Postgres | V9.4 | Extract value from json - sql

Question

I have a table that one of the columns is in type TEXT and holds a json object inside.
I what to reach a key inside that json and ask about it's value.
The column name is json_representation and the json looks like that:
{
"additionalInfo": {
"dbSources": [{
"user": "Mike"
}]
}
}
I want to get the value of the "user" and ask if it is equal to "Mike".
I tried the following:
select
json_representation->'additionalInfo'->'dbSources'->>'user' as singleUser
from users
where singleUser = 'Mike';
I keep getting an error:
Query execution failed
Reason:
SQL Error [42883]: ERROR: operator does not exist: text -> unknown
Hint: No operator matches the given name and argument type(s). You might need to add explicit type casts.
Position: 31
please advice
Thanks

Answer 1

The error message tells you what to do: you might need to add an explicit type cast:
And as you can not reference a column alias in the WHERE clause, you need to wrap it into a derived table:
select *
from (
select json_representation::json ->'additionalInfo'->'dbSources' -> 0 ->>'user' as single_user
from users
) as t
where t.single_user = 'Mike';
:: is Postgres' cast operator
But the better solution would be to change the column's data type to json permanently. And once you upgrade to a supported version of Postgres, you should use jsonb.