Postgres / SQL 数据库：如何强制键/值对的唯一组合

Question

a new Project requires a dynamic datamodel, meaning that the Properties for a record are stored in a seperate table like this:

Items:
ID | insertiondate
 1 | 2017-01-31


Properties:
ID | fk_Item_ID |     Key      |     Value
 1 |     1      |  referenceNr |       1
 2 |     1      |    office    |       O1

...

What i need now is a possibility to enforce that a "referenceNumber" in unique per "office".

so the insertion into this table with the 2 values (1, O2) is ok as well as (2, O1) - but (1, O1) has to violate the Constraint.

Is there a simple way to handle this?

Answer 1

Even if the project really asks for some key/value entries, this doesn't seem to be true for referencenr and office as you want to apply constraints on the pair. So simply put the two in your items table and add the constraint.

The only other option I see is to make the two one entry:

ID | fk_Item_ID | Key                  | Value
 1 |     1      | 'referenceNr/office' | '1/01'

I'd go for the first solution. Have key/value pairs only where absolutely necessary (and where the DBMS may be oblivious as to their content and mutual relations).