MySQL where JSON contains empty array

Question

How can I search a JSON column for an empty array?

Let's say I have a config column that sometimes has JSON equal to {"tier": []}.

WHERE JSON_CONTAINS(config, '', '$.tier') = 1

or

WHERE JSON_CONTAINS_PATH(config, 'one', '$.tier') = NULL

Aren't returning any results. Is there something I'm missing?

I've looked around for answers to this specifically, but I couldn't find anything on SO or the MySQL docs.

Answer 1

Here are two ways to do it, testing on MySQL 5.7.24:

mysql 5.7.24> select config from mytable 
  where json_contains(config, cast('[]' as json), '$.tier');
+--------------+
| config       |
+--------------+
| {"tier": []} |
+--------------+

mysql 5.7.24> select config from mytable 
  where json_contains_path(config, 'one', '$.tier');
+--------------+
| config       |
+--------------+
| {"tier": []} |
+--------------+

---

I found another solution, which helps to check strictly for an empty array:

First, see that I have two rows, and one has a non-empty array:

mysql 5.7.24> select config from mytable 
  where json_contains(config, json_array(), '$.tier');
+----------------------------------------+
| config                                 |
+----------------------------------------+
| {"tier": []}                           |
| {"tier": [{"name": "BK", "value": 8}]} |
+----------------------------------------+
2 rows in set (0.00 sec)

Now I make sure that the length of the array is 0 as a way of confirming that it is empty:

mysql 5.7.24> select config from mytable 
  where json_contains(config, json_array(), '$.tier') 
  and json_length(config, '$.tier') = 0; 
+--------------+
| config       |
+--------------+
| {"tier": []} |
+--------------+
1 row in set (0.00 sec)

Answer 2

If tier is guaranteed to be an array you can check if 0^th item exists using JSON_CONTAINS_PATH:

SELECT JSON_CONTAINS_PATH('{"tier": []}',             'one', '$.tier[0]'); -- 0
SELECT JSON_CONTAINS_PATH('{"tier": ["foo"]}',        'one', '$.tier[0]'); -- 1
SELECT JSON_CONTAINS_PATH('{"tier": ["foo", "bar"]}', 'one', '$.tier[0]'); -- 1

Alternately use JSON_LENGTH:

SELECT JSON_LENGTH('{"tier": []}',             '$.tier'); -- 0
SELECT JSON_LENGTH('{"tier": ["foo"]}',        '$.tier'); -- 1
SELECT JSON_LENGTH('{"tier": ["foo", "bar"]}', '$.tier'); -- 2