Extracting key from json string having null value - json

Question

I have following function
CREATE OR REPLACE FUNCTION public."test"(
_data text)
RETURNS integer
LANGUAGE 'plpgsql'
COST 100
VOLATILE
AS $BODY$
declare _sdate date:= null;
begin
select json_extract_path(_data:: json , 'sdate') into _sdate;
select _data:: json -> 'sdate' into _sdate;
END;
$BODY$;
I tried both way but it throws syntax error when sdate is null
I am calling like
select public."test"($${
"sdate":null
}$$)
but when I give sdate value it is working
select public."test"($${
"sdate":"2020-1-01"
}$$)
I also tried explicity parsing like
select _data:: json -> 'sdate'::date into _sdate;
But not working

Answer 1

You have many errors in your function. The first one being that you are not returning anything as you are missing a return statement.
Your expression: _data:: json -> 'sdate'::date is not working because the cast operator :: binds more strongly than the ->' operator and thus you are casting 'sdate'to a date, not the result of the->` operator.
You should also declare the function parameter as json, rather than casting it inside the function. And you need to use ->> to return the value as a text value because there is no direct cast from json to date.
You are casting the result to a date, but your function is declared to return integer which also doesn't match.
CREATE OR REPLACE FUNCTION public."test"(_data json)
RETURNS date --<< here
LANGUAGE plpgsql
stable
AS
$BODY$
declare
_sdate date := null;
begin
_sdate := (_data ->> 'sdate')::date;
return _sdate;
END;
$BODY$;
The way you formatted your code suggests that you think declare is needed for each variable. But declare starts a block that can contain multiple declarations. declarations, it's not something that needs to be repeated for each variable.