Can foreign key be different data type?
According to WL#148, a foreign key column must have the same data type + the same length + the same scale as the corresponding referenced column. With “comparable data types”, the restriction would be looser. For example, lengths could differ.
Can you reference multiple foreign keys in SQL?
A Foreign Key is a database key that is used to link two tables together. 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 to avoid create table with foreign key reference?
If you’d like to make sure that the SQL engine won’t insert a row with a foreign key that references a non-existent primary key, then you can add a FOREIGN KEY constraint in your CREATE TABLE statement. You can see the error that it results in by removing the comments from the fourth INSERT in the code below.
Why do I need a different datatype for foreign key?
Since the foreign key is pointing to another table’s primary key, this is why you need to specify the foreign key’s datatype. And it obviously needs to be the same datatype.
How are foreign keys defined in one to many relationships?
In a one-to-many relationship, the foreign key is defined on the table that represents the many end of the relationship. The many-to-many relationship involves defining a third table (called a junction or join table), whose primary key is composed of the foreign keys from both related tables.
How to set up a foreign key in SQL?
CREATE TABLE user ( id bigint NOT NULL, CONSTRAINT user_pkey PRIMARY KEY (id) ) WITH ( OIDS=FALSE ); CREATE TABLE book ( user integer, CONSTRAINT dependent_user_fkey FOREIGN KEY (user) REFERENCES user (id) MATCH SIMPLE ON UPDATE NO ACTION ON DELETE NO ACTION ) WITH ( OIDS=FALSE );