What relationship occurs when multiple records in a table are associated with multiple records in another table?
In this article, we are about to take a close look at the SQL Server relations, including one-to-one, many-to-many, and many-to-one relations, and get into details of creating those with the help of dbForge Studio for SQL Server. Show
Well-established relations in a database allow for clear table structures and help cut redundant data to a minimum. Building efficient relations in a database is extremely important— it helps enforce referential integrity which in its turn contributes to database normalization. There are five types of relations in the databases: one-to-one, one-to-many, many-to-one, many-to-many, and self-referencing relationships. So, what’s the difference between these database relationship types? In the article, we’ll examine each type separately and provide a working example of their usage. CONTENTS What is a relation in a database?Let’s start with some basic terminology. The term relation is sometimes used to refer to a table in a relational database. However, it is more often used to describe the relationships that exist between the tables in a relational database. A relationship between two database tables presupposes that one of them has a foreign key that references the primary key of another table. Database entity—strictly speaking — is a person, place, thing, object, or any item about which data is stored in the database. However, the term is usually used to refer to the database table as tables are, in fact, the physical implementation of entities. An entity-relationship diagram, also known as ERD, ER diagram, or ER model, comprises a
graphical representation of how entities relate to each other within a database. Let’s have a look at the entity-relationship diagram of the BicycleStore database with the help of the Database Diagram tool that comes with dbForge Studio for SQL Server. Types of relationships in a databaseThere are 3 main types of relationship in a database:
However, you may also encounter references to a many-to-one relationship which, in fact, is a special case of a one-to-many relationship and self-referencing relationship which occurs when only one table is involved. Let’s examine each relationship type in detail and consider the peculiarities of creating relationships in SQL. One-to-many relationshipLet’s start with a one-to-many relationship as it is the most commonly used type. So, what is one-to-many relationship in SQL? A one-to-many relationship occurs when one record in table 1 is related to one or more records in table 2. However, one record in table 2 cannot be related to more than one record in table 1. We can come up with hundreds of examples of such relations: pages and the book they belong to, pupils and their class, orders and the customer who placed them, etc. How to join tables with one-to-many relationship in SQL? INNER JOINs are considered to be the most effective way to combine data from two tables that have one-to-many relationship. Let’s query two SQL tables having one-to-many relationship. SELECT * FROM country c INNER JOIN city c1 ON c.country_id = c1.country_idExample of one-to-many relation in SQL ServerHow to implement one-to-many relationships when designing a database:
Step 1 CREATE TABLE dbo.city ( city_id int IDENTITY, city varchar(50) NOT NULL, country_id int NOT NULL, CONSTRAINT PK_city PRIMARY KEY CLUSTERED (city_id) ) ON [PRIMARY] GO CREATE TABLE dbo.country ( country_id int IDENTITY, country varchar(50) NOT NULL, CONSTRAINT PK_country PRIMARY KEY CLUSTERED (country_id) ) ON [PRIMARY] GOStep 2 ALTER TABLE dbo.city WITH NOCHECK ADD FOREIGN KEY (country_id) REFERENCES dbo.country (country_id) GOOne-to-one relationshipA one-to-one relationship in a database occurs when each row in table 1 has only one related row in table 2. For example, a department may have only one head manager, a husband — only one wife, an employee — one company car, etc. One-to-one relationship example in SQL: Implementing one-to-one relationHow to create one-to-one relationship in SQL Server? For this, you will need to create two tables, then define a simple primary foreign key relationship between them, and set the foreign key column to be unique. CREATE TABLE Employee ( ID int PRIMARY KEY, Name VARCHAR(50) ); CREATE TABLE Salary ( EmployeeID int UNIQUE NOT NULL, SalaryAmount int ); ALTER TABLE Salary ADD CONSTRAINT FK_Salary_Employee FOREIGN KEY (EmployeeID) REFERENCES Employee (ID);Many-to-many relationshipA many-to-many relationship occurs when multiple records in one table are related to multiple records in another table. For example, products and suppliers: one supplier may deliver one or many products and at the same time, the company may order one product from one or many suppliers. A many-to-many relationship example in SQL: The relationship between the Product entity and Order entity is many-to-many, as one product may be in many orders and many orders may contain the same product. Example of creating many-to-many relation in SQLRelational databases don’t support direct many-to-many relationships between two tables. Then, how to implement many-to-many relationships in SQL? To create a many-to-many relationship in a database, you’ll need to create a third table to connect the other two. This new table (also known as a linking, joining, bridging, or junction table) will contain the primary key columns of the two tables you want to relate and will serve as an intermediate table between them. Let’s consider the following example of how to create many-to-many relationship in SQL. Suppose, we want to establish a many-to-many relationship between two tables: films and category. First, we create the two tables. CREATE TABLE films ( film_id INT PRIMARY KEY ,title VARCHAR(50) ,director VARCHAR(50) ,year_released DATETIME ); CREATE TABLE category ( category_id INT PRIMARY KEY ,name VARCHAR(50) );Next, we create a junction table film_category that will map these two tables together by referencing the primary keys of both tables. CREATE TABLE film_category ( film_id INT ,category_id INT ,CONSTRAINT film_cat_pk PRIMARY KEY (film_id, category_id) ,CONSTRAINT FK_film FOREIGN KEY (film_id) REFERENCES films (film_id) ,CONSTRAINT FK_category FOREIGN KEY (category_id) REFERENCES category (category_id) );A many-to-many relationship between the films and category tables has been successfully established. Many-to-one relationshipMany experts don’t separate a many-to-one relationship as a class of its own as there is not much difference between one-to-many and many-to-one relationships. It’s just a matter of focus. For example, if one school class can consist of several pupils then, class to pupil is a one-to-many relationship (one class consists of many pupils), while pupil to class relationship is many-to-one (many pupils study in one class). Example of many-to-one relationship in SQL: Self-referencing relationshipsA self-referencing relationship (also known as a recursive relationship) in a database occurs when a column in a table relates to another column in the same table. In such a relationship, only one table is involved. For example, the Staff table contains information about company employees and their managers, however, managers themselves belong to staff too. A self-referencing relationship example in SQL: How to create a self-referencing relationship in a SQL Server databaseTo establish a self-referencing relationship on a table, we create a table, define a primary key, and then add a foreign key referencing that primary key. To learn more about establishing relationships in a database, please refer to our SQL Database Design Basics with Examples article. Create table relationships in SQL Server using SQL DesignerThe methods to define relationships in a SQL Server database described above are not so easy and straightforward. They involve a bit of coding and demand a certain level of SQL expertise. Is there an easier way to create relationships? Let’s have a look at the method to implement relations visually—using the SQL Designer that comes with dbForge Studio for SQL Server. Database Designer is a smart data modeling utility that allows you to visually build new databases and quickly analyze the structures of the existing ones. Thus, with dbForge Studio for SQL Server, you can effortlessly design your databases at both logical and physical levels and benefit from these capabilities:
How to view table relationships in SQL ServerUnderstanding and evaluating table relationships in a database are important at every stage of database development from design to maintenance. dbForge Studio for SQL Server allows visualizing and tracking relations between tables in a few clicks. With Virtual Relations Manager you can create and edit virtual relations between tables and then convert them into foreign keys. ConclusionThe article discusses the different types of entity relationships in a database: one-to-one, one-to-many, many-to-one, many-to-many, and self-referencing relationships. Managing database relations with bare hands can be a mind-blowing task. That’s where dbForge Studio for SQL Server hits the stage delivering the mighty Database Diagram tool that effectively handles even the most complex database relationships. Download a free 30-day trial of dbForge Studio for SQL Server right now and enjoy the Database Designer functionality along with many other robust features of the best SQL Server GUI tool you can find!
Database Designer, database diagram, relationships in SQL, SQL table relationships, types of relationship in database When creating a table What are the two types of relationships you can create?Relationship Types
There are three types of relationships that can exist between two tables: one-to-one. one-to-many. many-to-many.
How can you add a table to the relationship window?Create a table relationship by using the Relationships window. On the Database Tools tab, in the Relationships group, click Relationships. On the Design tab, in the Relationships group, click Add Tables (or Show Table in Access 2013). Select one or more tables or queries and then click Add.
Which type of form shows only one record at a time?Continuous Form shows only one record.
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