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This article will explain in detail why we need to use keys in the database. The editor thinks it is very practical, so I share it for you as a reference. I hope you can get something after reading this article.

Why do we need to use keys?

In practical applications, there are a large number of tables needed to store data, and different tables are related to each other.

In addition, a large amount of data is stored in the table. Tables typically extend to thousands of records stored in them, unclassified and unorganized.

Now to get any specific records from such a dataset, you will have to apply some conditions, but if there is duplicate data and error data occurs every time you try to get some data by applying certain conditions. How many experiments are there before getting the correct data?

To avoid all this, defining keys can easily identify any row of data in the table.

In a database, keys are used to establish and identify relationships between tables, as well as to uniquely identify any records or data rows in the table. It can be a single attribute or a set of attributes, where the combination can be used as a key.

Type of key

Here are some common types of keys. In order to let you know about these keys, let's introduce them through examples:

Student_idnameagecollege_idphone1

Xiaohua 20011561258xxx

2 Xiao Hong 21011771258xxx3 Xiao Ming 20021831258xxx

You can see that this is a simple Student table with fields student_id,name,subject_id,age and phone.

Candidate key (Candidate Key)

The candidate key is defined as the minimum set of fields that can uniquely identify each record in the table. It is an attribute or set of attributes that can act as the primary key of a table to uniquely identify each record in the table.

In the Student table above, student_id and phone are both candidate keys for the student table. Here are some characteristics of candidate keys:

The ● candidate key can never be NULL or empty; its value should be unique.

All attributes except the primary key in the ● table can be considered candidate keys, and a table can have multiple candidate keys.

The ● candidate key can be a combination of multiple columns (attributes).

Super key (Super Key)

A superkey is defined as a set of attributes in a table that uniquely identify each record in the table. A superkey is a superset of candidate keys.

In the table defined above, the superkey can be:

● student_id: because each row of student_id data is unique, it can be used to uniquely identify each row.

● (student_id, name): now two students can have the same name, but their student ID cannot be the same, so this combination can also be a key.

● phone: because each student's phone number is unique, phone can also be a key.

So they can all be superkeys.

Primary key (Primary Key)

The primary key is the most suitable candidate key to be the identification key of any table. It can uniquely identify each record in the table.

For Student tables, we can use the student_id column as the primary key.

Compound bond (Composite key)

A compound key is a key that consists of two or more attributes that uniquely identify any record in a table. When no single attribute of the table can uniquely identify the row, we need to use a compound key.

Note: the properties that jointly form compound keys are not independent or separate keys.

Student_idsubject_idexam_namemark102PHP course 90202MySQL course 80

Above is a Score table that stores students' scores in specific subjects.

In this table, student_id,subject_id will form the primary key, so it is a compound key.

Spare key (Alternative key)

Of all candidate keys, only one is selected as the primary key, and the remaining keys are called alternate or secondary keys.

Foreign key (Foreign Key)

Foreign keys are property values in a table that act as the primary key of another table. Therefore, foreign keys can be used to link two tables together.

Note: you need to be very careful about entering data in the foreign key column, because entering the wrong data may invalidate the relationship between the two tables.

About why we need to use keys in the database to share here, I hope the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

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