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Add foreign key definition and example (#22388)

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* Add foreign key definition and example

A very basic foreign key definition and example

* Fix typo as recommended

Line 66: as @nik recommendation
Line67: fix plural nouns

* grammar
This commit is contained in:
sirhung1993
2018-11-22 04:32:10 +07:00
committed by Christopher McCormack
parent aae77a7890
commit c2633cdefa
1 changed files with 31 additions and 0 deletions
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guide/english/computer-science/databases/relational-databases/index.md
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@ -61,6 +61,37 @@ Figure 4 - Example of Pet table with owner field linked.
A one-to-many relationship is one record linked to many other records, the example being the user Jerry having two pets. It could also be a many-to-many relationship where the tables could be books and authors, as authors could co-write many books. Finally the most common relationship type is one-to-one, a record that can only be linked to one, and only one, other record.
### Foreign key
To describe the Relationships above, Foreign keys(FK) should be used to link tables.
Each FK needs 3 parameters:
1. Referenced table: having candidate keys
2. Child table: having foreign keys
3. Constraint: having the columns which both tables have in common.
|Id |Name |Gender|
|-----|--------------|------|
|1 |Hung |1 |
|2 |Linh |2 |
|3 |Duong |0 |
|4 |Alice |2 |
|5 |Bob |1 |
Figure 5 - Example of child table containing FK Gender
|GenderId |Description |
|-----------|---------------------|
|0 | Not yet defined |
|1 | Male |
|2 | Female |
Figure 6 - Example of referenced table containing GenderId with candidate keys
```sql
ALTER TABLE <child table> ADD CONSTRAINT FK_Child_Gender_Ref_GenderId FOREIGN KEY(Gender) REFERENCES Child(GenderId);
```
Figure 7 - A constraint of both tables is Gender linking to GenderId
## Conclusion
This is just a brief intro into relational-databases. Below links are provided to resources that could help you further study the subject.