You also have NoSQL databases, where you can have arrays, embedding and you can often save yourself the CPU hit of joins.

many2one: so in this relationship you will have more than one record in one table which matches to only one record in another table. something like A <-- B. where (<–) is foreign key relationship. so B will have a column which will be mapped to more than one record of A.

no, the other way around

When B has a foreign key to A, many B records may relate to one A record. That’s the many2one part.

The fact that different B records can point to different A records is irrelevant to that.

one2many: same as many2one but instead now the foreign key constrain will look something like A --> B.

It’s the same, mirrored. Or mirrored interpretation / representation to be more specific. (No logical change.)

If you had B --> A for many2one, then the foreign key relationship is still B --> A. But if you want to represent it from A perspective, you can say one2many - even though A does not hold the foreign keys.

In relational database schemata, using foreign keys on a column means the definition order is always one to one, and only through querying for the shared id will you identify the many.

many2many: this one is interesting because this relationship doesn’t make use of foreign key directly. to have this relationship between A and B you have to make a third database something like AB_rel. AB_rel will hold values of primary key of A and also primary key of B. so that way we can map those two using AB_rel table.

Notably, we still make use of foreign keys. But because one record does not necessarily have only one FK value we don’t store it in a column but have to save it in a separate table.

This association table AB_rel will then hold the foreign keys to both sides.

Usually we don’t distinguish between many2one and one2many, since it’s the same just viewed from the other entity.

There is one more class though, which is one2one. That is, the entities have a direct relationship. Sometimes this also includes the case where you have zero or one, i.e. the relation is optional on one side. This can be accomplished with an FK plus unique constraint or by merging the tables.

One to many: my primary key is other table’s foreign keys.

Many to one: other table’s primary key is my foreign keys.

Many to many: my primary key and other table’s primary key are all foreign keys that connect each other in some other table.

So yeah, you’ve got it right. :)

to have this relationship between A and B you have to make a third database

Probably just a mistake here, but you make a third table, not a new database.

Apart from that (and the fact that one to many and many to one is the same thing), yeah, looks correct.