How to use REGEXP_SUBSTR properly? - sql

Question

Currently in my select statement I have id and value. The value is json which looks like this:
{"layerId":"nameOfLayer","layerParams":{some unnecessary data}
I would like to have in my select id and nameOfLayer so the output would be for example:
1, layerName
2, layerName2
etc.
The json looks always the same so the layerID is the first.
Could you tell me how can I use REGEXP_SUBSTR properly in my select query which looks like this now?
select
id,
value
from
...
where
table1.id = table2.bookmark_id
and ...;

Answer 1

In Oracle 11g, you can extract the layerId using the following regular expression, where js is the name of your JSON column:
regexp_replace(js, '^.*"layerId":"([^"]+).*$', '\1')
This basically extracts the string between double quotes after "layerId":.
In more recent versions, you would add a check constraint on the table to ensure that the document is valid JSON, and then use the dot notation to access the object attribute as follows:
create table mytable (
id int primary key,
js varchar2(200),
constraint ensure_js_is_json check (js is json)
);
insert into mytable values (1, '{"layerId":"nameOfLayer","layerParams":{} }');
select id, t.js.layerId from mytable t;
Demo on DB Fiddle:
ID | LAYERID
-: | :----------
1 | nameOfLayer

Answer 2

Don't use regular expressions; use a JSON_TABLE or JSON_VALUE to parse JSON:
Oracle 18c Setup:
CREATE TABLE test_data (
id INTEGER,
value VARCHAR2(4000)
);
INSERT INTO test_data ( id, value )
SELECT 1, '{"layerId":"nameOfLayer","layerParams":{"some":"unnecessary data"}}' FROM DUAL UNION ALL
SELECT 2, '{"layerParams":{"layerId":"NOT THIS ONE!"},"layerId":"nameOfLayer"}' FROM DUAL UNION ALL
SELECT 3, '{"layerId":"Name with \"Quotes\"","layerParams":{"layerId":"NOT THIS ONE!"}}' FROM DUAL;
Query 1:
SELECT t.id,
j.layerId
FROM test_data t
CROSS JOIN
JSON_TABLE(
t.value,
'$'
COLUMNS (
layerId VARCHAR2(50) PATH '$.layerId'
)
) j
Query 2:
If you only want a single value you could, alternatively, use JSON_VALUE:
SELECT id,
JSON_VALUE( value, '$.layerId' ) AS layerId
FROM test_data
Output:
Both output:
ID | LAYERID
-: | :-----------------
1 | nameOfLayer
2 | nameOfLayer
3 | Name with "Quotes"
Query 3:
You can try regular expressions but they do not always work as expected:
SELECT id,
REPLACE(
REGEXP_SUBSTR( value, '[{,]"layerId":"((\\"|[^"])*)"', 1, 1, NULL, 1 ),
'\"',
'"'
) AS layerID
FROM test_data
Output:
ID | LAYERID
-: | :-----------------
1 | nameOfLayer
2 | NOT THIS ONE!
3 | Name with "Quotes"
So if you can guarantee that no-one is going to put data into the database where the JSON is in a different order then this may work; however the JSON specification allows key-value pairs to be in any order so regular expressions are not a general solution that will parse every JSON string. You should be using a proper JSON parser and there are 3rd party solutions available for Oracle 11g or you can upgrade to Oracle 12c where there is a native solution.
db<>fiddle here

Answer 3

I think you can use regexp_substr like this:
regexp_substr(str, '[^"]+',1,2) as layer_id,
regexp_substr(str, '[^"]+',1,4) as layername
Db<>fiddle demo
Cheers!!