Please explain primary and foreign keys to me!

[DjRubbish]

Hello php-ers,

Can any of you illustrate the concept of primary and foreign keys in MySql databases by showing me an example of how they interact, please?

I've read my book's explanation 1,000,000 times and do you think I can understand it?

No.

I learn best by looking at an example and breaking it down to see how it works but my book is only explaining the concepts and it's all too abstract for me.

[mrhappiness]

primary key: a (normally numeric and auto_increment) column used to identify exactly one row of the table

foreign key: used for relations, it's another tables' primary key

example:

table user
user_id | username | usermal

table posts
post_id | user_id | topic | content

post_id and user_id are primary keys since they canbe used to identify one specific topic or user exactly

tu know which user created a topic the user_id is used as a foreign key in table posts

got it?

[DjRubbish]

in the first table there is only one row of user_id as it's unique

but in the second table one user can start many posts/ topics so the user_id value is repeated.

the user_id in the second table relates back to the first table so you can access details about that user_id

have I understood correctly?

[mrhappiness]

congrat's you got it 😃

[DjRubbish]

sweet.

The book I've got rabbits on for pages about this and you explained it in (much) less than 200 words.

Thanks a million! I can now move on.