I thought at first that it isn't a relational DB, but after I read that I can join tables and it was written on their site https://crate.io/overview/ (see Use cases), I'm not sure.
Especially I got confused by the senctence:
CrateDB is based on a NoSQL architecture, but features standard SQL.
Going by a Codd's 12 rules (which have been used to identify relational databases), CrateDB is not a relational database - yet. CrateDB's eventual consistency model does not prohibit that.
Rule 0: For any system that is advertised as, or claimed to be, a relational data base management system, that system must be able to manage data bases entirely through its relational capabilities.
CrateDB doesn't have another interface with which data can be inserted, retrieved, and updated.
Rule 1: All information in a relational data base is represented explicitly at the logical level and in exactly one way — by values in tables.
Exactly what can be found in CrateDB.
Rule 2: Each and every datum (atomic value) in a relational data base is guaranteed to be logically accessible by resorting to a combination of table name, primary key value and column name.
This is strictly enforced. Access through primary keys will even give you read-after-write consistency.
Rule 3: Null values (distinct from the empty character string or a string of blank characters and distinct from zero or any other number) are supported in fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of data type.
CrateDB supports null.
Rule 4: The data base description is represented at the logical level in the same way as ordinary data, so that authorized users can apply the same relational language to its interrogation as they apply to the regular data.
CrateDB has among other meta-tables, Information Schema tables
Rule 5: A relational system may support several languages and various modes of terminal use (for example, the fill-in-the-blanks mode). However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings and that is comprehensive in supporting all of the following items:
- Data definition.
- View definition.
- Data manipulation (interactive and by program).
- Integrity constraints.
- Authorization.
- Transaction boundaries (begin, commit and rollback).
CrateDB supports data definition and data manipulation parts and only a single integrity constraint (primary key). This is definitely incomplete.
Rule 6: All views that are theoretically updatable are also updatable by the system.
CrateDB does not support views yet.
Rule 7: The capability of handling a base relation or a derived relation as a single operand applies not only to the retrieval of data but also to the insertion, update and deletion of data.
CrateDB currently only does that for data retrieval...
Rule 8: Application programs and terminal activities remain logically unimpared whenever any changes are made in either storage representations or access methods.
CrateDB's use of SQL allows for this; performance/storage level improvements are even delivered via system upgrades.
Rule 9: Application programs and terminal activites remain logically unimpared when information-preserving changes of any kind that theoretically permit unimpairment are made to the base tables.
Parts of this are still missing (the views, inserts/updates on joins). However for retrieving data, this is already the case.
Rule 10: Integrity constraints specific to a particular relational data base must be definable in the relational data sublanguage and storable in the catalog, not in the application programs.
This is quite tricky for a distributed database, specifically the foreign key constraints. CrateDB only supports primary key constraints for now.
Rule 11: A relational DBMS has distribution independence.
In CrateDB any kind of sharding/partitioning/distribution is handled transparently for the user. Any kinds of constraints/settings for data distribution are applied on the data definition level.
Rule 12: If a relational system has a low-level (single-record-at-a-time) language, that low level cannot be used to subvert or bypass the integrity rules and constraints expressed in the higher level relational language (multiple-records-at-a-time).
One could argue that COPY FROM directly violates this rule since there is no type checking and conversion happening underneath. However there is no other command/language/API that would allow data manipulation otherwise.
While CrateDB certainly has some catching up to do, there is no reason why it wouldn't become a relational database in this sense soon. Its SQL support may not be on par with Oracle's or Postgres' but many people can live without some very use-case specific features.
As said above, all of the rules above are not directly violated, but rather not implemented yet in a satisfactory manner, so there is no reason why CrateDB can't become a fully relational database eventually.
(Disclaimer: I work there)