One-to-one relationship

One-to-one relationship is used when only one entity from a table can be related to only one entity in another table in a database. For example, a country can only have one capital city and capital city can only belong to one country.

To make sure that there’s one-to-one relationship, you have to make sure that in both tables values are unique.

CREATE table Country {  
	countryId INTEGER PRIMARY KEY,  
	countryName TEXT,  
  
	UNIQUE (countryId)  
}  
  
CREATE table CapitalCity {  
	capitalCityId INTEGER PRIMARY KEY,  
	capitalCityName TEXT  
  
	countryId INTEGER UNIQUE,  
	FOREIGN KEY (countryId) REFERENCES Country(countryId)  
}