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JanSQL Keys are defined as a set of attributes that are generally needed to access tuples from a table. These two SQL keys are a vital part of SQL keywords and also used to construct relationships between the two tables. In this blog, we will discuss SQL Primary keys , what is a candidate key and significant differences between the two.
Both SQL keys are responsible to identify a tuple within a table or a relation. The major difference is that there can be only one primary key that can be defined for a table. However, candidate keys can be used multiple times in a database relation.
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Let us learn the definition of both SQL keywords in brief below.
You have to be extra careful when deciding on the candidate or primary key. The incorrect selection may adversely impact the database structure and future Database normalization. To qualify the candidate key as the primary key, it should be Not Null and unique in the domain. When you start working on databases, you will realize that SQL primary keys are seldom changed.
Here are a few points to consider when converting a candidate key to the primary key:
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Here are the few points to consider when converting a candidate key to the primary key:
Other than the primary key and the candidate key, the next important concept to focus is a foreign key. A foreign key in the database is the key from another table that refers to the primary key in the table being used.
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Parameters for Comparison | Primary Key | Candidate Key |
Basic | There can be only one primary key in a relation | They can be multiple candidate keys in a relation. |
Specify | It is not mandatory to specify a primary key for each relation. | It is not possible to define any relation without the candidate key. |
Null | No attributes of the primary key can contain Null values. | The attributes of the candidate key can contain Null values. |
Feature | A primary key defines the most important attribute of a relation. | Candidate key features candidates that can be defined as the primary key. |
Vice-versa | A primary key is the candidate key. | Each candidate key is not the primary key. |
A primary key is an attribute or a set of attributes that uniquely identify each tuple in a relation. There is one primary key for each relation within a database. Keep in mind that the primary key should never contain NULL values. In case, one column contains Null values then it cannot be defined as the primary key. Further, the values of attributes of the primary key need to be static and the value of attributes changes rarely.
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One of the candidate keys gets qualified to become a primary key. The rules that are followed by the candidate key to qualify as the primary key include Null and unique parameters. If a relation contains an attribute that is defined as the primary key of some other relation then this attribute is called the foreign key. It is always advised to figure out the primary key of a database relation before you add other attributes to relation as a primary key identifies each tuple uniquely. It is always good to choose either a single attribute or a small number of attributes as the primary key that makes the relation handling easy. Let us understand the concept in deep with the help of an example below. Student {ID, First_name, Last_name, Age, Address}
First, find out the possible number of candidate keys for this relation. In this example, there are three candidate keys ID, First-name, Last-name. Out of these three, ID can be defined as the primary key because there are chances that two students share the same first and the last name but ID will remain unique in such cases.
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When you are working on complex database designs, a deep understanding of different SQL keys is necessary that includes a primary key, foreign key, candidate key, composite key, super key, unique key and more. There is a slight difference between them but you should clear with the differences otherwise it may lead to serious confusion later.
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A candidate key is an attribute or a set of attributes that uniquely defines the tuple within a relation. As we know, there is more than one candidate key in a relation. Out of all candidate keys, one candidate can qualify to become a primary key. Though each candidate key qualifies to become a primary key, only one should be chosen as the primary key.
The rules that a candidate key requires to become the primary key are the attribute values of the key can never be Null for any domain and it must be unique or static. If all candidate keys qualify to become a primary key then experienced DBA should take a decision to figure out the primary key. There is no relation possible to establish without a candidate key. Let us understand the candidate key with an example.
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If we add some more attributes to Student relation, I discussed above. Student {ID, First_name, Last_name, Age, Address, DOB, Department_name}
Here, we may identify two candidate keys that are ID and DOB. It will help you to understand the candidate keys are the one that uniquely identifies a tuple in a relation.
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You have to be extra cautious when defining SQL keywords or SQL key types mainly primary key and the candidate key for a relation within the database. All candidate keys can be a candidate key for the primary key and DBA should make a decision here about the primary key based on certain rules that need to be followed to qualify as SQL primary keys.
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Keep in mind that always choose the right primary key which is the very basic step in the database design. To know more about different SQL keywords, SQL Keys, and SQL shortcuts in detail, you should join the SQL certification program at the JanBask training to learn to use these keys practically.
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