What is the difference between a foreign key and a primary key?
PRIMARY KEY. FOREIGN KEY. 1. A primary key is used to ensure data in the specific column is unique. A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. 2.
What is a foreign key?
A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It is a column (or columns) that references a column (most often the primary key) of another table. Example: Refer the figure –.
What is primary key?
Primary Key: A primary key is used to ensure data in the specific column is unique. It is a column cannot have NULL values. It is either an existing table column or a column that is specifically generated by the database according to a defined sequence. Example: Refer the figure –.
Can a primary key be used in a table?
Only one primary key is allowed in a table. Whereas more than one foreign key are allowed in a table. It is a combination of UNIQUE and Not Null constraints. It can contain duplicate values and a table in a relational database. It does not allow NULL values.
What happens if a column is unique but not primary?
If the referenced column is unique, but is not the primary key, then your table may be in an "abnormal form" and the additional unique column may be redundant. The test for this is to ask yourself two questions:
What happens if a reference column is not unique?
1. The referencing column may not be unique, as you point out. If the referenced column is also not unique, you have created a many-to-many relationship, which is a right pain to implement and can cause problems maintaining data integrity. For example, I have two rows in table 1 with the same value in the referenced column. ...
Can you reference a non-primary key column?
Note that it is possible to reference a unique non-primary-key column or column group. However, since conceptually the primary key is " The " canonical identifier for the row, it makes more sense to reference the primary key.
Is a foreign key unique?
Foreign key is not unique - it may have duplicate entries. If it must be primary key of another table then why it is necessary?
Is a foreign key a parent table?
However, since conceptually the primary key is " The " canonical identifier for the row, it makes more sense to reference the primary key. Yes, foreign key has to be primary key of parent table. Yes, it may not be unique and may have duplicate entries in child table, but it must be unique and does not have any duplicate entries at the parent table ...
Can a city_id be unique in a city table?
But in city table, city_id (primary key) must be unique as each city must have only one city_id.
Can a foreign key have duplicate entries?
foreign key is not unique it may have duplicate entries. Yes, it may not be unique and may have duplicate entries in child table, but it must be unique and does not have any duplicate entries at the parent table ( as it is a primary key). For example consider below tables:
