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Can you have two primary keys?
No. You cannot use more than 1 primary key in the table. for that you have composite key which is combination of multiple fields. It needs to be a composite key.
Can a SQL table have 2 primary keys?
You can only have one primary key, but you can have multiple columns in your primary key. You can also have Unique Indexes on your table, which will work a bit like a primary key in that they will enforce unique values, and will speed up querying of those values.
Can there be multiple primary keys in a table justify your answer?
You cannot have multiple primary keys in a table. The reason is that only one field can be used as a unique identification feature for every record.
Can primary key be duplicate?
A primary key is a column of table which uniquely identifies each tuple (row) in that table. Only one primary key is allowed to use in a table. The primary key does not accept the any duplicate and NULL values.
Is it possible to reference fields other than the primary key?
In fact, you could reference fields other than the primary key, if all you need is to use the value for joins. However, a foreign key constraint tells the database to enforce the rule that for every foreign key value in a table, the referenced table has a record with that as it’s primary key.
How to define foreign key referencing two columns?
If the Primary Key you are referencing is specified as (Application, ID) then your foreign key must reference (Application, ID) and NOT (ID, Application) as they are seen as two different keys. the column order on table A should be –> A_ID then A_Name; defining the foreign key should follow the same order as well.
Can a single field be a foreign key to two different tables?
No, you can’t have a single field as a foreign key to two different tables. How would you tell where to look for the key? You would at least need a field that tells what kind of user it is, or two separate foreign keys.
Can a foreign key be used as a primary key?
You could also put the information that is common for all users in one table and have separate tables for the information that is specific for the user types, so that you have a single table with user id as primary key. A foreign key can only reference a single primary key, so no. However, you could use a bridge table: