SQL for JSON Rationalization Part 13: Comparison Operators for JSON Object and JSON Array

As promised in a previous blog the discussion of comparison operators in context of JSON object and JSON array is following in this blog.

Comparison Operators = and <>

Equality and inequality are very straightforward comparison operators and are discussed first. Both are defined on the paths to properties as well as the JSON types of properties.

JSON array equality is (recursively) defined as follows. Two JSON arrays are equal if

  • They have the same number of array indexes starting at index 0
  • The value of each array element is equal for the same index in each of the two JSON arrays

Implicitly this means that order matters in the sense that array elements are compared according to their index position.

JSON object equality is (recursively) defined as follows. Two JSON objects are equal if

  • They have the exact same set of paths
  • The same path in each document leads to the same value and the same JSON type

Implicitly this means that the order of properties in JSON objects does not matter. It is “only” necessary that both objects have the exact same set of paths in any order.

There is no implicit type conversion supported in JSON SQL. The JSON string “15” is considered different from the JSON number 15 as both are of different JSON type.

Sample Data Set

To illustrate equality the following collection compColl is introduced:

{"a":[15,true,{"p":"q"}],"b":[15,true,{"p":"q"}]}
{"a":[15,true,{"p":"q"}],"b":[15,true,{"p":"q"},null]}
{"a":[15,true,{"p":"q"}],"b":["15",true,{"p":"q"}]}
{"x":{"r":15,"s":[true,false]},"z":{"r":15,"s":[true,false]}}
{"x":{"r":15,"s":[true,false]},"z":{"r":15,"s":[[true,false]]}}
{"x":{"r":15,"s":[true,false]},"z":{"r":"15","s":[true,false]}}
{"e":15,"f":[14,15,16]}
{"e":15,"f":[16,15]}
{"e":15}

Sample Queries

An example query for equal JSON arrays is as follows.

select {*} from compColl where a = b

returns

{"a":[15,true,{"p":"q"}],"b":[15,true,{"p":"q"}]}

An example query for equal JSON objects is as follows.

select {*} from compColl where x = z

returns

{"x":{"r":15,"s":[true,false]},"z":{"r":15,"s":[true,false]}}

Inequality is defined as negation of equality. The following queries demonstrate this:

select {*} from compColl where a <> b

returns

{"a":[15,true,{"p":"q"}],"b":[15,true,{"p":"q"},null]}
{"a":[15,true,{"p":"q"}],"b":["15",true,{"p":"q"}]}
select {*} from compColl where x <> z

returns

{"x":{"r":15,"s":[true,false]},"z":{"r":15,"s":[[true,false]]}}
{"x":{"r":15,"s":[true,false]},"z":{"r":"15","s":[true,false]}}

Undefined Comparison Operators <, >, <= and >=

Several comparison operators are undefined for JSON array and JSON object: <, >, <= and >=. If during query processing these comparison operators are used in combination with JSON array and/or JSON object, then the JSON documents will not participate in the comparison and will not add any result document to the result set.

The following query demonstrates that only like JSON types are compared:

select {*} from compColl where a.[0] <= b.[0]

returns

{"a":[15,true,{"p":"q"}],"b":[15,true,{"p":"q"}]}
{"a":[15,true,{"p":"q"}],"b":[15,true,{"p":"q"},null]}

The following query demonstrates that the <= comparison on JSON array is not defined:

select {*} from compColl where a <= b

returns the empty result.

The reason that those four comparison operators are not implemented is that not all JSON types can be compared with each other. For example, a JSON Boolean and a JSON number cannot be compared and consequently the comparison of JSON array or JSON object might fail and return an undefined result. In fact, across all JSON types, only JSON string can be compared to JSON string and JSON number to JSON number by >, <, <= and >=; all other JSON type cannot be compared with each other or other JSON types with these operators.

In context of query processing a failing comparison operator would not be desirable as the query would fail. As a consequence, JSON SQL does not implement the four comparison operations <, >, <= and >= on JSON array and JSON object (actually, on any JSON type except JSON number and JSON string).

However, a user can compare JSON arrays and JSON objects by comparing their array elements or properties individually where applicable or necessary. This is called user-defined comparison and is based on individual restrictions.

User-defined Comparison

A user defines comparison by means of predicates. This supports the user in comparing only those JSON array elements or JSON object properties that need to be compared for the use case at hand and make sense in this context: a user is not forced to compare all JSON array elements or all JSON object properties by can do so selectively.

select {*} from compColl where a.[2].p >= b.[2].p

returns

{"a":[15,true,{"p":"q"}],"b":[15,true,{"p":"q"}]}
{"a":[15,true,{"p":"q"}],"b":[15,true,{"p":"q"},null]}
{"a":[15,true,{"p":"q"}],"b":["15",true,{"p":"q"}]}

As the example shows, there are JSON array elements or JSON object properties that cannot be compared, e.g., a.[2] or b.[2] (except for equal and not-equal).

Since JSON SQL supports JSON documents with varying schema, a user can ensure the presence and JSON type of certain properties that are relevant for comparison with the predicates exists_path and is_of_type. The former ensures the presence, the latter type compatibility.

The following query shows all documents where the property f does not have a second array index but has a property e. If a query compares e with f.[1] then this query shows which documents will not participate in the query.

select {*} 
from   compColl 
where  exists_path e 
       and not exists_path f.[1]

returns

{"e":15}

The analogous is possible with the is_of_type predicate that would show which documents are excluded because of type incompatibility.

Missing Paths

As the example shows, only those JSON documents are participating in the comparison that fulfill the constraints wrt. existence and type compatibility.

A missing path does not falsify the result, as this example shows: the document is simply not participating in the comparison:

The query

select {*} from compColl where e = f.[1]

returns

{"e":15,"f":[14,15,16]}
{"e":15,"f":[16,15]}

Total Order

A total order across all document in a collection is only possible if each document can be compared with every other document in the same collection. With the predicates exists_path and is_of_type it is possible to determine if any documents will be left out of a comparison and hence the documents of a collection cannot be totally ordered with the given predicates.

Summary

Even though the operators >, <, >= and <= cannot be implemented for several JSON types, clients can implement partial comparison of documents with combinations of individual restrictions. The predicates exists_path and is_of_type allow to determine the set of documents included in (or excluded from) the query.

Go [ JSON | Relational ] SQL!

Disclaimer

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.

SQL for JSON Rationalization Part 12: SERVER-831

There is an almost infamous bug in context of MongoDB, called SERVER-831. This blog puts SERVER-831 in context of JSON SQL.

Upfront

In order to not loose the context, the bug and one of the use cases were captured as PDFs and are attached here:

Anatomy of SERVER-831

Server 831 has two main aspects: a selection combined with an update. In the following only the selection is discussed; the update part follows down the road when update in context of JSON SQL will be introduced.

Basically, the selection selects based on the value of array elements without knowing the precise array index where the values are located. The array index is not part of the selection criteria, only the content of array elements.

Data Set

These are the documents in the collection server831 used for illustration in this blog:

{"a":[5,4,3,2,1]}
{"a":[5,10,15,20,25]}
{"a":[1,2,3,4,5]}
{"a":[{"_id":7},{"_id":8}]}
{"a":[{"_id":8},{"_id":7}]}
{"a":[null,[0,0,7],null]}
{"a":[true,false],"b":true}
{"a":[true,false],"b":false}
{"a":[true,[null],false]}

Predicating Array Elements by Array Index

So far it was possible to create a selection based on the array index. For example, the following query returns all documents that contain an array “a” where the value at index 0 is 5.

select {*} from server831 where a.[0] = 5

returns

{"a":[5,4,3,2,1]}
{"a":[5,10,15,20,25]}

Predicating Array Elements by Content

Sometimes there is the situation where the array index is unknown and the client wants to retrieve all documents where at least one array element has a specific value. Referring to every array element is denoted as [*] in the JSON SQL syntax (instead of referring to a specific array index).

For example, the following query selects all documents where at least one of the array elements of “a” has the value 5.

select {*} from server831 where a.[*] = 5

results in

{"a":[5,4,3,2,1]}
{"a":[5,10,15,20,25]}
{"a":[1,2,3,4,5]}

More interesting queries are the following examples that reach into the value(s) of array elements:

select {*} from server831 where a.[*]._id = 7

returns

{"a":[{"_id":7},{"_id":8}]}
{"a":[{"_id":8},{"_id":7}]}

Not only scalar literals can be used, but non-scalar literals as well.

select {*} from server831 where a.[*] = [0,0,7]

returns

{"a":[null,[0,0,7],null]}

Selection does not have to be based on literals:

select {*} from server831 where a.[*] = b

returns

{"a":[true,false],"b":true}
{"a":[true,false],"b":false}

The latter allows comparison of values inside documents.

Any number of levels can be referenced, for example

select {*} from server831 where a.[*].[*] = null

returns

{"a":[true,[null],false]}

SERVER-831 Use Case

The document provided in one of the use cases in SERVER-831 is as follows (with cleaned up syntax errors):

{ "_id": 1, 
  "name": "Dave Gahan", 
  "medications": [ 
  { "_id": 23, 
    "name": "Dilaudid", 
    "type": "Rx", 
    "prescriptions": [ 
    { "_id": 13, 
      "quantity": 60, 
      "started": "2009-01-01" }, 
    { "_id": 77, 
      "quantity": 45, 
      "started": "2009-02-01" } ] }, 
  { "_id": 41, 
    "name": "Oxycodone", 
    "type": "Rx" } ]}

The corresponding query in form of JSON SQL is (assuming the data is stored in collection uc831):

select {*} 
from uc831 
where _id = 1 
      and medications.[*]._id = 23 
      and medications.[*].prescriptions.[*]._id = 77

Summary

Selection based on array element values without the knowledge of the array index is an extremely powerful query feature of JSON SQL and is most likely useful in many different use cases.

The syntax of JSON SQL had to be modified only minimally without any restriction or loss of query expressiveness.

Go [ JSON | Relational ] SQL!

Disclaimer

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.

SQL for JSON Rationalization Part 11: JSON Specific Predicates

JSON SQL cannot assume collections containing only homogeneous documents; instead, it must support schemaless documents. This requires JSON specific predicates that are discussed in this blog.

Example Collection

The documents of the following collection “predColl” are used in this blog as examples.

{"a":"b"}
{"a":{"c":1,"d":2},"e":[77,{"x":"eightyeight"}]}
{"a":{"c":1,"d":2},"e":["seventyseven",{"x":88}]}

Definition: Path

A (complete) path in a JSON document is the list of property names and array indexes from the root all the way to a leaf separated by “.”. For example, a complete path is

e.[1].x

A partial path in a JSON document is the list of property names and array indexes from the root to an intermediary property (that is not a leaf property) separated by “.”. For example, a partial path is

e.[1]

It is possible that a path is a complete path in one document and a partial path in another document. For example, a path that is complete and partial at the same time is

a

Each path leads up to a property that is of a specific JSON type. For example,

  • the type of e.[1].x is JSON_STRING in one of the example documents and JSON_NUMBER in another one.
  • the type of e.[1] is JSON_OBJECT in both cases.

Document: Set of Paths

Based on the above definition, a JSON document can be represented as the set of its paths.

The document

{"a":{"c":1,"d":2},"e":["seventyseven",{"x":88}]}

can be represented as

a
a.c
a.d
e.[0]
e.[1]
e.[1].x

The corresponding types are

JSON_OBJECT
JSON_NUMBER
JSON_NUMBER
JSON_STRING
JSON_OBJECT
JSON_NUMBER

Schemaless Documents / Schema-per-Document

Traditionally, databases enforced the definition of a schema and many continue to do so. This means that data can only be stored successfully in a database if the data complies with the schema at that point in time.

Some database systems do not enforce a schema and as a consequence the data is not constrained by a schema. In context of JSON documents, if a document is not constrained by a schema, then it can have any valid structure (in terms of the rules the JSON standard defines).

These documents are termed “schemaless” documents. In reality there is a schema, an implicit  (intentional) one, for each document, termed “schema-per-document”.

In concrete terms “schemaless” in context of a database system means:

  • Any pair of documents within a collection of a database can potentially have different sets of paths
  • The same (complete or partial) path in two different documents can lead to a value of a different JSON type

As a consequence, queries must be able to test for the existence of paths as well as for the existence of a specific JSON type as these cannot be assumed (in contrast to a database that enforces schemas).

JSON Specific Predicates: exists_path and is_of_type

Two predicates are needed in order to check for the existence of paths or JSON types of values.

  • exists_path <path>
  • <path> is_of_type <JSON type>

The query

select {*} from predColl where exists_path a.d

returns

{"a":{"c":1,"d":2},"e":[77,{"x":"eightyeight"}]}
{"a":{"c":1,"d":2},"e":["seventyseven",{"x":88}]}

The query

select {*} from predColl where e.[1].x is_of_type JSON_number

returns

{"a":{"c":1,"d":2},"e":["seventyseven",{"x":88}]}

Negation

Negation of the predicates is useful to determine if there are document that do not have specific paths or paths do not refer to properties of specific JSON types.

select {*} from predColl where not exists_path e.[1].x

returns

{"a":"b"}

The query

select {*} from predColl where not a is_of_type JSON_object

returns

{"a":"b"}

Definition: (Partial) Homogeneity

Queries using the predicates exists_path or is_of_type can be used to determine if all documents have the required or expected paths or if there are documents that are missing specific paths or have paths leading to unexpected JSON types.

With these predicates it is possible to determine if the documents in a collection are homogeneous wrt. given set of paths.

Dynamic Schema Check

A developer can now implement first checking path existence or JSON type compliance before executing business logic related queries. If the required paths or JSON types are missing in specific documents, appropriate error handling can be implemented for these.

From a different viewpoint, a developer now has the tools to dynamically check or select all documents complying to a schema that is “imagined” at the type of querying.

Summary

While schemaless documents are convenient from a development perspective, this requires in general the ability to check for the existence or absence of paths as well as JSON type compliance.

The predicates exists_path and is_of_type provide the necessary querying tools in order to test for the variations in schemaless JSON documents.

Go [ JSON | Relational ] SQL!

Disclaimer

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.

SQL for JSON Rationalization Part 10: Complex Restriction

Until this blog only single restrictions were used; here is the discussion of complex restrictions in JSON SQL.

Example Data Set

In the following the collection “comrescoll” (for complex restriction collection) is used and it contains the following JSON documents:

{"a":{"c":"foo"},"b":[true,false,null]}
{"a":{"c":"foo"}}
{"b":[true,false,null]}

Single Restriction Semantics

As defined in a previous blog, a document only participates in a single restriction if the referenced properties are present.

For example,

select {*} 
from comrescoll 
where a.c = 'foo'

returns

{"a":{"c":"foo"},"b":[true,false,null]}
{"a":{"c":"foo"}}

as only those two object have the property “a.c” defined.

As the blog will show, a complex restriction applies to a document if all properties requiring evaluation are present.

Note that this previous sentence was really carefully worded. The following would not be correct: a complex restriction applies to a document if all properties that are referenced in the complex restriction are present.

The reason will be discussed later in detail.

AND

In an ANDed complex restriction, all properties that require evaluation have to be present in the document. A document will be included into the result set if the complex restriction evaluates to true for the document. The following query shows this case:

select {*} 
from comrescoll 
where a.c = 'foo' and b.[1] = false

returns

{"a":{"c":"foo"},"b":[true,false,null]}

… WHERE true, … WHERE false

A restriction always resulting in true can be implemented like this:

select {*} 
from comrescoll 
where 5 = 5

No complex restriction is required to accomplish this.

A restriction always resulting in false can be implemented like this:

select {*} 
from comrescoll 
where a.c = ‘foo’ and a.c = ‘bar’

However, an easier way is

select {*} 
from comrescoll 
where 5 = 6

This case does not require a complex restriction either.

OR

The OR operator behaves as expected:

select {*} 
from comrescoll 
where a.c = 'foo' or b.[1] = false

returns

{"a":{"c":"foo"},"b":[true,false,null]}
{"a":{"c":"foo"}}
{"b":[true,false,null]}

In case of a disjunction not all paths have to actually be present for a correct execution:

select {*} 
from comrescoll 
where d.[1] = false or a.c = 'foo'

returns

{"a":{"c":"foo"},"b":[true,false,null]}
{"a":{"c":"foo"}}

Even though the path “d.[1]” is not present, the evaluation takes place and is correct as the absence of a path does not necessarily fail the execution in a disjunction if this part of the complex restriction is unnecessary.

To force the presence of the path “d.[1]”, a predicate like “exists_path” would have to be added like this:

select {*} 
from comrescoll 
where d.[1] = false or a.c = 'foo' and exists_path d.[1]

This is not implemented yet, but it will be down the road at some point in time.

NOT

The operator NOT behaves as expected. The query

select {*} 
from comrescoll 
where not a.c = 'foo' and b.[1] = false

results in

{"b":[true,false,null]}

This might be surprising initially as the result contains a document that does not contain the path “a.c”. However, the absence of the path “a.c” means that “a.c” does not have the value ‘foo’.

Like above, the presence of “a.c” would be enforced by a conjunction of exists_path “a.c”.

Combination of AND, OR, NOT

The operators AND, OR and NOT can be combined as expected according to the SQL 92 standard. For example, the query

select {*} 
from comrescoll 
where not a.c = '' and not b.[0] = 0 or not b.[1] = 1

results in

{"a":{"c":"foo"},"b":[true,false,null]}
{"a":{"c":"foo"}}
{"b":[true,false,null]}

Caveat

Remember the “careful wording” from earlier? Consider the following query:

select {*} 
from comrescoll 
where a.c = 'foo' and b.[1] = false or d = 6 or d <> 6

On the surface of it, the paths “a.c”, “b.[1]” and “d” need to be present for the complex restriction to be evaluated. However, this query returns the following result:

{"a":{"c":"foo"},"b":[true,false,null]}

Upon closer inspection, the two restrictions involving the path “d” are insignificant. The path “d” is not needed for the evaluation even though the path “d” is referred to in the complex restriction.

Another example is

select {*} 
from comrescoll 
where a.c = 'foo' and b.[1] = false or (d = 6 and d <> 6)

resulting in

{"a":{"c":"foo"},"b":[true,false,null]}

What if the path “d” should be present? In this case a predicate like exists_path is necessary (to be discussed in one of the future blogs). With the additional predicate exists_path only those documents are included in the evaluation that actually contain that path.

Summary

Complex restrictions in JSON SQL are very similar to Relational SQL in their syntax and semantics except for the difference that paths that are not required for the evaluation of a complex restriction do not have to be present in the documents even though they are mentioned in the complex restriction.

Go [ JSON | Relational ] SQL!

Disclaimer

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.

 

SQL for JSON Rationalization Part 9: Restriction – Arrays

This installment reviews restriction in JSON SQL based on JSON array literals (all other JSON types except JSON array have been discussed in previous blogs).

JSON Object Notation

JSON SQL follows the JSON object notation as defined in the JSON standard. An empty JSON array is denoted as [] and a non-empty JSON array has one or more comma separated values, including JSON array.

A JSON array literal is either an empty JSON array or a non-empty JSON array. A JSON array literal is not enclosed in quotes. The only JSON literal enclosed in quotes is JSON string. If a JSON array is enclosed in quotes then it is not a JSON array, but a JSON string.

Sample Document Set

The following document set is used in this blog and the documents are stored in a collection called “arrayColl”.

select {*} from arrayColl

results in

{"one":[{"a":1},{"b":2}]}
{"one":"[{\"a\": 1}, {\"b\": 2}]"}
{"three":[{"b":[{"c":null},{"d":true}]}]}
{"four":[{"x":8,"y":9}]}
{"five":[]}

Restriction based on JSON Object Literal

Starting with the empty JSON array literal, the following two queries product the same result.

select {*} from arrayColl where five = []

and

select {*} from arrayColl where [] = five

result in

{"five":[]}

In the following, queries show the JSON array literal on the right side of the operator, however, it can be on either side.

Operators = And <>

The operators = and <> are defined for a JSON array literal. JSON SQL regards two JSON arrays as equal if both have the same (equal) values in the same order; and not equal otherwise.

The query (restriction using JSON array literal)

select {*} from arrayColl where one = [{"a": 1}, {"b": 2}]

returns

{"one":[{"a":1},{"b":2}]}

The query (restriction using JSON string literal(!))

select {*} from arrayColl where one = '[{"a": 1}, {"b": 2}]'

returns

{"one":"[{\"a\": 1}, {\"b\": 2}]"}

A restriction can reach into the JSON array as well using the path notation. The query

select {*}
from arrayColl
where three.[0].b = [{"c": null}, {"d": true}]

returns

{"three":[{"b":[{"c":null},{"d":true}]}]}

Operators <, >, <= And >=

The operators <, >, <= and >= could be defined recursively for convenience with some restrictions. For example, a JSON array could be considered less than another JSON array if both have the same values and if the corresponding values are less than another.

However, JSON true, JSON false and JSON null would not be able to participate in the operator <, >, <= or >=, only JSON object, JSON array, JSON number and JSON string.

Those four operators are currently not directly implemented in JSON SQL since it is possible to achieve the same by writing a complex conjunctive restriction (details on this approach will be discussed in a subsequent blog as well as strategies of what to do if any of JSON true, JSON false or JSON null are present).

Canonical Interpretation

The order of the values inside a JSON array is significant, but not within a JSON object. The query

select {*} from arrayColl where four = [{"y": 9, "x": 8}]

therefore results in

{"four":[{"x":8,"y":9}]}

Summary

Restriction by JSON array is provided by JSON SQL without problem and the syntax extends the Relational SQL syntax naturally.

Go [ JSON | Relational ] SQL!

Disclaimer

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.

SQL for JSON Rationalization Part 8: Restriction – Objects

This installment reviews restriction in JSON SQL based on JSON object literals (all other JSON types except JSON array have been discussed in previous blogs).

JSON Object Notation

JSON SQL follows the JSON object notation as defined in the JSON standard. An empty JSON object is denoted as {} and a non-empty JSON object has one or more comma separated pairs (a pair is a tuple of string and JSON type separated by a colon – also referred to as property).

A JSON object literal is either an empty JSON object or a non-empty JSON object. A JSON object literal is not enclosed in quotes. The only JSON literal enclosed in quotes is JSON string. If a JSON object is enclosed in quotes then it is not a JSON object, but a JSON string.

Sample Document Set

The following document set is used in this blog and the documents are stored in a collection called “objectColl”.

select {*} from objectColl

results in

{"one": {"a": 1}}
{"one": "{\"a\": 1}"}
{"three": {"b": {"c": null}}}
{"four": {"x": 8, "y": 9}}
{"five": {}}

Restriction based on JSON Object Literal

Starting with the empty JSON object literal, the following two queries product the same result.

select {*} from objectColl where five = {}

and

select {*} from objectColl where {} = five

result in

{"five": {}}

In the following, queries show the JSON object literal on the right side of the operator, however, it can be on either side.

Operators = And <>

The operators = and <> are defined for a JSON object literal. JSON SQL regards two JSON objects as equal if both have the same pairs (recursively), in any order; and not equal otherwise.

The query (restriction using JSON object literal)

select {*} from objectColl where one = {"a": 1}

returns

{"one": {"a": 1}}

The query (restriction using JSON string literal(!))

select {*} from objectColl where one = '{"a": 1}'

returns

{"one": "{\"a\": 1}"}

A restriction can reach into the JSON object as well using the path notation. The query

select {*} from object Coll where three.b = {"c": null}

returns

{"three": {"b": {"c": null}}}

Operators <, >, <= And >=

The operators <, >, <= and >= could be defined recursively for convenience with some restrictions. For example, a JSON object could be considered less than another JSON object if both have the same pairs and if the values of the corresponding pairs are less than another.

However, JSON true, JSON false and JSON null would not be able to participate in the operator <, >, <= or >=, only JSON object, JSON array, JSON number and JSON string.

Those four operators are currently not directly implemented in JSON SQL since it is possible to achieve the same by writing a complex conjunctive restriction (details on this approach will be discussed in a subsequent blog as well as strategies of what to do if any of JSON true, JSON false or JSON null are present).

Canonical Interpretation

The order of the pairs inside a JSON object is not significant (according to the JSON standard). The query

select {*} from objectColl where four = {"y": 9, "x": 8}

therefore results in

{"four": {"x": 8, "y": 9}}

Summary

Restriction by JSON object is provided by JSON SQL without problem and the syntax extends the Relational SQL syntax naturally.

Go [ JSON | Relational ] SQL!

Disclaimer

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.

SQL for JSON Rationalization Part 7: Restriction – True, False and Null

The last blog (Part 6) introduced the general notion of restriction and focused on JSON String and JSON Number. This blog will extend the discussion covering JSON true, JSON false and JSON null.

Sample Data Set

The following set of documents stored in the collection “boolcoll” is used in this blog:

select {*} from boolcoll

returns

{"a":true}
{"a":false}
{"true":false}
{"true":"null"}

Textual Representation

Even though the JSON standard defines that JSON true, JSON false and JSON null are all lowercase, in context of JSON SQL all combinations of upper and lower case characters are permitted:

select {*} from boolcoll where a = TruE

returns

{"a":true}

Operators

Not all of the default operators are defined on JSON true, JSON false or JSON null. Defined are = and <>, undefined are <, >, <= and >=. If one of the undefined operators is used in conjunction with JSON true, JSON false or JSON null, a semantic query analysis error is returned before the query is executed.

Restriction Syntax

As in the last blog, the property name can be on either side of the operator. The following two queries return the same result:

select {*} from boolcoll where a <> false

and

select {*}  from boolcoll where false <> a

return

{"a":true}

Execution Semantics

If the specified property is present, the restriction is evaluated and the document is added to the result set if it fulfills the restriction. If the property is not present, no evaluation and consequently no inclusion into the result set takes place.

select {*} from boolcoll where null = null

returns all documents as null = null is always true.

Disambiguation

As shown in the example data set for this blog, it is possible that a property name is “true”, “false” or “null”. So far only the short form of property names was used with JSON SQL queries, i.e., property names without being enclosed in double quotes (contrary to the JSON standard definition).

However, as soon as property names can be the same as keywords like JSON true, JSON false or JSON null, disambiguation has to take place. This is accomplished by following the JSON standard: enclosing the property name in double quotes. For example,

select {*} from boolcoll where "true" = false

returns all documents with a property name of “true” that has the value JSON false.

{"true":false}

Since Relational SQL does not use double quotes, there cannot be any confusion:

Along the same lines,

select {*} from boolcoll where "true" = 'null'

returns all documents where the property “true” has the value ‘null’ (string).

{"true":"null"}

This is not ambiguous, either, as Relational SQL uses single quotes to denote String literals.

Needless to say that double quotes can be used outside a disambiguation. For example, one of the above queries could be specified as

select {*} from boolcoll where "a" = TruE

returning

{"a":true}

Double quotes can be used in the projection clause as well.

select "a" from boolcoll where "a" = tRUe

returns

|a                        |
+-------------------------+
|true                     |

Summary

JSON true, JSON false and JSON null can be used in JSON SQL queries without restriction and in a well-defined way. Disambiguation is not interfering with either the syntax as defined by the JSON standard, or the regular Relational SQL syntax. Great!

Go [ JSON | Relational ] SQL!

Disclaimer

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.