Contents
Is foreign key necessary for join?
You need two columns of the same type, one on each table, to JOIN on. Whether they’re primary and foreign keys or not doesn’t matter. You don’t need a FK, you can join arbitrary columns. But having a foreign key ensures that the join will actually succeed in finding something.
What is the purpose of foreign keys?
A foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between the data in two tables to control the data that can be stored in the foreign key table.
Can foreign key be repeated?
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table).
Can we join two tables without common columns?
Yes, you can! The longer answer is yes, there are a few ways to combine two tables without a common column, including CROSS JOIN (Cartesian product) and UNION. The latter is technically not a join but can be handy for merging tables in SQL. In this article, I’ll guide you through the different solutions with examples.
What’s the difference between a join and a foreign key?
FOREIGN KEYs and JOINs don’t do the same thing! A FOREIGN KEY enforces data integrity, making sure the data confirms to some rules when it is added to the DB. A JOIN is used when you extract/query data from the DB by giving rules how to select the data. JOINs work if there are FK or not. FK’s work if you extract data with or without JOINs.
Is it necessary for a foreign key to be a primary key?
is it necessary for a foreign key to be primary key of another table Yes, foreign key has to be primary key of parent table. foreign key is not unique it may have duplicate entries
Why do you need foreign keys in SQL?
Foreign keys are just constraints to enforce referential integrity. You will still need to use JOINs to build your queries. Foreign keys guarantee that a row in a table order_details with a field order_id referencing an orders table will never have an order_id value that doesn’t exist in the orders table.
When do table names have to be unique?
Table names have to be unique, but only within the context of the schema that contains the tables, etc. When you query columns you need to reference the schema, table and column that you are interested in, unless one or more of these can be inferred.