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JSON and PL/SQL 12.2: Get Values From JSON Documents With API

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JSON and PL/SQL 12.2: Get Values From JSON Documents With API

Learn about pre-defined PL/SQL object types to perform fine-grained programmatic construction and manipulation of in-memory JSON data.

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With the release of Oracle Database 12c, Oracle SQL entered the Age of JSON. You can use SQL to join JSON data with relational data. You can extract content from within the JSON document and make it available for relational processes and tools. You can even query, right from within the database, JSON data that is stored outside Oracle Database in an external table. Check out my post on JSON resources for more guidance on all of the above.

And that was all possible in 12.1. In Oracle Database 12c Release 2, we added several pre-defined PL/SQL object types to perform fine-grained programmatic construction and manipulation of in-memory JSON data. I'll publish occasional posts on what you can do with these types. Here's the first one, showing you how to get the value for a specific key in a JSON document.

Suppose I've created and populated my species table as follows:

CREATE TABLE json_species
(
   id     NUMBER NOT NULL PRIMARY KEY,
   info   CLOB CONSTRAINT is_json CHECK (info IS JSON ) )
/

BEGIN
   INSERT INTO json_species
        VALUES (1, '{"name":"Spider"}');

   INSERT INTO json_species
        VALUES (2, '{"name":"Elephant", "trunk_length":"10"}');

   INSERT INTO json_species
        VALUES (3, '{"name":"Shark", "fin_count":"4"}');

   COMMIT;
END;
/

If I want to see the values for names in the JSON documents in this table, I can use SQL:

SELECT sp.info.name
  FROM json_species sp

And if this is all you need, there's no reason to bring in the PL/SQL types. If, however, you need to get values in order to perform additional processing on them and perhaps even change the values, the PL/SQL API comes in handy.

Here's how I can get those same values (and display them) in PL/SQL:

DECLARE
   CURSOR species_cur
   IS
      SELECT id, sp.info
        FROM json_species sp
       ORDER BY id;

   l_species   json_object_t;
   l_name      json_element_t;
BEGIN
   FOR rec IN species_cur
   LOOP
      l_species := json_object_t (rec.info);
      l_name := l_species.get ('name');
      DBMS_OUTPUT.put_line (l_name.to_string);
   END LOOP;
END;
/

An explanation of this code:

  • Declare an instance of the JSON_OBJECT_T type.
  • Declare an instance of the JSON_ELEMENT_T type.
  • For every row in the table, get the JSON document (info column) — which, remember is a CLOB, not a JSON document.
  • Create a JSON document from that CLOB by calling the JSON_OBJECT_T constructor function.
  • Call the GET member function of the JSON_OBJECT_T type on the l_species object to get the element for "name".
  • Call the TO_STRING member function of the JSON_ELEMENT_T type to convert the JSON element into a string.
  • Display that string.
And I see this output:
"Spider"
"Elephant"
"Shark"
"Ostrich"

(Well, I will see all that text, but maybe not in that exact order since I didn't specify an ORDER BY clause in my SELECT. Always good to remember that!)

That was the step-by-step approach, which you will want to use when you are just getting started with these types. If, however, all I wanted to do in PL/SQL was display the value (which I could, remember, do with "pure" SQL), I could write and execute nothing more than this:

BEGIN
   FOR rec IN (SELECT sp.info FROM json_species sp)
   LOOP
      dbms_output.put_line (
         json_object_t (rec.info).get ('name').to_string);
   END LOOP;
END;
/

Sure, nothing fancy there. And there's lots more to explore. So:

  1. If you reading this post before February 8, 2018, be sure to sign up for a CodeTalk session on JSON and PL/SQL I am doing with Darryl Hurley of Implestrat. He knows a lot more about this stuff than me! (For now, anyway!)
  2. Check out my LiveSQL script with the above code and more.
  3. I will be publishing more on this in the weeks to come.

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Topics:
database ,tutorial ,json ,pl/sql ,api

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