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I have a table, persons, which contains two columns, an id and a JSONB-based data column (this table has just been made for demonstrational purposes to play around with PostgreSQL's JSON support).

Now, supposed it contains two records:

1, { name: 'John', age: 30 }
2, { name: 'Jane', age: 20 }

Now, supposed I want to get the name of every person older than 25. What I have tried is:

select data->'name' as name from persons where data->'age' > 25

Unfortunately, this results in an error. I can solve it by using ->> instead of ->, but then comparisons don't work as expected any more, since not the numbers are compared, but their representations as strings:

select data->'name' as name from persons where data->>'age' > '25'

I then figured out that I can actually solve the issue by using -> and a cast to int:

select data->'name' as name from persons where cast(data->'age' as int) > 25

This works, but it's not that nice that I have to know the actual type (the type of age in the JSON document is number anyway, so why can't PostgreSQL figure out that by itself?).

I then figured out that if I manually convert to text using the :: syntax, everything works as expected, too - although we are now comparing strings again.

select data->'name' as name from persons where data->'age'::text > '25'

If I then try this with the name instead of the age, it doesn't work:

select data->'name' as name from persons where data->'name'::text > 'Jenny'

This results in an error:

invalid input syntax for type json

Pretty obviously, I don't get something here. Unfortunately, it's quite hard to find any real-world examples of using JSON with PostgreSQL.

Any hints?

  • 1
    In data->'name'::text, you are casting the 'name' string to text, not the result. You don’t get an error when comparing to '25' because 25 is a valid JSON literal; but Jenny is not (although "Jenny" would be). – chirlu Feb 5 '16 at 15:19
  • Thanks, that's the solution :-). I confused 'Jenny' with '"Jenny"'. – Golo Roden Feb 5 '16 at 18:32
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This does not work because it's trying to cast a jsonb value to integer.

select data->'name' as name from persons where cast(data->'age' as int) > 25

This would actually work:

SELECT data->'name' AS name FROM persons WHERE cast(data->>'age' AS int) > 25;

Or shorter:

SELECT data->'name' AS name FROM persons WHERE (data->>'age')::int > 25;

And this:

SELECT data->'name' AS name FROM persons WHERE data->>'name' > 'Jenny';

Seems like confusion with the two operators -> and ->> and operator precedence. The cast :: binds stronger than the json(b) operators.

Figure out type dynamically

This is the more interesting part of your question:

the type of age in the JSON document is number anyway, so why can't PostgreSQL figure out that by itself?

SQL is a strictly typed language, it does not allow the same expression to evaluate to integer in one row and to text in the next. But since you are only interested in the boolean result of the test, you can get around this restriction with a CASE expression that forks depending on the result of jsonb_typeof():

SELECT data->'name'
FROM   persons
WHERE  CASE jsonb_typeof(data->'age')
        WHEN 'number'  THEN (data->>'age')::numeric > '25' -- treated as numeric
        WHEN 'string'  THEN data->>'age' > 'age_level_3'   -- treated as text
        WHEN 'boolean' THEN (data->>'age')::bool           -- use boolean directly (example)
        ELSE FALSE                                         -- remaining: array, object, null
       END;

An untyped string literal to the right of the > operator is coerced to the respective type of the value to the left automatically. If you put a typed value there, the type has to match or you have to cast it explicitly - unless there is adequate implicit cast registered in the system.

If you know that all numeric values are actually integer, you can also:

... (data->>'age')::int > 25 ...
  • what is the sqlalchemy core expression for the above comparison of select statement eg. s=select([issues]).where(issues.c.id == mid).select_from(issues,..... outerjoin(issues.c.data['type_id'] == mtypes.c.id)) ...Here issues.c.data jsonb data type and is being compared with mtypes.c.id of integer type – user956424 Jul 16 at 11:08

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