Escape square brackets in Oracle and PostgreSQL - sql

Question

I'm working on a project where the database could be an instance of oracle or Postgres.
I have the need to write a query with a like that work on both dbs.
The query works on a text column containing a JSON string, for example:
{"ruleName":"r2_an","divisionNameList":["div1"],"names":["name1"],"thirdTypeLabels":[],"secondTypeLabels":[],"firstTypeLabels":[]}
I need to select the lines with empty thirdTypeLabels.
select *
from my_table
where JSON like '%thirdTypeLabels%[]%';
On Oracle, for example, does not extract anything, even if in "my_table" there is more than one line matching.
The query is inside a Java software, using JDBC, because we need performace.
Have you any suggestion?

Answer 1

You should use a proper JSON parser otherwise there is no guarantee that %thirdTypeLabels%[]% will restrict the match of the empty array to the thirdTypeLabels key-value pair.
So for Oracle 18c you can use:
SELECT id,
thirdTypeLabelsCount
FROM mytable t
CROSS JOIN
JSON_TABLE(
t.json,
'$'
COLUMNS(
thirdTypeLabelsCount NUMBER PATH '$.thirdTypeLabels.size()'
)
)
WHERE thirdTypeLabelsCount = 0;
or
SELECT *
FROM mytable
WHERE JSON_EXISTS( json, '$ ? (#.thirdTypeLabels.size() == 0) ' )
db<>fiddle

Answer 2

For Postgres you have two choices to make this query work properly:
select *
from the_table
where jsonb_array_length(json::jsonb -> 'thirdTypeLabels') = 0;
Or - starting with Postgres 12 - using a JSON Path expression
select *
from the_table
where jsonb_path_exists(json::jsonb, '$.thirdTypeLabels.size() ? (# == 0)' );
Or use the same JSON path expression as in Oracle:
select *
from the_table
where jsonb_path_exists(json::jsonb, '$' ? (#.thirdTypeLabels.size() == 0)');
In Postgres you should also use a column defined as jsonb rather than text (or varchar)