Postgres | V9.4 | 从 json 中提取值

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 .