Operator does not exist: json = json

Question

when I try to select some record from a table

    SELECT * FROM movie_test WHERE tags = ('["dramatic","women", "political"]'::json)

The sql code cast a error

LINE 1: SELECT * FROM movie_test WHERE tags = ('["dramatic","women",...
                                        ^
HINT:  No operator matches the given name and argument type(s). You might      need to add explicit type casts.

********** 错误 **********

ERROR: operator does not exist: json = json
SQL 状态: 42883
指导建议:No operator matches the given name and argument type(s). You might need to add explicit type casts.
字符:37

Did I miss something or where I can learn something about this error.

Answer 1

You cannot compare json values. You can compare text values instead:

SELECT * 
FROM movie_test 
WHERE tags::text = '["dramatic","women","political"]'

Note however that values of type json are stored as text in a format in which they are given. Thus the result of comparison depends on whether you consistently apply the same format:

SELECT 
    '["dramatic" ,"women", "political"]'::json::text =  
    '["dramatic","women","political"]'::json::text      -- yields false!

In Postgres 9.4+ you can solve this problem using type jsonb, which is stored in a decomposed binary format. Values of this type can be compared:

SELECT 
    '["dramatic" ,"women", "political"]'::jsonb =  
    '["dramatic","women","political"]'::jsonb           -- yields true

so this query is much more reliable:

SELECT * 
FROM movie_test 
WHERE tags::jsonb = '["dramatic","women","political"]'::jsonb

Read more about JSON Types.