It is added only for the purpose of unique row identification. The Id column is the surrogate primary key of the Customer table. You can find more insights on surrogate keys in this article. If you have trouble deciding between using a natural and a surrogate primary key, check out this article. The CustomerId column is the primary key for the Customer table and a foreign key to the Order table.
The OrderId column is the primary key for the Order table. It is also a foreign key and part of a composite primary key in the OrderProduct table.
The other half of this composite primary key is the ProductId column, which is a foreign key to the Product table. The CourseEnrollment table has a composite primary key.
Its primary key consists of the CourseId and StudentId columns. It means that the combination of the CourseId and StudentId columns must be unique for each row. However, both the CourseId and the StudentId columns can contain duplicate values. Hence, the values of the StudentId column can repeat.
Also, each course can be attended by many students, so the values of the CourseId column can repeat as well. However, one student can attend a given course only once.
Thus, the combination of the CourseId and StudentId columns must be unique. And that is shown in the table above. Now you know how crucial the primary key is. And all of that is based on the simple idea of unique row identification. When choosing a table's primary key column, keep in mind that all its values must be unique — unless you choose to use a composite primary key, in which case the combination of values must be unique. Another important point to keep in mind when deciding on a primary key is that its values cannot be null.
The primary key column must be unique and must contain non-null values. Primary key types: composite, natural, and surrogate keys. Why primary keys are so important. Primary key syntax and usage in SQL. Typical primary key use cases. How to set primary keys in Vertabelo.
The primary key concept is critical to an efficient relational database. Without the primary key and closely related foreign key concepts, relational databases would not work.
In fact, since a table can easily contain thousands of records including duplicates , a primary key is necessary to ensure that a table record can always be uniquely identified. All keys that come from real-world observables and attributes are called natural primary keys, as opposed to surrogate primary keys that are, instead, arbitrarily assigned to each record.
Almost all individuals deal with natural primary keys frequently but unknowingly in everyday life. For example, students are routinely assigned unique identification ID numbers, and all U. Street addresses or driver license numbers are examples of primary keys used to uniquely identify respectively locations or cars. As another example, a database must hold all of the data stored by a commercial bank.
To uniquely identify customers, a column or combination of columns is selected to guarantee that two customers never have the same unique value. Thus, certain columns are immediately eliminated, e. A good primary key candidate is the column that is designated to hold Social Security numbers. The next logical option is to use a combination of columns, such as adding the surname to the date of birth to the email address, resulting in a long and cumbersome primary key.
Then, the database automatically generates a unique number each time a customer is added, guaranteeing unique identification. As this key is created, the column is designated as the primary key within the SQL script that creates the table, and all null values are automatically rejected. For example, a customer may be asked to provide his surname when conducting a bank query.
A common surname such as Smith query is likely to return multiple results. A primary key is used to uniquely identify each row in a table. It can either be part of the actual record itself , or it can be an artificial field one that has no meaning other than being an identifier of the record. How to set primary keys is an important factor in database design, as the choices of primary key can have significant impact on the performance, usability, and the extensibility of the entire database.
A primary key can consist of one or more columns on a table.
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