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Flip to reveal answersWhat does an ERD show?
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All 5 Flashcards — ERDs
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Question
What does an ERD show?
Answer
The entities a database stores and the relationships between them. Each box is one kind of thing that will become a table; each line is a relationship, labelled with the verb joining them.
Question
What is the difference between cardinality and modality?
Answer
Cardinality is how many — one or many, drawn as a bar or a crow's foot. Modality is whether the relationship is compulsory — mandatory drawn as a second bar, optional as a circle.
Question
How do you work out a relationship's cardinality?
Answer
Say it as a sentence in both directions. 'A teacher teaches many classes' and 'a class is taught by one teacher' together give one-to-many, with the crow's foot on the Class end.
Question
Why must a many-to-many relationship be resolved?
Answer
A column holds only one value, so neither table can hold several foreign keys. It is replaced by a linking entity with one row per pair, keyed on both foreign keys together.
Question
Where does the foreign key go in a one-to-many relationship?
Answer
In the table on the many side. A Class holds TeacherID, because the alternative would need a Teacher row to hold an unknown number of class columns.
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Topic 3.2 hub
Database design
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Computer Science exam skills
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