Second, because the (early days) table inheritance feature didn’t really support foreign keys either. It makes your database data consistent. Primary Key vs Foreign Key Summarized For your specific example, it looks like bigserial autoincrements, so you may want to use bigint as the foreign key in the second table, as you won't want it autoincrementing as a foreign key. We will also see how primary key and foreign key are implemented in PostgreSQL. A primary key uniquely identifies a tuple in a table whereas a foreign establishes a relationship between two tables. Two reasons: first, when partitioned tables were first introduced in PostgreSQL 10, they didn’t support foreign keys at all; you couldn’t create FKs on partitioned tables, nor create FKs that referenced a partitioned table. By default, a relationship will be created when there is a navigation property discovered on a type. A foreign key can also constrain and reference a group of columns. If you have a column that references another column in the database, add a foreign key constraint. Foreign keys allow us to keep our data normalized by referencing an object from one table in another so the second table has access to the first table’s keys and values. 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. PostgreSQL databases can use foreign keys. The relation between task and user injects the user_id foreign key on tasks, and marks it as a reference to the User table. Use foreign keys. FOREIGN KEY (col1,col2) REFERENCES c(col1,col2) -- multi-column foreign key SQL99: If MATCH FULL or MATCH PARTIAL is specified for a referential constraint and if As usual, it then needs to be written in table constraint form. I've only done as much Postgresql as I've had to, but in general terms, the foreign key can be whatever datatype best suits the data. Post.Blog is the inverse navigation property of Blog.Posts (and vice versa) Conventions. In this article, we will do the comparison between primary key and foreign in relational databases. Foreign Keys. Choosing between MongoDB and PostgreSQL. If a foreign key on the dependent entity is not nullable, then Code First sets cascade delete on the relationship. Post.Blog is a reference navigation property. Don’t cascade deletes. If a foreign key on the dependent entity is nullable, Code First does not set cascade delete on the relationship, and when the principal is deleted the foreign key will be set to null. Here is a contrived syntax example: CREATE TABLE t1 ( a integer PRIMARY KEY, b integer, c integer, FOREIGN KEY (b, c) REFERENCES other_table (c1, c2) ); By default user_id will be set to NULL if the referenced user is deleted, and updated if the id of the user id updated. Post.BlogId is the foreign key. The PostgreSQL FOREIGN KEY is a combination of columns with values based on the primary key values from another table. There are reasons to not use foreign keys at scale, but we are not at scale and we can drop these in the future if they become a problem. The Italian team of 2ndQuadrant has been working since last year on adding a new feature to PostgreSQL: support of referential integrity between the elements of an array in a table (referencing) and the records of another table (referenced). Blog.Posts is a collection navigation property. Foreign key constraints should not cascade deletes for a few reasons: SQL foreign key constraint is used to make sure the referential integrity of the data parent to match values in the child table. A foreign key can be used to match a column or combination of columns with primary key in a parent table. A foreign key enables you to link two or more tables together. 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