PostgreSQL

PostgreSQL is:

open sourced
supports both relational and non-relational (JSON) queries
for dynamic websites and web applications

-- Get Version
select version();

-- List all tables
select * from pg_catalog.pg_tables;

-- Inheritance
create table cities (
  name text,
  population real,
  altitude int
);

create table capitals (
  state char(2)
) inherits (cities); -- muliple inheritance is possible

-- Result:

SELECT name, altitude
    FROM cities
    WHERE altitude > 500;
+----------+----------+
|name      | altitude |
+----------+----------+
|Las Vegas | 2174     |
+----------+----------+
|Mariposa  | 1953     |
+----------+----------+
|Madison   | 845      |
+----------+----------+

SELECT name, altitude
    FROM ONLY cities
    WHERE altitude > 500;
+----------+----------+
|name      | altitude |
+----------+----------+
|Las Vegas | 2174     |
+----------+----------+
|Mariposa  | 1953     |
+----------+----------+
    

SELECT * FROM capitals;
+----------+----------+-----------+
|name      | altitude | state     |
+----------+----------+-----------+
| Madison  | 845      | Wisconsin |
+----------+----------+-----------+