mysql -->Searching and Modifying JSON Values

Searching and Modifying JSON Values

A JSON path expression selects a value within a JSON document.

Path expressions are useful with functions that extract parts of or modify a JSON document, to specify where within that document to operate. For example, the following query extracts from a JSON document the value of the member with the name key:

mysql> SELECT JSON_EXTRACT('{"id": 14, "name": "Aztalan"}', '$.name');
+---------------------------------------------------------+
| JSON_EXTRACT('{"id": 14, "name": "Aztalan"}', '$.name') |
+---------------------------------------------------------+
| "Aztalan"                                               |
+---------------------------------------------------------+

Path syntax uses a leading $ character to represent the JSON document under consideration, optionally followed by selectors that indicate successively more specific parts of the document:

• A period followed by a key name names the member in an object with the given key. The key name must be specified within double quotation marks if the name without quotes is not legal within path expressions (for example, if it contains a space).

• [N] appended to a path that selects an array names the value at position N within the array. Array positions are integers beginning with zero. If path does not select an array value, path[0] evaluates to the same value as path:

  mysql> SELECT JSON_SET('"x"', '$[0]', 'a');
  +------------------------------+
  | JSON_SET('"x"', '$[0]', 'a') |
  +------------------------------+
  | "a"                          |
  +------------------------------+
  1 row in set (0.00 sec)

• [M to N] specifies a subset or range of array values starting with the value at position M, and ending with the value at position N.

  last is supported as a synonym for the index of the rightmost array element. Relative addressing of array elements is also supported. If path does not select an array value, path[last] evaluates to the same value as path, as shown later in this section (see Rightmost array element).

• Paths can contain * or ** wildcards:

  • .[*] evaluates to the values of all members in a JSON object.

  • [*] evaluates to the values of all elements in a JSON array.

  • prefix**suffix evaluates to all paths that begin with the named prefix and end with the named suffix.

• A path that does not exist in the document (evaluates to nonexistent data) evaluates to NULL.

Let $ refer to this JSON array with three elements:

[3, {"a": [5, 6], "b": 10}, [99, 100]]

Then:

• $[0] evaluates to 3.

• $[1] evaluates to {"a": [5, 6], "b": 10}.

• $[2] evaluates to [99, 100].

• $[3] evaluates to NULL (it refers to the fourth array element, which does not exist).

Because $[1] and $[2] evaluate to nonscalar values, they can be used as the basis for more-specific path expressions that select nested values. Examples:

• $[1].a evaluates to [5, 6].

• $[1].a[1] evaluates to 6.

• $[1].b evaluates to 10.

• $[2][0] evaluates to 99.

As mentioned previously, path components that name keys must be quoted if the unquoted key name is not legal in path expressions. Let $ refer to this value:

{"a fish": "shark", "a bird": "sparrow"}

The keys both contain a space and must be quoted:

• $."a fish" evaluates to shark.

• $."a bird" evaluates to sparrow.

Paths that use wildcards evaluate to an array that can contain multiple values:

mysql> SELECT JSON_EXTRACT('{"a": 1, "b": 2, "c": [3, 4, 5]}', '$.*');
+---------------------------------------------------------+
| JSON_EXTRACT('{"a": 1, "b": 2, "c": [3, 4, 5]}', '$.*') |
+---------------------------------------------------------+
| [1, 2, [3, 4, 5]]                                       |
+---------------------------------------------------------+
mysql> SELECT JSON_EXTRACT('{"a": 1, "b": 2, "c": [3, 4, 5]}', '$.c[*]');
+------------------------------------