Extract values from JSON type

Introduction#

MySQL 5.7.8+ supports native JSON type. While you have different ways to create json objects, you can access and read members in different ways, too.

Main function is JSON_EXTRACT, hence -> and ->> operators are more friendly.

Syntax#

• JSON_EXTRACT(json_doc,path[,…])
• JSON_EXTRACT(json_doc,path)
• JSON_EXTRACT(json_doc,path1,path2)

Parameters#

• Multiple matched values by path argument(s)

If it is possible that those arguments could return multiple values, the matched values are autowrapped as an array, in the order corresponding to the paths that produced them. Otherwise, the return value is the single matched value.

• NULL Result when:
  • any argemunt is NULL
  • path not matched

Returns NULL if any argument is NULL or no paths locate a value in the document.

Read JSON Array value

Create @myjson variable as JSON type (read more):

 SET @myjson = CAST('["A","B",{"id":1,"label":"C"}]' as JSON) ;

SELECT some members!

 SELECT
   JSON_EXTRACT( @myjson , '$[1]' ) ,
   JSON_EXTRACT( @myjson , '$[*].label') ,
   JSON_EXTRACT( @myjson , '$[1].*' ) ,
   JSON_EXTRACT( @myjson , '$[2].*')
 ;
 -- result values:
 '\"B\"', '[\"C\"]', NULL, '[1, \"C\"]'
 -- visually:
 "B", ["C"], NULL, [1, "C"]

JSON Extract Operators

Extract path by -> or ->> Operators, while ->> is UNQUOTED value:

 SELECT
   myjson_col->>'$[1]' , myjson_col->'$[1]' ,
   myjson_col->>'$[*].label' ,
   myjson_col->>'$[1].*' ,
   myjson_col->>'$[2].*'
 FROM tablename ;
  -- visuall:
     B, "B" , ["C"], NULL, [1, "C"]
  --^^^ ^^^

So col->>path is equal to JSON_UNQUOTE(JSON_EXTRACT(col,path)) :

As with ->, the ->> operator is always expanded in the output of EXPLAIN, as the following example demonstrates:

mysql> EXPLAIN SELECT c->>'$.name' AS name   
    ->     FROM jemp WHERE g > 2\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: jemp
   partitions: NULL
         type: range
possible_keys: i
          key: i
      key_len: 5
          ref: NULL
         rows: 2
     filtered: 100.00
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select
json_unquote(json_extract(`jtest`.`jemp`.`c`,'$.name')) AS `name` from
`jtest`.`jemp` where (`jtest`.`jemp`.`g` > 2)
1 row in set (0.00 sec)

Read about inline path extract(+)