Example of a strong and weak entity types

Question

I ve tried to look on Google about a decent explanation of weak and strong entity type  but I haven t fully understood them   Could someone give me an example of a strong and weak entity type

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Just to play with it  question is strong entity type and answer is weak  Question is always there  but an answer requires a question to exist   Example  Don t ask  Why   if Your Dad s a Chemistry Professor

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A company insurance policy insures an employee and any dependents  the DEPENDENT cannot exist without the EMPLOYEE  that is  a person cannot get insurance coverage as a dependent unless the person is a dependent of an employee DEPENDENT is the weak entity in the relationship  EMPLOYEE has DEPENDENT

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First Strong Weak Reference types are introduced in ARC  In Non ARC assign retain are being used  A strong reference means that you want to  own  the object you are referencing with this property variable  The compiler will take care that any object that you assign to this property will not be destroyed as long as you points to it with a strong reference  Only once you set the property to nil  the object get destroyed   A weak reference means you signify that you don t want to have control over the object s lifetime or don t want to  own  object  The object you are referencing weakly only lives on because at least one other object holds a strong reference to it  Once that is no longer the case  the object gets destroyed and your weak property will automatically get set to nil   The most frequent use cases of weak references in iOS are for IBOutlets  Delegates etc   For more info Refer   http   www informit com articles article aspx p 1856389 amp seqNum 5

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After browsing search engines for a few hours I came across a site with a great ERD example here  http   www exploredatabase com 2016 07 description-about-weak-entity-sets-in-DBMS html  I ve recreated the ERD  Unfortunately they did not specify the primary key of the weak entity     If the building could only have one and only one apartment  then it seems the partial discriminator room number would not be created  i e  discarded

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Weak Entity Type  An entity whose instances cannot exits without being linked with instances of some other entity is called weak entity type  It cannot exist independently  For example  Our PC is depend on us it will not open or close with its own   Strong Entity Type  An entity whose linked to the instances of any other entity type is called strong entity type  It can exit independently  For example  A person can do every thing can go everywhere and use ever thing

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A weak entity is the entity which can t be fully identified by its own attributes and takes the foreign key as an attribute  generally it takes the primary key of the entity it is related to  in conjunction   Examples  The existence of rooms is entirely dependent on the existence of a hotel  So room can be seen as the weak entity of the hotel  Another example is the bank account of a particular bank has no existence if the bank doesn t exist anymore

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A weak entity is one that can only exist when owned by another one   For example  a ROOM can only exist in a BUILDING  On the other hand  a TIRE might be considered as a strong entity because it also can exist without being attached to a CAR

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Database DataModels RelationalDataModel WeakEntity  It probably can be written in two factors    DEPENDENCE  Depends on the existence of an identifying entity set  total  one-to-many relationship   IDENTIFICATION  Does not have a primary key  It has a partial key  or discriminator   It needs to use the primary key of another table for identification    If we would think of a database holding questions and answers  then the questions would be the strong entity and the answers would be the weak entity  So  Question  id  text  and Answer  number  question id  text  would be our tables  But why is the Answer s table a weak entity    Dependence to the question table  Every answer is connected to one question  assumption  and so it cannot be on its own  That is why we have people who ask one question and answer it themselves so that they can help other people and get some extra likings  Identification from the primary key of the question  One would not be able to identify an answer  assuming that its id is a number identifier  because a question might be answered by answers whose identifier might exist in other questions too  Primary key of the answer table   number  question id

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Weak entity exists to solve the multi-valued attributes problem     There are two types of multi-valued attributes  One is the simply many values for an objects such as a  hobby  as an attribute for a student  The student can have many different hobbies  If we leave the hobbies in the student entity set   hobby  would not be unique any more  We create a separate entity set as hobby  Then we link the hobby and the student as we need  The hobby entity set is now an associative entity set  As to whether it is weak or not  we need to check whether each entity has enough unique identifiers to identify it  In many opinion  a hobby name can be enough to identify it   The other type of multi-valued attribute problem does need a weak entity to fix it  Let s say an item entity set in a grocery inventory system  Is the item a category item or the actually item  It is an important question  because a customer can buy the same item at one time and at a certain amount  but he can also buy the same item at a different time with a different amount  Can you see it the same item but of different objects  The item now is a multi-valued attribute  We solve it by first separate the category item with the actual item  The two are now different entity sets  Category item has descriptive attributes of the item  just like the item you usually think of  Actual item can not have descriptive attributes any more because we can not have redundant problem  Actual item can only have date time and amount of the item  You can link them as you need  Now  let s talk about whether one is a weak entity of the other  The descriptive attributes are more than enough to identify each entity in the category item entity set  The actual item only has date time and amount  Even if we pull out all the attributes in a record  we still cannot identify the entity  Think about it is just time and amount  The actual item entity set is a weak entity set  We identify each entity in the set with the help of duplicate prime key from the category item entity set

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Weak entities are also called dependent entities  since it s existence depends on other entities  Such entities are represented by a double outline rectangle in the E-R diagram   Strong entities are also called independent entities

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A data object that can exist without depending upon the existence of another data object is known as Strong Data Object

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Strong entity  It can exist without any other entity   Example  Customer customerid  name  surname    Weak entity  It depends on a dominant entity  and it cannot exist without a strong entity   Example  Adress addressid  adressName  customerid

[database] Example of a strong and weak entity types

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