What is a key in a table?
What are Keys in DBMS?KEYS in DBMS is an attribute or set of attributes which helps you to identify a row(tuple) in a relation(table). They allow you to find the relation between two tables. Keys help you uniquely identify a row in a table by a combination of one or more columns in that table. Key is also helpful for finding unique record or row from the table. Database key is also helpful for finding unique record or row from the table. Show
Example:
In the above-given example, employee ID is a primary key because it uniquely identifies an employee record. In this table, no other employee can have the same employee ID. In this tutorial, you will learn:
Why we need a Key?Here are some reasons for using sql key in the DBMS system.
Types of Keys in DBMS (Database Management System)There are mainly Eight different types of Keys in DBMS and each key has it’s different functionality:
Let’s look at each of the keys in DBMS with example:
What is the Super key?A superkey is a group of single or multiple keys which identifies rows in a table. A Super key may have additional attributes that are not needed for unique identification. Example:
In the above-given example, EmpSSN and EmpNum name are superkeys. What is a Primary Key?PRIMARY KEY in DBMS is a column or group of columns in a table that uniquely identify every row in that table. The Primary Key can’t be a duplicate meaning the same value can’t appear more than once in the table. A table cannot have more than one primary key. Rules for defining Primary key:
Example: In the
following example,
What is the Alternate key?ALTERNATE KEYS is a column or group of columns in a table that uniquely identify every row in that table. A table can have multiple choices for a primary key but only one can be set as the primary key. All the keys which are not primary key are called an Alternate Key. Example: In this table, StudID, Roll No, Email are qualified to become a primary key. But since StudID is the primary key, Roll No, Email becomes the alternative key.
What is a Candidate Key?CANDIDATE KEY in SQL is a set of attributes that uniquely identify tuples in a table. Candidate Key is a super key with no repeated attributes. The Primary key should be selected from the candidate keys. Every table must have at least a single candidate key. A table can have multiple candidate keys but only a single primary key. Properties of Candidate key:
Candidate key Example: In the given table Stud ID, Roll No, and email are candidate keys which help us to uniquely identify the student record in the table.
Candidate Key in DBMS What is the Foreign key?FOREIGN KEY is a column that creates a relationship between two tables. The purpose of Foreign keys is to maintain data integrity and allow navigation between two different instances of an entity. It acts as a cross-reference between two tables as it references the primary key of another table. Example:
In this key in dbms example, we have two table, teach and department in a school. However, there is no way to see which search work in which department. In this table, adding the foreign key in Deptcode to the Teacher name, we can create a relationship between the two tables.
This concept is also known as Referential Integrity. What is the Compound key?COMPOUND KEY has two or more attributes that allow you to uniquely recognize a specific record. It is possible that each column may not be unique by itself within the database. However, when combined with the other column or columns the combination of composite keys become unique. The purpose of the compound key in database is to uniquely identify each record in the table. Example:
In this example, OrderNo and ProductID can’t be a primary key as it does not uniquely identify a record. However, a compound key of Order ID and Product ID could be used as it uniquely identified each record. What is the Composite key?COMPOSITE KEY is a combination of two or more columns that uniquely identify rows in a table. The combination of columns guarantees uniqueness, though individually uniqueness is not guaranteed. Hence, they are combined to uniquely identify records in a table. The difference between compound and the composite key is that any part of the compound key can be a foreign key, but the composite key may or maybe not a part of the foreign key. What is a Surrogate key?SURROGATE KEYS is An artificial key which aims to uniquely identify each record is called a surrogate key. This kind of partial key in dbms is unique because it is created when you don’t have any natural primary key. They do not lend any meaning to the data in the table. Surrogate key in DBMS is usually an integer. A surrogate key is a value generated right before the record is inserted into a table.
Above, given example, shown shift timings of the different employee. In this example, a surrogate key is needed to uniquely identify each employee. Surrogate keys in sql are allowed when
Difference Between Primary key & Foreign keyFollowing is the main difference between primary key and foreign key:
Summary
What is a keys in a database?What are the keys in DBMS? A key refers to an attribute/a set of attributes that help us identify a row (or tuple) uniquely in a table (or relation). A key is also used when we want to establish relationships between the different columns and tables of a relational database.
What is key in SQL table?An SQL key is either a single column (or attribute) or a group of columns that can uniquely identify rows (or tuples) in a table. SQL keys ensure that there are no rows with duplicate information. Not only that, but they also help in establishing a relationship between multiple tables in the database.
Can a table have a key?Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key. The primary key of one table may also help to identify records in other tables, and be part of the second table's primary key.
What is Keys and its types?Keys in RDBMS allow you to establish a relationship between and identify the relation between tables. Eight types of key in DBMS are Super, Primary, Candidate, Alternate, Foreign, Compound, Composite, and Surrogate Key. A super key is a group of single or multiple keys which identifies rows in a table.
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