Mysql temp table insert from json can't handle null values
The solution is to ommit the property which you want nulled.

After searching for a while I found this bug report from over a year ago that explains how a json string that contains the value NULL is actually a value and not a null.

{"item":1,"quantity":4} --> item = 1, quantity = 4

{"item":1,"quantity":null} --> item = 1, quantity = 'null' ( Some kind of unfriendly json null )

{"item":1} --> item = 1, quantity = NULL ( Your good old sql null ! )

After seeing this, I thought "there's gotta be a way to cast this unfriendly json null into a nice null". I tried many things which worked, but none of them were insertable into a temporary table.

This was my final attempt which should work, based on what I stated above. I manage to detect if the value is null using an in-line IF statement, then I try to cast an actual SQL NULL into a SIGNED int. I demonstrate how you can select the values but that it's impossible to insert them for some reason.

The code itself :

DROP TABLE IF EXISTS json_temp_table;
CREATE TEMPORARY TABLE json_temp_table (
  item_id int NOT NULL PRIMARY KEY,
  model_number varchar(100),
  quantity int NULL
)
ENGINE = INNODB;

INSERT INTO json_temp_table
SELECT X.item_id,X.model_number,CAST(IF(x.quantity_is_null=1,NULL, X.quantity) AS SIGNED) 'quantity' FROM (
SELECT
  json_tb.item_id,
  json_tb.model_number,
  json_tb.quantity,
  if(json_type(json_extract(JSON_OBJECT('b',json_tb.quantity),'$.b')) = 'NULL',1 ,0) 'quantity_is_null'
FROM JSON_TABLE
(
'[{"item_id":1,"model_number":"MFJA53","quantity":4},{"item_id":2,"model_number":"HSRHJN5","quantity":null},{"item_id":3,"model_number":"FAFAF1","quantity":345}]'
, "$[*]"
COLUMNS
(
item_id int PATH "$.item_id",
model_number varchar(100) PATH "$.model_number",
quantity int PATH "$.quantity"
)
) json_tb) x;


Executed on the CLI mysql client

mysql&gt; DROP TABLE IF EXISTS json_temp_table;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql&gt; CREATE TEMPORARY TABLE json_temp_table (
    -&gt;   item_id int NOT NULL PRIMARY KEY,
    -&gt;   model_number varchar(100),
    -&gt;   quantity int NULL
    -&gt; )
    -&gt; ENGINE = INNODB;
Query OK, 0 rows affected (0.00 sec)

mysql&gt; SELECT X.item_id,X.model_number,CAST(IF(x.quantity_is_null=1,NULL, X.quantity) AS SIGNED) 'quantity' FROM (
    -&gt; SELECT
    -&gt;   json_tb.item_id,
    -&gt;   json_tb.model_number,
    -&gt;   json_tb.quantity,
    -&gt;   if(json_type(json_extract(JSON_OBJECT('b',json_tb.quantity),'$.b')) = 'NULL',1 ,0) 'quantity_is_null'
    -&gt; FROM JSON_TABLE
    -&gt; (
    -&gt; '[{"item_id":1,"model_number":"MFJA53","quantity":4},{"item_id":2,"model_number":"HSRHJN5","quantity":null},{"item_id":3,"model_number":"FAFAF1","quantity":345}]'
    -&gt; , "$[*]"
    -&gt; COLUMNS
    -&gt; (
    -&gt; item_id int PATH "$.item_id",
    -&gt; model_number varchar(100) PATH "$.model_number",
    -&gt; quantity int PATH "$.quantity"
    -&gt; )
    -&gt; ) json_tb) x;
+---------+--------------+----------+
| item_id | model_number | quantity |
+---------+--------------+----------+
|       1 | MFJA53       |        4 |
|       2 | HSRHJN5      |     NULL |
|       3 | FAFAF1       |      345 |
+---------+--------------+----------+
3 rows in set, 1 warning (0.00 sec)

mysql&gt; INSERT INTO json_temp_table
    -&gt; SELECT X.item_id,X.model_number,CAST(IF(x.quantity_is_null=1,NULL, X.quantity) AS SIGNED) 'quantity' FROM (
    -&gt; SELECT
    -&gt;   json_tb.item_id,
    -&gt;   json_tb.model_number,
    -&gt;   json_tb.quantity,
    -&gt;   if(json_type(json_extract(JSON_OBJECT('b',json_tb.quantity),'$.b')) = 'NULL',1 ,0) 'quantity_is_null'
    -&gt; FROM JSON_TABLE
    -&gt; (
    -&gt; '[{"item_id":1,"model_number":"MFJA53","quantity":4},{"item_id":2,"model_number":"HSRHJN5","quantity":null},{"item_id":3,"model_number":"FAFAF1","quantity":345}]'
    -&gt; , "$[*]"
    -&gt; COLUMNS
    -&gt; (
    -&gt; item_id int PATH "$.item_id",
    -&gt; model_number varchar(100) PATH "$.model_number",
    -&gt; quantity int PATH "$.quantity"
    -&gt; )
    -&gt; ) json_tb) x;
ERROR 3156 (22018): Invalid JSON value for CAST to INTEGER from column quantity at row 1
mysql&gt;