A foreign key is a column of data that links two tables together. It always references another primary key elsewhere, commonly called the parent table.
A primary key is a unique data value that uniquely identifies each row in a database table. It cannot accept duplicate values or null values.
What is a primary key?
A primary key is a column or set of columns uniquely identifying each row in a database table. It’s the most essential and fundamental aspect of any database, as it permits the establishment of relationships between different tables using foreign keys.
A table’s primary key is typically the first column you use when creating a new table. This key must be unique for each record in the table and cannot appear more than once in a row.
In most cases, you should choose a number for your primary key; this makes the database management system’s job simpler. Furthermore, ensure that this key is non-nullable to avoid any potential relationship issues when updating data in the table.
Selecting a primary key for your table can be an arduous task, particularly if the data being tracked has shifting values. This includes people’s names and addresses that may shift over time.
One possible primary key for an individual is their Social Security number. This unique identifier is the only way most Americans can identify themselves legally within America, making it a prime candidate for use as a primary key.
Another option is using a person’s date of birth as an identity key. While this can serve as a primary key, it may not always be accurate since some people’s dates of birth change over time.
Finally, you should decide how to utilize this primary key when creating relationships between different tables. For instance, you could use the date of birth to establish a connection between an employee and their employer.
Database primary keys can be composed of either one attribute or multiple attributes combined, known as a composite primary key. These keys consist of two or more columns and are only valid if their values don’t repeat within the table.
What is a foreign key?
A foreign key in a database is an organized set of columns that links data between tables. It uniquely identifies each record within the relational database and helps create and sustain relationships between tables.
A primary key is a column in a table that uniquely identifies each row. It must be unique and cannot contain duplicate values; null values are also prohibited for primary keys. When deleting a row with its primary key value, you must first delete all references to that value from the table itself.
Foreign keys can create both one-to-many and many-to-many relationships between tables. For instance, if there are sales and customer tables, the customer table might contain a foreign key attribute that references the sales table’s primary key (company name, department name, and address) in order to decide which customers receive which sales.
When creating a foreign key, the key you select must reference all data in both tables. Otherwise, referential integrity can be compromised–especially for 1:M relationships where multiple rows in one table refer to exactly one row in another.
Additionally, ensure the data type of a foreign key column matches that of its corresponding primary key in the table you are creating it for. Doing this helps prevent errors from arising if entered data does not match what exists in the referenced table.
Another essential feature of a foreign key is its constrained nature. This allows you to regulate what happens with data after it has been deleted from its parent table, using options such as SET NULL, NO ACTION and CASCADE to ensure no further deletions occur in the child table.
SQL allows for adding a foreign key constraint as part of the CREATE TABLE statement or the REFERENCES clause to an existing table. Adding this clause is simpler than creating the constraint itself since all required is the table name and referenced primary key column names.
How do foreign keys work?
Database foreign keys are used to connect data across tables. They may be defined using either one primary key, multiple primary keys, or a combination.
The primary key serves as a unique identifier for every row in the table, preventing duplication of records. Null values are not permitted within a primary key.
The table’s primary key can be composed of text, numbers, or any other format supported by the database. It stores one value and allows grouping values together to identify certain rows within a table.
Each table has its own primary key. This original table, known as a parent or referenced table, may be the target of multiple foreign keys from other tables (known as child tables).
One primary purpose of a foreign key is to maintain referential integrity between two tables, commonly referred to as referential integrity. Referential constraints must be enforced whenever a row in the referencing table is updated or deleted to accomplish this goal. When this takes place, all foreign key column values in the referencing table must also be set to null to guarantee they still correspond to data values present in the original table.
Foreign keys also serve to manage a relational data model. This type of modeling allows administrators to organize data points according to defined relationships.
Consider a company’s database with two tables. One contains all customer details, while the other houses orders. In this scenario, each order must be associated with an individual customer. In such cases, a foreign key in the orders table points directly at a primary key in customers.
This helps the company guarantee that each customer’s order is always linked back to a specific individual. While customer name and address are stored in the orders table, each order is given its own ‘customer number’ in the customers table for unique identification.
The customer number is the primary key in the customers table, while its counterpart in the orders table serves as a foreign key that refers back to that same primary key from earlier.
What are the benefits of foreign keys?
Databases use foreign keys to connect tables, enabling queries between them. A foreign key is a column in one table that links to another through its primary key; this primary key uniquely identifies each row within the other table.
A foreign key column can have either a single value or multiple values. Typically, it must match the primary key of another table to ensure consistency between rows in both tables. A foreign key constraint helps enforce integrity between two databases by guaranteeing values in one row are consistent with those in another table.
This is essential, as it guarantees that each column in a row in one table is linked to its corresponding row in another. This concept of referential integrity ensures this occurs.
Referential integrity helps safeguard databases by preventing invalid data from entering the system and ensuring all records are consistently created or updated. Not only does this reduce processing time for queries, but it also allows efficient querying of interlinked data sets.
Foreign keys combined with join criteria allow the database engine to use an index to expedite finding related data faster. This is especially helpful when two tables being joined have different data types or formats, and comparison between each must take place.
Foreign keys can also safeguard a referenced column’s integrity by guaranteeing all values are unique. This is especially useful if the data in that column has been modified or deleted from its original location.
However, one drawback of foreign keys is that the CPU cost of checking each row for its corresponding primary key is added to the processing time for INSERT operations. This issue is common and could potentially cause delays in overall INSERT processing.
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