If entries can be relations themselves it is not possible afaik. For example
User | Telephone Numbers
-----+------------------
A | 123, 456 <- not atomic; more than 1 number (i.e. a set)
B | 789
Now there are academic operators to convert to and from a purely relational system, but I don't think they are implemented/in the standard. I forgot what they are called, however.
In general you don't want a universal query language. Depending on the shape of the data you want different things to be easily expressible. You can, for example express queries on tree-shaped data with SQL (see xPath-Accelerator), but it is quite cumbersome and its meaning is lost to the reader. I.e.: It's fine when computer-generated, but there is too much noise for a human to read/write themselves.
I'd be glad to be proven wrong here, but as time has shown, there is no one size fits all for programming languages. The requirements for different applications just vary too much.
In general you don't want a universal query language. Depending on the shape of the data you want different things to be easily expressible. You can, for example express queries on tree-shaped data with SQL (see xPath-Accelerator), but it is quite cumbersome and its meaning is lost to the reader. I.e.: It's fine when computer-generated, but there is too much noise for a human to read/write themselves. I'd be glad to be proven wrong here, but as time has shown, there is no one size fits all for programming languages. The requirements for different applications just vary too much.