14 Clauses Used in SQL Query Functions and Conditions
Clauses RETURNING
, wrapper, error, and empty-field are described. Each is used in one or more of the SQL functions and conditions json_value
, json_query
, json_table
, json_serialize
, json_mergepatch
, is json
, is not json
, json_exists
, and json_equal
.
- RETURNING Clause for SQL Query Functions
SQL functionsjson_value
,json_query
,json_serialize
, andjson_mergepatch
accept an optionalRETURNING
clause, which specifies the data type of the value returned by the function. This clause and the default behavior (noRETURNING
clause) are described here. - Wrapper Clause for SQL/JSON Query Functions JSON_QUERY and JSON_TABLE
SQL/JSON query functionsjson_query
andjson_table
accept an optional wrapper clause, which specifies the form of the value returned byjson_query
or used for the data in ajson_table
column. This clause and the default behavior (no wrapper clause) are described here. Examples are provided. - Error Clause for SQL Query Functions and Conditions
Some SQL query functions and conditions accept an optional error clause, which specifies handling for a runtime error that is raised by the function or condition. This clause and the default behavior (no error clause) are summarized here. - Empty-Field Clause for SQL/JSON Query Functions
SQL/JSON query functionsjson_value
,json_query
, andjson_table
accept an optionalON EMPTY
clause, which specifies the handling to use when a targeted JSON field is absent from the data queried. This clause and the default behavior (noON EMPTY
clause) are described here. - ON MISMATCH Clause for JSON_VALUE
When theRETURNING
clause specifies a user-defined object-type or collection-type instance, functionjson_value
accepts an optionalON MISMATCH
clause, which specifies handling to use when a targeted JSON value does not match the specified SQL return value. This clause and its default behavior (noON MISMATCH
clause) are described here.
Parent topic: Query JSON Data
14.1 RETURNING Clause for SQL Query Functions
SQL functions json_value
, json_query
,
json_serialize
, and json_mergepatch
accept an optional
RETURNING
clause, which specifies the data type of the value
returned by the function. This clause and the default behavior (no
RETURNING
clause) are described here.
For json_value
, you can use any of these predefined SQL data types in a RETURNING
clause: VARCHAR2
, NUMBER
, DATE
, TIMESTAMP
, TIMESTAMP WITH TIME ZONE
, SDO_GEOMETRY
, and CLOB
. You can also use a user-defined object type or a collection type.
Note:
An instance of Oracle SQL data type
DATE
can include a time component. And in your JSON data you
can use a string that represents an ISO 8601 date-with-time value, that is, it can
have a time component. But json_value
with RETURNING
DATE
always returns a SQL DATE
value that is
an ANSI date, that is, the value has no time component.
To return a SQL DATE
value that has a time component
use RETURNING TIMESTAMP
and then cast the returned
value to DATE
. For
example:
SELECT cast(json_value('{"a" : "2019-01-02T12:34:56"}', '$.a' RETURNING TIMESTAMP) AS DATE)
FROM DUAL;
For
json_query
, json_serialize
, and
json_mergepatch
you can use VARCHAR2
,
CLOB
, or BLOB
. A BLOB
result
is in the AL32UTF8 character set. Whatever the data type returned by
json_serialize
, the returned data represents textual JSON
data.
You can optionally specify a length for
VARCHAR2
(default: 4000
) and a precision and
scale for
NUMBER
.
The default behavior (no RETURNING
clause) is to use
VARCHAR2(4000)
.
Data type SDO_GEOMETRY
is for Oracle Spatial and Graph
data. In particular, this means that you can use json_value
with
GeoJSON data, which is a format for encoding geographic data in JSON.
The RETURNING
clause also accepts two optional
keywords, PRETTY
and ASCII
. If both are present
then PRETTY
must come before ASCII
. Keyword
PRETTY
is not allowed for json_value
.
The effect of keyword PRETTY
is to pretty-print
the returned data, by inserting newline characters and indenting. The default
behavior is not to pretty-print.
The effect of keyword
ASCII
is to automatically escape all non-ASCII
Unicode characters in the returned data, using standard ASCII Unicode escape
sequences. The default behavior is not to escape non-ASCII Unicode
characters.
Tip:
You can pretty-print the entire context item by using only
$
as the path expression.
If
VARCHAR2
is specified in a RETURNING
clause
then scalars in the value are represented as follows:
-
Boolean values are represented by the lowercase strings
"true"
and"false"
. -
The
null
value is represented by SQLNULL
. -
A JSON number is represented in a canonical form. It can thus appear differently in the output string from its representation in textual input data. When represented in canonical form:
-
It can be subject to the precision and range limitations for a SQL
NUMBER
. -
When it is not subject to the SQL
NUMBER
limitations:-
The precision is limited to forty (40) digits.
-
The optional exponent is limited to nine (9) digits plus a sign (
+
or-
). -
The entire text, including possible signs (
-
,+
), decimal point (.
), and exponential indicator (E
), is limited to 48 characters.
-
The canonical form of a JSON number:
-
Is a JSON number. (It can be parsed in JSON data as a number.)
-
Does not have a leading plus (
+
) sign. -
Has a decimal point (
.
) only when necessary. -
Has a single zero (
0
) before the decimal point if the number is a fraction (between zero and one). -
Uses exponential notation (
E
) only when necessary. In particular, this can be the case if the number of output characters is too limited (by a smallN
forVARCHAR2(N)
).
-
Oracle extends the SQL/JSON standard in the case when the returning
data type is VARCHAR2(N)
, by allowing optional keyword
TRUNCATE
immediately after the data type. When
TRUNCATE
is present and the value to return is wider than
N
, the value is truncated — only the first
N
characters are returned. If
TRUNCATE
is absent then this case is treated as an error,
handled as usual by an error clause or the default error-handling behavior.
Related Topics
See Also:
-
Oracle Database SQL Language Reference for information about SQL data types
DATE
andTIMESTAMP
-
Oracle Database SQL Language Reference for information about SQL data type
NUMBER
-
Oracle Spatial and Graph Developer's Guide for information about using Oracle Spatial and Graph data
Parent topic: Clauses Used in SQL Query Functions and Conditions
14.2 Wrapper Clause for SQL/JSON Query Functions JSON_QUERY and JSON_TABLE
SQL/JSON query functions json_query
and json_table
accept an optional wrapper clause, which specifies the form of the value returned by json_query
or used for the data in a json_table
column. This clause and the default behavior (no wrapper clause) are described here. Examples are provided.
The wrapper clause takes one of these forms:
-
WITH WRAPPER
– Use a string value that represents a JSON array containing all of the JSON values that match the path expression. The order of the array elements is unspecified. -
WITHOUT WRAPPER
– Use a string value that represents the single JSON object or array that matches the path expression. Raise an error if the path expression matches either a scalar value (not an object or array) or more than one value. -
WITH CONDITIONAL WRAPPER
– Use a string value that represents all of the JSON values that match the path expression. For zero values, a single scalar value, or multiple values,WITH CONDITIONAL WRAPPER
is the same asWITH WRAPPER
. For a single JSON object or array value, it is the same asWITHOUT WRAPPER
.
The default behavior is WITHOUT WRAPPER
.
You can add the optional keyword UNCONDITIONAL
immediately after keyword WITH
, if you find it clearer: WITH WRAPPER
and WITH UNCONDITIONAL WRAPPER
mean the same thing.
You can add the optional keyword ARRAY
immediately before keyword WRAPPER
, if you find it clearer: WRAPPER
and ARRAY WRAPPER
mean the same thing.
Table 14-1 illustrates the wrapper clause possibilities. The array wrapper is shown in bold
.
Table 14-1 JSON_QUERY Wrapper Clause Examples
JSON Values Matching Path Expression | WITH WRAPPER | WITHOUT WRAPPER | WITH CONDITIONAL WRAPPER |
---|---|---|---|
|
|
|
|
|
|
|
|
|
|
Error (scalar) |
|
|
|
Error (multiple values) |
|
none |
|
Error (no values) |
|
Consider, for example, a json_query
query to retrieve a JSON object. What happens if the path expression matches a JSON scalar value instead of an object, or it matches multiple JSON values (of any kind)? You might want to retrieve the matched values instead of raising an error. For example, you might want to pick one of the values that is an object, for further processing. Using an array wrapper lets you do this.
A conditional wrapper can be convenient if the only reason you are using a wrapper is to avoid raising an error and you do not need to distinguish those error cases from non-error cases. If your application is looking for a single object or array and the data matched by a path expression is just that, then there is no need to wrap that expected value in a singleton array.
On the other hand, with an unconditional wrapper you know that the resulting array is always a wrapper — your application can count on that. If you use a conditional wrapper then your application might need extra processing to interpret a returned array. In Table 14-1, for instance, note that the same array ([42, "a", true]
) is returned for the very different cases of a path expression matching that array and a path expression matching each of its elements.
Parent topic: Clauses Used in SQL Query Functions and Conditions
14.3 Error Clause for SQL Query Functions and Conditions
Some SQL query functions and conditions accept an optional error clause, which specifies handling for a runtime error that is raised by the function or condition. This clause and the default behavior (no error clause) are summarized here.
By default, SQL functions and conditions for JSON avoid raising runtime errors. For example, when JSON data is syntactically invalid, json_exists
and json_equal
return false and json_value
returns NULL
.
But in some cases you can also specify an error clause, which overrides the default behavior. The error handling you can specify varies, but each SQL function and condition for JSON that lets you specify error handling supports at least the ERROR ON ERROR
behavior of raising an error.
The optional error clause can take these forms:
-
ERROR ON ERROR
– Raise the error (no special handling). -
NULL ON ERROR
– ReturnNULL
instead of raising the error.Not available for
json_exists
. -
FALSE ON ERROR
– Return false instead of raising the error.Available only for
json_exists
andjson_equal
, for which it is the default. -
TRUE ON ERROR
– Return true instead of raising the error.Available only for
json_exists
andjson_equal
. -
EMPTY OBJECT ON ERROR
– Return an empty object ({}
) instead of raising the error.Available only for
json_query
. -
EMPTY ARRAY ON ERROR
– Return an empty array ([]
) instead of raising the error.Available only for
json_query
. -
EMPTY ON ERROR
– Same asEMPTY ARRAY ON ERROR
. -
DEFAULT '
literal_return_value
' ON ERROR
– Return the specified value instead of raising the error. The value must be a constant at query compile time.Not available:
-
For
json_exists
,json_equal
,json_serialize
,json_mergepatch
, or ajson_table
column value clause that hasjson_exists
behavior -
For
json_query
or ajson_table
column value clause that hasjson_query
behavior -
For row-level error-handing for
json_table
-
When
SDO_GEOMETRY
is specified either as theRETURNING
clause data type forjson_value
or as ajson_table
column data type
-
The default behavior is NULL ON ERROR
, except for conditions json_exists
and json_equal
.
Note:
There are two levels of error handling for json_table
, corresponding to its two levels of path expressions: row and column. When present, a column error handler overrides row-level error handling. The default error handler for both levels is NULL ON ERROR
.
Note:
An ON EMPTY
clause overrides the behavior specified by ON ERROR
for the error of trying to match a missing field.
Note:
The ON ERROR
clause takes effect only for runtime errors that arise when a syntactically correct SQL/JSON path expression is matched against JSON data. A path expression that is syntactically incorrect results in a compile-time syntax error; it is not handled by the ON ERROR
clause.
Related Topics
See Also:
-
Oracle Database SQL Language Reference for detailed information about the error clause for SQL functions for JSON
-
Oracle Database SQL Language Reference for detailed information about the error clause for SQL conditions for JSON
Parent topic: Clauses Used in SQL Query Functions and Conditions
14.4 Empty-Field Clause for SQL/JSON Query Functions
SQL/JSON query functions json_value
, json_query
, and json_table
accept an optional ON EMPTY
clause, which specifies the handling to use when a targeted JSON field is absent from the data queried. This clause and the default behavior (no ON EMPTY
clause) are described here.
You generally handle errors for SQL/JSON functions and conditions using an error clause (ON ERROR
). However, there is a special case where you might want different handling from this general error handling: when querying to match given JSON fields that are missing from the data. Sometimes you do not want to raise an error just because a field to be matched is absent. (A missing field is normally treated as an error.)
You typically use a NULL ON EMPTY
clause in conjunction with an accompanying ON ERROR
clause. This combination specifies that other errors are handled according to the ON ERROR
clause, but the error of trying to match a missing field is handled by just returning NULL
. If no ON EMPTY
clause is present then an ON ERROR
clause handles also the missing-field case.
In addition to NULL ON EMPTY
there are ERROR ON EMPTY
and DEFAULT
... ON EMPTY
, which are analogous to the similarly named ON ERROR
clauses.
If only an ON EMPTY
clause is present (no ON ERROR
clause) then missing-field behavior is specified by the ON EMPTY
clause, and other errors are handled the same as if NULL ON ERROR
were present (it is the ON ERROR
default). If both clauses are absent then only NULL ON ERROR
is used.
Use NULL ON EMPTY for an Index Created on JSON_VALUE
NULL ON EMPTY
is especially useful for the case of a functional index created on a json_value
expression. The clause has no effect on whether or when the index is picked up, but it is effective in allowing some data to be indexed that would otherwise not be because it is missing a field targeted by the json_value
expression.
You generally want to use ERROR ON ERROR
for the queries that populate the index, so that a query path expression that results in multiple values or complex values raises an error. But you sometimes do not want to raise an error just because the field targeted by a path expression is missing — you want that data to be indexed. Example 26-5 illustrates this use of NULL ON EMPTY
when creating an index on a json_value
expression.
14.5 ON MISMATCH Clause for JSON_VALUE
When the RETURNING
clause specifies a user-defined
object-type or collection-type instance, function json_value
accepts an
optional ON MISMATCH
clause, which specifies handling to use when a
targeted JSON value does not match the specified SQL return value. This clause and its
default behavior (no ON MISMATCH
clause) are described here.
Note:
Clauses ON
ERROR
and ON EMPTY
apply only when no JSON data
matches the path expression. Clause ON MISMATCH
applies when
the JSON data that matches the path expression does not match the specified
object or collection return type. Clause ON MISMATCH
is allowed
only when the return type is an instance of a user-defined object type or
collection type. If you use it with another return type then a query
compile-time error is raised.
When you return an
instance of a SQL object or collection type that reflects the JSON data targeted by
function json_value
, the definitions of that targeted data and the
object or collection to be returned must match, or else a query compile-time error
applies.
The default handling of such an error is just to
ignore it. But you can instead handle such an error in various ways, by
providing one or more ON MISMATCH
clauses, as follows:
-
IGNORE ON MISMATCH
— Explicitly specify the default behavior: ignore the mismatch. The object or collection instance returned can contain one or more SQLNULL
values because of mismatches against the targeted JSON data. -
NULL ON MISMATCH
— Return SQLNULL
as the object-type or collection-type value. -
ERROR ON MISMATCH
— Raise a query compile-time error for the mismatch.
Each of these ON MISMATCH
clause types can also be
followed, in parentheses ((
…)
), by
one or more clauses that each indicates a kind of mismatch to handle, separated by
commas (,
). These are the possible mismatch kinds:
-
MISSING DATA
— Some JSON data was needed to match the object-type or collection-type data, but it was missing. -
EXTRA DATA
— One or more JSON fields have no corresponding object-type or collection-type data. For example, for JSON fieldaddress
there is no object-type attribute with the same name (matching case-insensitively, by default). -
TYPE ERROR
— A JSON scalar value has a data type that is incompatible with the corresponding return SQL scalar data type. This can be because of type incompatibility, as put forth in Table 16-1, or because the SQL data type is too constraining (e.g.,VARCHAR(2)
is two short for JSON string"hello"
).
If no such kind-of-mismatch clause (e.g. EXTRA
DATA
) is present for a given handler (e.g. NULL ON
MISMATCH
) then that handler applies to all kinds of
mismatch.
You can have any number of ON MISMATCH
clauses of
different kinds, but if two or more such contradict each other then a query
compile-time error is raised.
Parent topic: Clauses Used in SQL Query Functions and Conditions