database - What is the difference between Max Cardinality and Min Cardinality? -

-

i having hard time understanding difference between max , min cardinalities when trying design database.

remember cardinality relationship thing.

max cardinality(cardinality) 1 or many. class has relationship package b cardinality of one, means @ there can 1 occurance of class in package. opposite package has max cardnality of n, mean there can n number of classes

min cardinality(optionality) means "required." 0 or 1. 0 mean 0 or more, 1 ore more

there tons of articles out there explain this, including explain how property "diagram". thing can search cardinality/optionality (omg terms) explains same thing, optionality "min" cardinality "max",

from http://www.databasecentral.info/faq.htm

q: can see how maximum cardinality used when creating relationships between data tables. however, don't see how minimal cardinality applies database design. missing?

a: correct in noticing maximum cardinality more important characteristic of relationship minimum cardinality is. minimum cardinality tells minimum allowed number of rows table must have in order relationship meaningful. example, basketball team must have @ least 5 players, or not basketball team. minimum cardinality on player side 5 , minimum cardinality on team side one.

one can argue person cannot player unless on team, , minimum cardinality of team mandatory. organization cannot basketball team unless has @ least 5 players. minimum cardinality of players mandatory also. 1 argue in opposite direction too. when player quits team, cease team until replacement recruited? cannot engage in games, cease team? example of fact each individual situation must evaluated on own terms. truth in particular instance? next time similar situation arises, decision might different, due different circumstances.


Comments

Post a Comment

Popular posts from this blog

linux - Mailx and Gmail nss config dir -

-
i trying send mail linux command line using mailx command. can send local domain no problem want set mail send gmail account receive mail sent gmail account. after configuring mail.rc so: account gmail { set smtp=smtps://smtp.gmail.com:587 set smtp-auth=login set smtp-auth-user=username@gmail.com set smtp-auth-password=password set ssl-verify=ignore } i error: resolving host smtp.gmail.com . . . done. connecting 74.125.25.109 . . . connected. missing "nss-config-dir" variable. "/home/username/dead.letter" 11/354 . . . message not sent. after looking "nss-config-dir" here , located certn.db , keyn.db files , added mail.rc so: account gmail { set smtp=smtps://smtp.gmail.com:587 set smtp-auth=login set smtp-auth-user=username@gmail.com set smtp-auth-password=password set ssl-verify=ignore set nss-config-dir=/home/user/.mozilla/firefox/location.default } now when try send mail using command: echo "sent gmail account" | mailx -v -a
Read more

c# - Is it possible to remove an existing registration from Autofac container builder? -

-
something along lines: builder.registertype<mytype>().as<itype>(); builder.registertype<mytype2>().as<itype>(); builder.deregistertype<mytype>().as<itype>() var container = builder.build(); var types = container.resolve<ienumerable<itype>>(); assert.istrue(types.count == 1); assert.istrue(types[0].gettype == typeof(mytype2)); scenario: go through bunch of assemblies , go register types want make sure have 1 implementation of given type. need before create container. track on own nice if autofac me bit. this cannot done directly using containerbuilder , unless start on new one. mind you, having first built container should able construct new container filtering away unwanted types , reusing registrations first container. this: ... var container = builder.build(); builder = new containerbuilder(); var components = container.componentregistry.registrations .where(cr => cr.activator.limittype != typ
Read more

php - Mysql PK and FK char(36) vs int(10) -

-
i have read mysql can handle indexes , searches better if tables keys integers rather char(36) uuid's. is worth converting int(10)? note: use 1 server , have have plans use multiple databases concurrently therefore removing 1 of big reasons using uuid. our tables innodb if makes difference. thanks in advance. usually rdbms can handle integer keys pk more efficient, other datatypes. reason how build index column, yes: long dont require string (or other typed) keys, should use integers. however: char(36) , int(10) far away being equal, because int(10) much smaller, char(36). dont know, if require many different keys, should keep in mind. update, complete last paragraph: int(10) 32-bit, char(36) 36 - ^byte^ (= 288-bit). not mean, int consumes less space, means, char(36) has 4 times more different keys.
Read more