Does a relational database need a primary key?
In order for a table to qualify as a relational table, it must have a primary key. The primary key consists of one or more columns whose data contained within are used to uniquely identify each row in the table.
Can you have a relational database without primary and foreign keys?
A primary key is not required. A foreign key is not required either. You can construct a query joining two tables on any column you wish as long as the datatypes either match or are converted to match. No relationship needs to explicitly exist.
What Cannot be done if the table has no primary key?
Even if you do not add a primary key to an InnoDB table in MySQL, MySQL adds a hidden clustered index to that table. If you do not define a primary key, MySQL locates the first UNIQUE index where all the key columns are NOT NULL and InnoDB uses it as the clustered index.
What is the difference between a primary key and a foreign key?
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. It refers to the field in a table which is the primary key of another table.
Can a table have two foreign key without primary key?
There is no problem having a table that consists of foreign keys only. In your particular example it would be OK to remove the OrderID. Relational theory states that each table should have a candidate key so, ideally, the business rule you mention would be in place to make the foreign key only table a “good” table.
Is it good to have primary key in database?
Also, a primary key is a constraint used by the query optimiser that should improve performance for lookups and joins. The only time you wouldn’t have a primary key is in a “reporting” table, probably in a denormalised data warehouse. Yes, it is good practise to have a primary key on every table.
Can a table have the same primary key?
No two rows in a table can have the same primary key value. You cannot enter NULL for any column in a primary key. We recommend using a small, integer column as a primary key. Each table should have a primary key. A column or combination of columns that qualify as a primary key value is referred to as a candidate key.”.
Can a reporting table not have a primary key?
The only time you wouldn’t have a primary key is in a “reporting” table, probably in a denormalised data warehouse. Yes, it is good practise to have a primary key on every table. But, NOT every table should have a single auto number id column.
Is it ” OK ” to have a table that only contain foreign keys?
1 Answer 1. There is no problem having a table that consists of foreign keys only. In your particular example it would be OK to remove the OrderID. Relational theory states that each table should have a candidate key so, ideally, the business rule you mention would be in place to make the foreign key only table a “good” table.