JSON_VALUE的作用，简单来说，就是从JSON到SQL：
SQL/JSON function JSON_VALUE selects JSON data and returns a SQL scalar or an instance of a user-defined SQL object type or SQL collection type (varray, nested table)
所以，JSON_VALUE是JSON到SQL间的桥梁。JSON_VALUE只返回一个标量。
JSON_VALUE需2个参数，第一个为JSON文档，第二个为JSON路径。
{"PONumber" : 1,"Reference" : "MSULLIVA-20141102","Requestor" : "Martha Sullivan","User" : "MSULLIVA","CostCenter" : "A50","ShippingInstructions" :{"name" : "Martha Sullivan","Address" :{"street" : "200 Sporting Green","city" : "South San Francisco","state" : "CA","zipCode" : 99236,"country" : "United States of America"},"Phone" :[{"type" : "Office","number" : "979-555-6598"}]},"Special Instructions" : "Surface Mail","LineItems" :[{"ItemNumber" : 1,"Part" :{"Description" : "Run Lola Run","UnitPrice" : 19.95,"UPCCode" : 43396040144},"Quantity" : 7},{"ItemNumber" : 2,"Part" :{"Description" : "Felicia's Journey","UnitPrice" : 19.95,"UPCCode" : 12236101345},"Quantity" : 1},{"ItemNumber" : 3,"Part" :{"Description" : "Lost and Found","UnitPrice" : 19.95,"UPCCode" : 85391756323},"Quantity" : 8},{"ItemNumber" : 4,"Part" :{"Description" : "Karaoke: Rock & Roll Hits of 80's & 90's 8","UnitPrice" : 19.95,"UPCCode" : 13023009592},"Quantity" : 8},{"ItemNumber" : 5,"Part" :{"Description" : "Theremin: An Electronic Odyssey","UnitPrice" : 19.95,"UPCCode" : 27616864451},"Quantity" : 8}]
}
我们使用标准的示例数据。
查看字段的数据类型：
SQL> select j.po_document."User".type() from j_purchaseorder j where j.po_document.PONumber = 1;J.PO_DOCUMENT."USER".TYPE()
----------------------------------------------------------------------------------------------------
stringSQL> select j.po_document.PONumber.type() from j_purchaseorder j where j.po_document.PONumber = 1;J.PO_DOCUMENT.PONUMBER.TYPE()
----------------------------------------------------------------------------------------------------
number从下例可知，JSON文档的字段名是区分大小写的：
SQL> select json_value(po_document, '$.user') from j_purchaseorder where id =1;JSON_VALUE(PO_DOCUMENT,'$.USER')
---------------------------------------SQL>
select json_value(po_document, '$.User') from j_purchaseorder where id =1;
-- 或
select json_value(po_document, '$.User') from j_purchaseorder where json_value(po_document, '$.PONumber') = 1;
-- 或
select json_value(po_document, '$.User') from j_purchaseorder j where j.po_document.PONumber = 1;
-- 或
select j.po_document."User" from j_purchaseorder j where j.po_document.PONumber = 1;JSON_VALUE(PO_DOCUMENT,'$.USER')
--------------------------------------
MSULLIVA
其中的$是JSON路径表达式，语法说明参见17.2 SQL/JSON Path Expression Syntax
返回null的情形：
JSON_VALUE(PO_DOCUMENT,'$.SHIPPINGINSTRUCTIONS.LINEITEMS')
---------------------------------------------------------------------------
这个也返回空，不明白：
select json_value(po_document, '$.LineItems[1].Part.UnitPrice') from j_purchaseorder where id =1;
出错的情形：
SQL> select json_value(po_document, '$.ShippingInstructions.name').type() from j_purchaseorder where id =1;
select json_value(po_document, '$.ShippingInstructions.name').type() from j_purchaseorder where id =1*
ERROR at line 1:
ORA-22806: not an object or REF
Help: https://docs.oracle.com/error-help/db/ora-22806/
RETURNING 子句
文档说：
Use the RETURNING clause to specify the data type of the return value. If you omit this clause, then JSON_VALUE returns a value of type VARCHAR2(4000).
总之，返回的是SQL Data Type。
例如，以下返回的是VARCHAR2(4000)：
SQL> select json_value(po_document, '$.User') from j_purchaseorder where id =1;JSON_VALUE(PO_DOCUMENT,'$.USER')
---------------------------------------------
MSULLIVA
结合文档，以下证明其确实为VARCHAR2类型：
SQL> select dump(json_value(po_document, '$.User')) from j_purchaseorder where id =1;DUMP(JSON_VALUE(PO_DOCUMENT,'$.USER'))
--------------------------------------------------
Typ=1 Len=8: 77,83,85,76,76,73,86,65
指定RETURNING子句，以下显示了准确的数据类型：
create table test as select json_value(po_document, '$.User' RETURNING VARCHAR2(32)) as result from j_purchaseorder j where 1=2;DESC test;SQL> desc test;Name Null? Type----------------------------------------- -------- ----------------------------RESULT VARCHAR2(4000)
本例还可以转换为NUMBER类型：
create table test as select json_value(po_document, '$.User' RETURNING NUMBER) as result from j_purchaseorder j where 1=2;SQL> desc test;Name Null? Type----------------------------------------- -------- ----------------------------RESULT NUMBER返回BOOLEAN的例子，23ai才支持：
SQL> SELECT json_value(po_document, '$.AllowPartialShipment' RETURNING BOOLEAN) as aps
FROM j_purchaseorder j where j.po_document.AllowPartialShipment is not null;APS
-----
true
还可以将true/false转换为数字1/0：
SELECT json_value(po_document, '$.AllowPartialShipment'RETURNING NUMBERALLOW BOOLEAN TO NUMBER CONVERSION)FROM j_purchaseorder j where j.po_document.AllowPartialShipment is not null;JSON_VALUE(PO_DOCUMENT,'$.ALLOWPARTIALSHIPMENT'RETURNINGNUMBERALLOWBOOLEANTONUMBERCONVERSION)
---------------------------------------------------------------------------------------------1
返回标量是最常见的场景，还可以返回User-Defined Object-Type or Collection-Type Instance。
参考
- JSON Developer’s Guide - SQL/JSON Function JSON_VALUE
- SQL Language Reference - JSON_VALUE
- The new SQL/JSON Query operators (Part1): JSON_VALUE
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