15.1.17 CREATE JSON DUALITY VIEW Statement

CREATE [OR REPLACE]
    [ALGORITHM = {UNDEFINED | MERGE}]
    [DEFINER = user]
    [SQL SECURITY {DEFINER | INVOKER}]
    JSON [RELATIONAL] DUALITY VIEW 
    [IF NOT EXISTS] [schema_name.]view_name 
    AS json_duality_select_statement
    
json_duality_select_statement: 
    SELECT json_duality_object_expression
    FROM [schema_name.]root_table_name [AS table_alias]

json_duality_object_expression: 
    JSON_DUALITY_OBJECT(...)

This statement creates a JSON duality view named view_name. Using OR REPLACE causes any existing JSON duality view of that name to be replaced by a new JSON duality view having the same name. Specifying IF NOT EXISTS causes view creation to be attempted only if there is no existing JSON duality view with the same name, rather than returning an error.

JSON duality views use the same namespace as SQL views. This means that you cannot create a JSON duality view having the same name as an existing SQL view; it also means that you cannot create an SQL view with the same name as an existing JSON duality view. CREATE OR REPLACE does not work to replace an SQL view with a JSON duality view, or a JSON duality view with an SQL view.

DEFINER and SQL SECURITY work with this statement as they do for CREATE VIEW. For ALGORITHM, using TEMPTABLE returns an error.

The RELATIONAL keyword is optional, has no effect, and is omitted from our examples.

schema_name, if used with the view name, must be the name of an existing schema. If the schema name is omitted, the JSON duality view is created in the current schema; if no schema is currently selected and none is specified, the statement is rejected with an error. schema_name and view_name must conform to the rules for MySQL identifiers; see Section 11.2, “Schema Object Names”, as well as Section 11.2.1, “Identifier Length Limits”, for information about these rules.

The WITH ... CHECK OPTION clause works with CREATE JSON DUALITY VIEW as it does with CREATE VIEW. See the description of that statement for more information.

json_duality_select_statement selects a JSON object expression (json_duality_object_expression) constructed using columns from the table root_table_name in schema schema_name. If schema_name is omitted, MySQL assumes that the table is in the current schema; if no schema is specified and none is currently selected, the statement is rejected with an error. Both schema_name and root_table_name must follow the usual rules for MySQL identifiers.

json_duality_select_statement must contain one and only one JSON_DUALITY_OBJECT() expression and one FROM clause. Set operations (UNION, INTERSECT, EXCEPT) and common table expressions (WITH) are not supported. The FROM clause must reference a single table. WHERE, JOIN, GROUP BY, ORDER BY, HAVING, WINDOW, and LIMIT clauses are not supported.

json_duality_object_expression is a value returned by JSON_DUALITY_OBJECT(). See the description of that function for information about its arguments.

The JSON_DUALITY_OBJECT() function returns a JSON duality object for use in CREATE JSON DUALITY VIEW or ALTER JSON DUALITY VIEW. Attempting to invoke it in any other context results in an error.

JSON_DUALITY_OBJECT() takes one or two arguments: an optional table annotations expression, and a set of key-value pairs in JSON object format.

Requirements:

• It must include a key named _id in the root object representing the primary key of the root table. Absence of this key results in an error. No sub-key may be named _id.

• All participating tables, including the root table and any tables referenced within nested_descendent_json_objects and singleton_descendent_json_object, must be base tables and have a primary key.

• The table projection must include the primary key of every participating table.

• Child tables being projected can be related to parent tables in one of two ways:

  • PK - FK relationship: If a child table is projected as singleton_descendent_json_object, the WHERE clause must enforce the format child_table.PK = parent_table.FK. If a child table is projected as nested_descendent_json_objects, the WHERE clause must enforce the format child_table.FK = parent_table.PK.

  • PK - Any Column relationship: If a child table is projected as singleton_descendent_json_object, the WHERE clause must enforce the format child_table.PK = parent_table.any_column. If a child table is projected as nested_descendent_json_objects, the WHERE clause must enforce the format child_table.any_column = parent_table.PK.

The complete syntax for the arguments to this function is shown here, with additional notes following:

table_annotations:
    WITH (table_annotation[, table_annotation]...)

table_annotation:
    INSERT | UPDATE | DELETE

json_duality_key_value_pairs: 
    json_duality_key_value_pair[, json_duality_key_value_pair]...

json_duality_key_value_pair:
    key_name:value_expression

value_expression:
    column_name  
    | (singleton_descendent_json_object) 
    | (nested_descendent_json_objects) 

singleton_descendent_json_object: 
    SELECT json_duality_object_expression
    FROM child_table_name [AS table_alias]
    WHERE json_duality_join_condition

nested_descendent_json_objects:
    SELECT JSON_ARRAYAGG(json_duality_object_expression [json_constructor_null_clause]) 
    FROM child_table_name [AS table_alias] 
    WHERE json_duality_join_condition

json_constructor_null_clause:
    NULL ON NULL | ABSENT ON NULL 

json_duality_join_condition:
    [schema_name.]child_table_name.column_name
    = [schema_name.]parent_table_name.column_name

json_duality_object_expression: 
    JSON_DUALITY_OBJECT( 
        [table_annotations_expression] json_duality_key_value_pairs 
    )

json_duality_key_value_pairs is a set of key-value pairs in key_name:value_expression format. There must be a key named _id in the root object, and it must correspond to a primary key column of the table being projected; sub-keys named _id are not allowed.

value_expression must be one of: a column name; an object returned by JSON_DUALITY_OBJECT() (singleton descendant); an object returned by JSON_ARRAYAGG() (nested descendant).

column_name must reference a valid column in the table that is being projected (root_table_name or current_table_name). The same column_name cannot be used more than once in a single invocation of JSON_DUALITY_OBJECT(). Functions and operators cannot be used with column_name. Columns of types JSON, VECTOR, and GEOMETRY (including all derivatives such as POINT, LINESTRING, and POLYGON) are not supported, nor are generated columns. The column having the key _id in the root table for json_duality_key_value_pairs must be a primary key of that table.

The singleton_descendent_json_object consists of a SELECT statement with a FROM clause. The SELECT list and FROM clause follow the same rules as those described for the top-level query in a CREATE JSON DUALITY VIEW statement.

nested_descendent_json_objects selects a single expression (json_duality_object_expression) using JSON_ARRAYAGG(), which must contain a non-empty JSON_DUALITY_OBJECT(). The select list and FROM clause follow the same rules as those described for singleton_descendent_json_object. The optional json_constructor_null_clause specifies the behavior of this function when json_duality_object_expression evaluates to null. It takes either of the values ABSENT ON NULL or NULL ON NULL (the default). NULL ON NULL returns the JSON null value; ABSENT ON NULL causes the value to be omitted from the output JSON array.

singleton_descendent_json_object and nested_descendent_json_objects also support a WHERE clause. This must contain one expression only, having the form shown here:

[schema_name.]child_table_name.column_name 
    = [schema_name.]parent_table_name.column_name

No types of conditions other than equality are supported in this WHERE clause. Multiple conditions using AND or OR operators are also not supported.

JSON_DUALITY_OBJECT() takes an optional table_annotations_expression. This expression consists of a comma-separated list of one or more of the annotation values INSERT, UPDATE, and DELETE, in any order. No annotation value may be listed more than once. The function returns a mapping between columns of table and the JSON collection defined by json_duality_key_value_pairs. The value used with each key can be one of three types:

• The name of a column in table. This must be the name of the column only, and cannot be an expression.

• A singleton_descendent_json_object which consists of a SELECT with a FROM clause. The SELECT list and FROM clause follow the same rules as those described for the top-level query in CREATE JSON DUALITY VIEW.

• A set of nested_descendent_json_objects selects an expression using JSON_ARRAYAGG(), which in turn contains JSON_DUALITY_OBJECT().

If the table is projected multiple times, the set of columns projected must be consistent across all instances of the table projection.

See Section 27.7, “JSON Duality Views”, for more information and examples.