JSON_ARRAY(Name, District, Population) FROM City;

And the output looks like:


'[\"Kabul\", \"Kabol\", 1780000]'
'[\"Qandahar\", \"Qandahar\", 237500]'
...

JSON_OBJECT wants key/value pairs and will complain if the key name is NULL or you have an odd number of objects. If we try SELECT JSON_OBJECT(Name, District, Population) FROM City; we will get Error Code: 1582. Incorrect parameter count in the call to native function 'JSON_OBJECT'. This fuctions sees the odd number of arguments as a 'key' and the evens as the 'value' in key/value pairs and therefore we should not have an odd number of arguments. We could stick in a dummy literal string into the select but odds are that we want the three fields specified but need to turn them into key/value pairs. So lets add 'keys' and let the database supply the values.

SELECT JSON_OBJECT('City', Name, 'Dist', District, 'Pop', Population) FROM City;,

And the output looks like:


'{\"Pop\": 1780000, \"City\": \"Kabul\", \"Dist\": \"Kabol\"}'
'{\"Pop\": 237500, \"City\": \"Qandahar\", \"Dist\": \"Qandahar\"}'
...

Conclusion

Developers need to work smarter and not harder. And I have been harping on letting the database do the heavy lifting for years. This is an example of letting the database format your information for you rather than feeding it into a function within your application. Sure you can do it but this saves you a step or two and reduces the complexity of your application.

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