http://qs1969.pair.com?node_id=11134857


in reply to Re^2: [OT] Re^3: Examples or tutorials for Perl grammars?
in thread Examples or tutorials for Perl grammars?

Can I express xpath like queries, like find all elements which are descendants of another element but the direct parent is matching a special criterion?

How would that look like in SQL?

Edit

I'm aware that xpath is for XML and not JSON. Just ignore the missing axes...

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

  • Comment on Re^3: [OT] Re^3: Examples or tutorials for Perl grammars?

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Re^4: [OT] Re^3: Examples or tutorials for Perl grammars?
by erix (Prior) on Jul 09, 2021 at 19:03 UTC

    The SQL-standard's SQL/JSON description uses the term JSON PATH but does not refer to XPath. The postgres docs however say that jsonpath is inspired by XPath.

    "SQL/JSON path expressions specify the items to be retrieved from the JSON data, similar to XPath expressions used for SQL access to XML"

    Here is an example. The JSON table that I use here consists of millions of rows, each row with a so-called JSON document, not very large, and not very deeply nested. It seems to me that this is/will be typical usage. I retrieved test-JSON data from https://extendsclass.com/json-generator.html (h/t choroba).

    One example row with its JSON content: 

    {                                       
    "age": 63,                          
    "city": "New York City",            
    "date": "1986-04-03",               
    "array": [                          
        "Lila",                         
        "Brandise",                     
        "Jsandye"                       
    ],                                  
    "country": "Iceland",               
    "friends": [                        
        {                               
            "country": "CA",            
            "lastname": "Yusuk",        
            "firstname": "Deirdre"      
        },                              
        {                               
            "country": "AZ",            
            "lastname": "Suanne",       
            "firstname": "Elfreda"      
        },                              
        {                               
            "country": "FM",            
            "lastname": "Magdalen",     
            "firstname": "Britte"       
        }                               
    ],                                  
    "lastname": "Hailee",               
    "firstname": "Jaime",               
    "countryCode": "IE",                
}

    [download]

    Here is a query example. The @@ is a postgresql operator, but the part in the single quotes is a jsonpath expression which is SQL/JSON-compliant. 

    select id, js->'friends' as friends
from jsontable
where js @@ '$.friends[*].country == "AZ"
  && $.friends[*].lastname starts with "Y" ' ;


   id    |               friends              
---------+-------------------------------------
  110877 | [                                   
         |     {                               
         |         "country": "CA",            
         |         "lastname": "Yusuk",        
         |         "firstname": "Deirdre"      
         |     },                              
         |     {                               
         |         "country": "AZ",            
         |         "lastname": "Suanne",       
         |         "firstname": "Elfreda"      
         |     },                              
         |     {                               
         |         "country": "FM",            
         |         "lastname": "Magdalen",     
         |         "firstname": "Britte"       
         |     }                               
         | ]
  116322 | [                                   
         |     {                               
         |         "country": "AZ",            
         |         "lastname": "Ietta",        
         |         "firstname": "Wendi"        
         |     },                              
         |     {                               
         |         "country": "NG",            
         |         "lastname": "Yuille",       
         |         "firstname": "Giustina"     
         |     },                              
         |     {                               
         |         "country": "MS",            
         |         "lastname": "Tengdin",      
         |         "firstname": "Adelle"       
         |     }                               
         | ]
  117778 | [                                   
         |     {                               
         |         "country": "CK",            
         |         "lastname": "Fancie",       
         |         "firstname": "Fanny"        
         |     },                              
         |     {                               
         |         "country": "JM",            
         |         "lastname": "Jerald",       
         |         "firstname": "Mureil"       
         |     },                              
         |     {                               
         |         "country": "AZ",            
         |         "lastname": "Yam",          
         |         "firstname": "Wileen"       
         |     }                               
         | ]

[[...]]

    [download]

    This query returns ~1000 rows (above only 3 are shown) from a 3 GB table with 3 M rows. Thanks to an index on the JSON column (js), the query returns in 120 ms.

    YMMV. It is probably more interesting to try on a real instance. I did some fiddle-ing:

    db-fiddle, postgres 13

    https://www.postgresql.org/docs/current/datatype-json.html#DATATYPE-JSONPATH

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      Great!

      «The Crux of the Biscuit is the Apostrophe»

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