Category: Keys

The SQL Alternate Key SQL Alternate Keys in a database table are candidate keys that are not currently selected as a primary key. They can be used to uniquely identify a tuple(or a record) in a table. There is no specific query or syntax to set the alternate key in a table. It is just a column…

The SQL Composite Key An SQL Composite Key is a key that can be defined on two or more columns in a table to uniquely identify any record. It can also be described as a Primary Key created on multiple columns. Composite Keys are necessary in scenarios where a database table does not have a single column…

The SQL Foreign Key In SQL, a Foreign Key is a column in one table that matches a Primary Key in another table, allowing the two tables to be connected together. A foreign key also maintains referential integrity between two tables, making it impossible to drop the table containing the primary key (preserving the connection between the…

The SQL Primary Key The SQL Primary Key is a column (or combination of columns) that uniquely identifies each record in a database table. The Primary Key also speeds up data access and is used to establish a relationship between tables. Even though a table can only have one Primary Key, it can be defined on one…

The SQL Unique Key The SQL Unique Key (or, Unique constraint) does not allow duplicate values in a column of a table. It prevents two records from having same values in a column. Unique Key is just an alternative to the Primary Key; as both Unique and Primary Key constraints ensure uniqueness in a column of the…