There Are Only Four Types of Relational Keys Candidate Key As Stated

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There are only four types of relational keys Candidate Key As stated above, a candidate key is any set of one or more columns whose combined values are unique among all occurrences (i.e., tuples or rows). Since a null value is not guaranteed to be unique, no component of a candidate key is allowed to be null. There can be any number of candidate keys in a table . Relational pundits are not in agreement whether zero candidate keys is acceptable, since that would contradict the (debatable) requirement that there must be a primary key. Primary Key The primary key of any table is any candidate key of that table which the database designer arbitrarily designates as "primary". The primary key may be selected for convenience, comprehension, performance, or any other reasons. It is entirely proper (albeit often inconvenient) to change the selection of primary key to another candidate key. As we will see below, there is no property of a primary key which is not shared by all other candidate keys in of the table except this arbitrary designation. Unfortunately E-R methodologies and RDBMS products have come to rely on the primary key almost to the exclusion of the concept of candidate keys, which are rarely supported by software. Alternate Key The alternate key s of any table are simply those candidate keys which are not currently selected as the primary key. According to { Date95 } (page 115), "... exactly one of those candidate keys [is] chosen as the primary key [and] the remainder, if any, are then called alternate keys ." An alternate key is a function of all candidate keys minus the primary key. Foreign Key A foreign key is a set of one or more columns in any table (not necessarily a candidate key, let alone the primary key, of that table) which may hold the value(s) found in the primary key column(s) of some other table. So we must have a primary key to match the foreign key. .

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Keys Are, As Their Name Suggests, a Key Part of a Relational Database
The key is defined as the column or attribute of the database table. For example if a table has id, name and address as the column names then each one is known as the key for that table. We can also say that the table has 3 keys as id, name and address. The keys are also used to identify each record in the database table . Primary Key:- • Every database table should have one or more columns designated as the primary key . The value this key holds should be unique for each record in the database. For example, assume we have a table called Employees (SSN- social security No) that contains personnel information for every employee in our firm. We’ need to select an appropriate primary key that would uniquely identify each employee. Primary Key • The primary key must contain unique values, must never be null and uniquely identify each record in the table. • As an example, a student id might be a primary key in a student table, a department code in a table of all departments in an organisation. Unique Key • The UNIQUE constraint uniquely identifies each record in a database table. • Allows Null value. But only one Null value. • A table can have more than one UNIQUE Key Column[s] • A table can have multiple unique keys Differences between Primary Key and Unique Key: • Primary Key 1. A primary key cannot allow null (a primary key cannot be defined on columns that allow nulls). 2. Each table can have only one primary key. • Unique Key 1. A unique key can allow null (a unique key can be defined on columns that allow nulls.) 2.
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