Practice on organizing databases

There are seas, rivers and their tributaries. Rivers can be tributaries of other rivers or flow directly into the sea. Make a structure of the tables.

User, message exchange between users. Create a table structure.

A site with dates of football games. Each game contains the date of the game, the first team and the second team. There are players, each of whom belongs to one team. Make a structure of tables.

Forum, categories in it, topics in them (a topic belongs to only one category), posts in topics. A topic has an author. Users can exchange personal messages. Create a table structure.

A social network with users, their friends, a wall, comments on wall posts. Users can exchange personal messages. Create a table structure.

Family tree. User, his grandparents, mother, father, brothers, sisters, children. You can find any relative in any generation (for example, great-great-great-grandfather). You need to store and receive family connections. Make a table structure.