Is it possible to combine data from different tables in one column - sql

Question

I'm trying to achieve the following JSON structure with a SQL-Schema:
"data_set": [
{
"id": 0,
"annotation": "foo",
"value":
{
"type": "number",
"value": 10.0,
"unit": "m"
}
},
{
"id": 1,
"annotation": "bar",
"value":
{
"type": "text",
"value": "Hello World"
}
}
]
The tricky part is that I not only want to includes values of one type, but different types. My thought was to have different tables for each value e.g.:
numeric_value: id {PK} | type | value | unit
text_values: id {PK} | type | value
and include them in the data_set table via foreign key:
data_set: id {PK} | annotation | value {FK}
My problem is that I'm not sure how to reference ids from different tables in one column using keys, of if I'm taking a totally wrong approach in tackling this problem.

Answer 1

You can do this in different ways. One of them is to use only one table, and have it defined in such a way that, when your value_type is number you are forced to fill in the values for numeric_value and unit (and nothing else), and when it is text, you must then fill in the text_value column (and not fill in the rest, but leave NULL).
This definition could do it:
CREATE TABLE data_set
(
data_set_id integer PRIMARY KEY,
annotation text,
value_type text NOT NULL,
numeric_value numeric,
unit text,
text_value text,
/* Constraint to make sure values are consistent with definition */
CHECK (CASE WHEN value_type = 'text' THEN
numeric_value IS NULL AND unit IS NULL AND text_value IS NOT NULL
WHEN value_type = 'number' THEN
numeric_value IS NOT NULL AND unit IS NOT NULL AND text_value IS NULL
ELSE
false
END)
) ;
The following inserts would be valid:
INSERT INTO
data_set
(data_set_id, annotation, value_type, numeric_value, unit)
VALUES
(1, 'height', 'number', 10.0, 'm') ;
INSERT INTO
data_set
(data_set_id, annotation, value_type, text_value)
VALUES
(2, 'brand', 'text', 'BigBrand') ;
These ones would not:
INSERT INTO
data_set
(data_set_id, annotation, value_type, text_value)
VALUES
(3, 'brand', 'strange_type', 'misstake') ;
INSERT INTO
data_set
(data_set_id, annotation, value_type, text_value)
VALUES
(4, 'brand', 'number', 'misstake') ;