What is the foreign key and how is it used?
A foreign key is a key used to link two tables. A foreign key is a column or combination of columns whose values match a primary key in a different table. The relationship between 2 tables matches the primary key in one of the tables with a foreign key in the second table.
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Is it good to use foreign key?
Yes, you should. Foreign keys are just constraints that help you establish relationships and make sure you have the correct information in your database. You should use them to prevent incorrect data entry of any kind. There is actually a growing move away from foreign key based relational databases at the moment.
What are the conditions for the foreign key?
Creating a foreign key constraint requires the REFERENCES privilege on the parent table. The corresponding columns in the foreign key and the referenced key must have similar data types. The size and sign of fixed precision types, such as INTEGER and DECIMAL, must be the same. The length of the string types does not need to be the same.
Why do we use foreign keys?
As we mentioned, the primary purpose of the foreign key is to provide referential integrity between the parent and child table. In SQL Server, we can specify delete and update rules for foreign keys so that we can determine the behavior of the child data when we want to update or delete some data from the parent table.
Why should you avoid the foreign key?
Reasons not to use Foreign Keys:
- it’s making the database work harder on each CRUD operation because it has to check FK consistency.
- when enforcing relationships, FKs specify an order in which you should add/remove things, which can cause the database to refuse to do what you want.
What is the point of SQL foreign keys?
A foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between data in two tables to control what data can be stored in the foreign key table.
Does foreign key allow duplicate values?
Unlike primary keys, foreign keys can contain duplicate values. Also, it’s okay for them to contain NULL values. Although they are not automatically created for foreign keys, it is a good idea to define them. You can define multiple foreign keys within a table.
Why do we need to use foreign keys?
– Physically define the business by avoiding data integrity issues in your database. (eg, they logically document the business by showing how all the data relates to each other). Foreign keys are native to SQL Server and are designed to prevent data integrity issues.
What is the main use of the foreign key?
Dear, foreign key is used to make relation of one table with another table, while updating the table will affect both at the same time. Basically foreign key is used to establish relationship between tables, it is used to reduce data redundancy and data integrity.
Can a unique key be used as a foreign key?
Yes, there can be a foreign key that is a unique key in another table, since the unique key is a subset of the primary key, but not the exact primary key. So it is possible that the foreign key is a unique key in another table. The general standard answer is no. It is only possible if the foreign key refers to any column uniquely in another table.
Does a foreign key have to be unique?
According to the SQL standard, a foreign key must reference the primary key or a unique key of the primary table. If the primary key has multiple columns, the foreign key must have the same number and order of columns. Therefore, the foreign key refers to a single row in the main table; there can be no duplicates.