Let's say I have an entity CLIENT, which can be either PERSON or ORGANIZATION. Depending on which type it is, I have to choose attributes (address, name for organization, date_of_birth,first_name,last_name for person). I have created all three entities, but how can I make the attributes type-dependent?
Seen Database design: objects with different attributes, didn't help...
One typical choice is a 1:1 extension table:
create table client (id int primary key);
create table person (id int foreign key references client(id), ...columns...);
create table organization (id int foreign key references client(id), ...columns...);
However, my preferred choice is to include all columns in the client table. You can have a column for type that is either person or organization. Columns that are not relevant for the row's type can be null. Your queries will be much simpler that way.
Either you use 3 tables or you use 1 table and leave the not needed columns null. Which design is superior depends on the use case. Using only 1 table gives simpler queries but requires to change the table for each new subclass. Using multiple tables allows to add more types easily but gives more complicated queries. In doubt I would start with only 1 table but your mileage may vary.