SQL PRIMARY KEY
A SQL PRIMARY KEY is a field or combination of fields in a database table that uniquely identifies each record in the table. It ensures that each row in the table is uniquely identifiable and serves as a reference point for establishing relationships with other tables. The PRIMARY KEY constraint also enforces the uniqueness and non-nullability of the specified column or columns.
3 Key excerpts on "SQL PRIMARY KEY"
eBook - ePubData Analysis for Corporate Finance
Building financial models using SQL, Python, and MS PowerBI
- Mariano F. Scandizzo CFA CQF(Author)
- 2021(Publication Date)
- Fulton Books, Inc.(Publisher)
...7: Table Design & Layout In general, each table is modeled after a real-world entity, in this case, the information related to the bike shops reselling AdventureWorks products. Then, the columns of the table represent the attributes of the entity, such as name, address, phone number, etc. Each row of the table represents one instance of the table. Values are stored at the intersection of each row and column, sometimes referred to as a cell. I guess by now you have realized that a tool such as Microsoft Excel has been purposely created to provide you with a simplified, user-friendly interphase to create tables. Typically, a table contains one or more columns that uniquely identifies each row in a table, called primary key of the table. A primary key is a table field which uniquely identifies each and every row/record in the table. Primary keys must obey the use of unique values to ensure no duplicated records. Furthermore, a primary key column cannot have null values. A table can have only one primary key. Such key could be formed by a single or the combination of multiple fields. A primary key conformed by several fields is called a composite key. As mentioned before, if a table has a primary key defined on any field(s), then the table cannot have two records using the same primary key value. For instance, the primary key of the resellers table is the ResellerKey column. You do not have to guess the structure of a table. SSMS can easily provide you with such information. Fig. 8: Obtaining Table Design Information If you go to the object explorer in SSMS, click on the full database, followed by selecting tables, to then select the specific table you want to explore...
eBook - ePubInformation Systems
What Every Business Student Needs to Know, Second Edition
- Efrem G. Mallach(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...Columns are also called attributes of the entity that the table represents. Example: A column in a student table might store the student’s major. “Major” is an attribute of the entity “student.” Every row in every table has a unique way to identify it, called a primary key. Example: Student ID number. There may be no duplicate keys. (Databases can enforce this rule.) Student name is not a good primary key. Names are not necessarily unique. Phone number is not a good primary key. A student might not have a phone, or might share one with another student. (Likely? No. Possible? Yes.) Keys connect records in different tables with each other. Example: An athletic team roster, instead of storing team members’ majors, stores their student IDs. These IDs identify records in the student table. Those records have the students’ majors. Suppose students’ majors were in both the student table and the volleyball team table. Now Laura switches from English to History. The registrar’s office will update her registration record. The team data will be wrong, perhaps for a long time. Storing the same information more than once, such as having Laura’s major in two tables, is called redundancy. Redundancy leads to errors. Good database design minimizes redundancy. A Note on Language You may hear that these databases are called relational because they relate tables to each other. They do, but that’s not where this name comes from. It comes from mathematics and means, in effect, that all the fields in a record relate to one real-world entity. Had Dr. Edgar Codd, who invented relational databases at IBM in the 1970s, foreseen how much confusion this term would cause, he might have picked another—but it’s too late to change. We’re stuck with that name. Facebook, for example, is a giant database at heart with a “front end” program that takes data from that database and arranges it on web pages. Think about the information Facebook has...
eBook - ePub- John R. Talburt(Author)
- 2011(Publication Date)
- Morgan Kaufmann(Publisher)
...Conversely, any given Course entity may be related to (have in it) several Student entities, or none. Figure 1.1 Example of a Simple ERD Each entity type also has a set of attributes that describes the entity. For example, the Instructor entity type has the three attributes FacultyID, Name, and Department. Assigning values to these attributes defines a particular instructor, called an instance of the Instructor entity. By the previous definition, an instance of an entity is also an entity reference. A fundamental rule of ERM is that every instance of an entity should have a unique identifier. Codd (1970) called this the Entity Identity Rule. A primary key is an identity attribute or group of identity attributes selected by the data modeler because the combination of values taken on by these attributes will be unique for each entity instance. However, at the design stage, it is not always clear that a particular combination of descriptive attributes will have this property, or it if does, that the combination will continue to be unique as more instances of the entity are acquired. For this reason data modelers often play it safe by adding another attribute to an entity type that does not describe any intrinsic characteristic of the entity but is simply there to guarantee that each instance of the entity has a primary key. For example, in Figure 1.1, with only name and department as the identity attributes for the Instructor entity, it is conceivable that a department could have two instructors with the same name. If this were to happen, the combination of name and department would no longer meet the requirements to form a primary key. By adding a FacultyID attribute as a third attribute and by controlling the values assigned to FacultyID, it is possible to guarantee that each instance of the Instructor entity has a unique primary key value...
