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Can we create primary key and foreign key on same column?
The FOREIGN KEY constraint differs from the PRIMARY KEY constraint in that, you can create only one PRIMARY KEY per each table, with the ability to create multiple FOREIGN KEY constraints in each table by referencing multiple parent table.
How do you create a foreign key on a composite primary key?
A referential constraint must have a one-to-one relationship between referencing and referenced columns. In other words, if the primary key is a set of columns (a composite key), then the foreign key also must be a set of columns that corresponds to the composite key.
Can a foreign key be a composite key?
A composite foreign key is a foreign key that consists of two or more columns. It is important to note that all the columns in a single foreign key must point to the same table. In other words, it is not possible to have a foreign key that references to a column in Table 1 and a column in Table 2.
Can a composite foreign key be used as a primary key?
You can create a composite foreign key just as you would create a single foreign key, except that instead of specifying just one column, you provide the name of two or more columns, separated by a comma. Here’s an example of a database using a composite foreign key (and a composite primary key).
Can a foreign key point to another table?
When you create a foreign key, the key that you “point to” in the other table must be a UNIQUE or PRIMARY KEY constraint. You cannot establish a foreign key that points to a column that allow duplicate values. It would be very hard to imagine how the data should “act” if you update one of the duplicate values in the other table (for instance).
How to establish a PK with a foreign key?
If you have a compound PK made up from three columns, then any child table that wants to establish a foreign key relationship must ALSO have all those 3 columns and use all 3 columns to establish the FK relationship.
Do you need a unique ID for a foreign key?
You probably want to assign a unique ID to each person, not relying on their name to be unique or for an address to be required. That ID will be your Primary key and foreign key. Your purchase table should also have its own id for its primary key — otherwise because primary keys must be unique, each person can only have one purchase.