A type of integrity constraint used in a RDBMS platform to ensure that a value in a column matches one of a range of key values from another table.

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24
votes
3answers
1k views

Is using multiple foreign keys separated by commas wrong, and if so, why?

There are two tables: Deal and DealCategories. One deal can have many deal categories. So the proper way should be to make a table called DealCategories with the following structure: DealCategoryId ...
5
votes
1answer
1k views

Many to Many and Weak Entities

I have an entity that cannot exist without being defined by another, and I want this entity to participate in a many to many relationship. Example: An artist has an album (the album cannot exist ...
0
votes
2answers
102 views

Primary key for link/junction table with time-dependent data

I want to link two tables in such a way that for any given time period the relationship is one-to-one, but over time the relationship can change. As an minimal example, suppose I want to list the ...
6
votes
2answers
3k views

Non-unique multicolumn foreign key

I have a "comments" table that models a conversation on a topic, like this: id serial topic_id integer parent_comment_id integer body text So, every comment has a reference to its topic AND ...
4
votes
3answers
18k views

How do I insert a row which contains a foreign key?

Using PostgreSQL v9.1. I have the following tables: CREATE TABLE foo ( id BIGSERIAL NOT NULL UNIQUE PRIMARY KEY, type VARCHAR(60) NOT NULL UNIQUE ); CREATE TABLE bar ( id BIGSERIAL ...
-2
votes
1answer
167 views

How to represent it with UNIQUE indexes and foreign keys?

"Group" is a group of pupils. "Topic" is a topic of lessons. To every group corresponds exactly one topic. (So there may be multiple groups per topic.) To a pupil corresponds multiple groups, but ...
9
votes
2answers
980 views

What is the purpose of SET NULL in Delete/Update Foreign Keys constraints?

I am probably being narrow minded, but if I create a foreign key constraint and a row gets updated or deleted, I lose that connection if the child table's column gets set to NULL. What is the purpose ...
4
votes
1answer
275 views

How to relate two rows in the same table

I have a table where the rows can be related to each other, and logically, the relationship goes both ways (basically, is directionless) between the two rows. (And if you're wondering, yes, this ...
3
votes
1answer
112 views

Enforcing constraints “two tables away”

I ran into some trouble modeling an electrical schematic in SQL. The structure I'd like to capture is part ←────────── pin ↑ ↑ part_inst ←───── pin_inst where "inst" is short ...
3
votes
2answers
134 views

Pair of 2-tuple composite FKs pointing to same table (closure table pattern)

I don't have much experience with DB design, and having a hard time figuring out what's wrong with the following: Table father has a composite PK (col1, col2) Table closure has ancestor(a-col1, ...
0
votes
1answer
42 views

Why use foreign keys if cascade action is not need?

I'm testing some queries with MySQL and tried to create two tables were there should be a reference between IDs. Something like: CREATE TABLE user ( id int(11) UNSIGNED AUTO_INCREMENT, name ...
13
votes
1answer
2k views

How to write a query which finds all circular references when a table references itself?

I have the following schema (names changed), which I cannot change: CREATE TABLE MyTable ( Id INT NOT NULL PRIMARY KEY, ParentId INT NOT NULL ); ALTER TABLE MyTable ADD FOREIGN KEY ...
6
votes
2answers
8k views

Good explanation of cascade (ON DELETE/UPDATE) behavior

I don't design schemas everyday, but when I do, I try to setup cascade updates/deletes correctly to make administration easier. I understand how cascades work, but I can never remember which table is ...
9
votes
3answers
195 views

Are there any database engines which will intuit the join condition based on the existing foreign keys?

It seems strange to me that, when I've defined a foreign key, the engine cannot use this information to automatically figure out the correct JOIN expressions, but instead requires me to re-type the ...
3
votes
2answers
224 views

Both Primary and Foreign Key Needed on Table?

Someone recently asked this question: There are two tables: Deal and DealCategories. One deal can have many deal categories. So the proper way should be to make a table called ...
3
votes
1answer
985 views

Foreign Key constraint on fixed value field

Note: I am a developer... I have an Asset table that has many codes that are foreign keys into a second table OutlineFiles. The OutlineFiles table has Type and Code as the primary key. The foreign ...
1
vote
1answer
799 views

Is it possible to force drop of mysql tables with FK?

For dropping a table in which a Foreign Key is used in other tables, it is necessary to drop all other tables having FK connection with the corresponding table. Is there a short way to drop a table, ...