How many clubs can one pupil join?
One form has many pupils. One pupil has one locker. A pupil can join many clubs, and a club has many pupils.
These three sentences are three different kinds of link. Each kind is stored in its own way.
Key words
| Word | What it means |
|---|---|
| Relationship | a link between two entities |
| Cardinality | how many records on one side link to records on the other side |
| One-to-one | one record links to exactly one record, written 1:1 |
| One-to-many | one record links to many records, written 1:M |
| Many-to-many | many records link to many records, written M:M |
| Link table | an extra table that turns a many-to-many link into two one-to-many links |
| Crow's foot | the three-pronged mark at the end of a line that means many |
| Primary key | the field that gives every record its own unique id |
| Foreign key | a field that holds the primary key of a record in another table |
| Database | an organised store of tables that you can ask questions |
| SQL | the language used to ask a database for data |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can name the three kinds of relationship, show them with crow's foot lines, and say how each is stored.
Starter
Have a go
Starter
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The three kinds of relationship
| Kind | Example | Drawn as | Stored by |
|---|---|---|---|
| One-to-one (1:1) | A pupil and a locker | A plain line | A foreign key, unique, on either table |
| One-to-many (1:M) | A form and its pupils | A crow's foot at the many end | A foreign key on the many side |
| Many-to-many (M:M) | Pupils and clubs | A crow's foot at both ends | A link table with a composite key |
The cells students fill in are left blank in this preview.
Read the lines
Have a go
Read the lines
3 questionsHave a go: your answers are marked here and then forgotten. Sign in if you would like your score kept, and to pick up where you left off.Sign in to keep your score
Your answers are marked here and are not kept. A college account keeps every student’s marks.