16 SQL/JSON Function JSON_QUERY

SQL/JSON function json_query selects one or more values from JSON data and returns a string (VARCHAR2, CLOB, or BLOB instance) that represents the JSON values. You can thus use json_query to retrieve fragments of a JSON document.

The first argument to json_query is a SQL expression that returns an instance of a scalar SQL data type (that is, not an object or collection data type). It can be of data type VARCHAR2, CLOB, or BLOB. It can be a table or view column value, a PL/SQL variable, or a bind variable with proper casting. The result of evaluating the SQL expression is used as the context item for evaluating the path expression.

The second argument to json_query is a SQL/JSON path expression followed by optional clauses RETURNING, WRAPPER, ON ERROR, and ON EMPTY. The path expression can target any number of JSON values.

In the RETURNING clause you can specify data type VARCHAR2, CLOB, or BLOB. A BLOB result is in the AL32UTF8 character set. (VARCHAR2 is the default.) The value returned always contains well-formed JSON data. This includes ensuring that non-ASCII characters in string values are escaped as needed. For example, an ASCII TAB character (Unicode character CHARACTER TABULATION, U+0009) is escaped as \t. Keywords FORMAT JSON are not needed (or available) for json_query — JSON formatting is implicit for the return value.

The wrapper clause determines the form of the returned string value.

The error clause for json_query can specify EMPTY ON ERROR, which means that an empty array ([]) is returned in case of error (no error is raised).

Example 16-1 shows an example of the use of SQL/JSON function json_query with an array wrapper. For each document it returns a VARCHAR2 value whose contents represent a JSON array with elements the phone types, in an unspecified order. For the document in Example 4-2 the phone types are "Office" and "Mobile", and the array returned is either ["Mobile", "Office"] or ["Office", "Mobile"].

Note that if path expression $.ShippingInstructions.Phone.type were used in Example 16-1 it would give the same result. Because of SQL/JSON path-expression syntax relaxation, [*].type is equivalent to .type.

See Also:

Oracle Database SQL Language Reference for information about json_query

Example 16-1 Selecting JSON Values Using JSON_QUERY

SELECT json_query(po_document, '$.ShippingInstructions.Phone[*].type'
                               WITH WRAPPER)
  FROM j_purchaseorder;

• JSON_QUERY as JSON_TABLE
  SQL/JSON function json_query can be viewed as a special case of function json_table.

16.1 JSON_QUERY as JSON_TABLE

SQL/JSON function json_query can be viewed as a special case of function json_table.

Example 16-2 illustrates the equivalence: the two SELECT statements have the same effect.

In addition to perhaps helping you understand json_query better, this equivalence is important practically, because it means that you can use either function to get the same effect.

In particular, if you use json_query more than once, or you use it in combination with json_exists or json_value (which can also be expressed using json_table), to access the same data, then a single invocation of json_table presents the advantage that the data is parsed only once.

Because of this, the optimizer often automatically rewrites multiple invocations of json_exists, json_value and json_query (any combination) to fewer invocations of json_table.

Example 16-2 JSON_QUERY Expressed Using JSON_TABLE

SELECT json_query(column, json_path
                  RETURNING data_type array_wrapper error_hander ON ERROR)
  FROM table;

SELECT jt.column_alias
  FROM table,
       json_table(column, '$' error_handler ON ERROR
         COLUMNS ("COLUMN_ALIAS" data_type FORMAT JSON array_wrapper
                  PATH json_path)) AS "JT";