9.6.17

microsoft sql server every entity has 1 primary key composite and compound // cancatinated - primary key w/ mult. attr. primary key attributes 5 primary keys, 1 per entity. cancatinated primary keys (or the key is composite) common attributes between clas and clst amount to 6 (matching attributes) baseball example agent-me-ownership general manager-player-agent degree 3 and degree 2 relationships, rather not have 3 connecting relationships, but 2 connecting to 1 for entities (table in database) to be related (relationship line), look for common attributes (shows relation) the way two entities are related is through common attributes very end symbol crows foot (or dash) ((not circle)) at end of relationship has meaning crow foot = at most many dash = at most 1 (0 or 1) farside or nearside far is crows foot near is either 1 or 0 prof related to clas and clas related to prof bidiretional relationships, two uni directional relationships one entity instance of prof is .... analyzing data model starts with one. One employee... One prof... So how many over the other entity? One employee and how many cars? (using relationship such as at most many cars or at most 1 car) one employee can buy at most many cars (crow foot) we are in a class and not a course circle = atleast none 1 or none one employee does not have to buy a car (because of circle just before dash) one employee can have atleat 0 cars a car must be bought by atleast 1 employee a coarse can have at most many classes a class has at most 1 course a classlist has at most many students a student can have at most many classlist a classlist has atleast 1 class a class has a most many classlist