Semi-structured
Semi-structured Data Types
| Name | Build From Values | Description |
|---|---|---|
| ARRAY | [1,2,3] | Zero-based indexed list, each value can have difference data type. |
| OBJECT | {"a":1,"b":{"c":2}} | Collection of key-value pairs, each key is a VARCHAR, and each value is a VARIANT. |
| VARIANT | [1,{"a":1,"b":{"c":2}}] | Collection of elements of different data types., including ARRAY and OBJECT. |
Array Data Types
ARRAY in Databend is similar to an array in many other programming languages, but the value in an ARRAY types can be different, each value in an Array is VARIANT type.
Example
CREATE TABLE array_table(arr ARRAY NULL);
DESC array_table;
Insert a value into the table, [1,2,3,["a","b","c"]]:
INSERT INTO array_table VALUES(parse_json('[1,2,3,["a","b","c"]]'));
Get the element at index 0 of the array:
SELECT arr[0] FROM array_table;
+--------+
| arr[0] |
+--------+
| 1 |
+--------+
Get the element at index 3 of the array:
SELECT arr[3] FROM array_table;
+---------------+
| arr[3] |
+---------------+
| ["a","b","c"] |
+---------------+
arr[3] value is a ARRAY type too, we can get the sub elements like:
SELECT arr[3][0] FROM array_table;
+-----------+
| arr[3][0] |
+-----------+
| "a" |
+-----------+
Object Data Types
Databend OBJECT is a data type likes a "dictionary”, “hash”, or “map” in other programming languages. An OBJECT contains key-value pairs.
In a Databend OBJECT, each key is a VARCHAR, and each value is a VARIANT.
Example
Create a table with OBJECT type:
CREATE TABLE object_table(obj OBJECT NULL);
Desc the object_table:
DESC object_table;
+-------+--------+------+---------+
| Field | Type | Null | Default |
+-------+--------+------+---------+
| obj | Object | YES | NULL |
+-------+--------+------+---------+
Insert a value into the table, {"a":1,"b":{"c":2}}:
INSERT INTO object_table VALUES(parse_json('{"a":1,"b":{"c":2}}'));
Get the value by key a:
SELECT obj:a FROM object_table;
+-------+
| obj:a |
+-------+
| 1 |
+-------+
Get the value by key b:
SELECT obj:b FROM object_table;
+---------+
| obj:b |
+---------+
| {"c":2} |
+---------+
Get the sub value by the key b:c:
SELECT obj:b:c FROM object_table;
+---------+
| obj:b:c |
+---------+
| 2 |
+---------+
Variant Data Types
A VARIANT can store a value of any other type, including ARRAY and OBJECT, likes "Struct" in other languages.
Example
Create a table:
CREATE TABLE variant_table(var VARIANT NULL);
Insert a value with different type into the table:
INSERT INTO variant_table VALUES(1),(1.34),(true),(parse_json('[1,2,3,["a","b","c"]]')),(parse_json('{"a":1,"b":{"c":2}}'));
Query the result:
SELECT * FROM variant_table;
+-----------------------+
| var |
+-----------------------+
| 1 |
| 1.34 |
| true |
| [1,2,3,["a","b","c"]] |
| {"a":1,"b":{"c":2}} |
+-----------------------+
Data Type Conversion
By default, elements retrieved from a VARIANT column are returned. To convert a returned element to a specific type, add the :: operator and the target data type (e.g. expression::type).
SELECT Arr[0]::INT FROM array_table
+---------------+
| arr[0]::Int32 |
+---------------+
| 1 |
+---------------+
Let's do a more complex query:
SELECT sum(arr[0]::INT) FROM array_table GROUP BY arr[0]::INT;
+--------------------+
| sum(arr[0]::Int32) |
+--------------------+
| 1 |
+--------------------+