Table of Contents
What is a junction table SQL?
A junction table contains the primary key columns of the two tables you want to relate. You then create a relationship from the primary key columns of each of those two tables to the matching columns in the junction table. In the pubs database, the titleauthor table is a junction table.
How do junction tables work?
An associative (or junction) table maps two or more tables together by referencing the primary keys (PK) of each data table. In effect, it contains a number of foreign keys (FK), each in a many-to-one relationship from the junction table to the individual data tables.
Why do we need a junction table?
Because in order to have a book with more than one author, you will have to create a duplicate row in the Books table. In your example this is not so bad, but imagine if the Books table had 10 columns, or 20 or more.
How does a junction table handle a many to many relationship?
When you need to establish a many-to-many relationship between two or more tables, the simplest way is to use a Junction Table. A Junction table in a database, also referred to as a Bridge table or Associative Table, bridges the tables together by referencing the primary keys of each data table.
Can you create a relationship between two tables?
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.
How do I create a many-to-many table in SQL?
How do you create a one-to-many relationship?
To create a one-to-many relationship The field on the one side (typically the primary key) of the relationship must have a unique index. This means that the Indexed property for this field should be set to Yes (No Duplicates). The field on the many side should not have a unique index.
How do I create a junction table?
The junction table is created by using the primary key from the Classrooms and Students tables. We have now created a table with columns for the StudentID and the ClassroomID. This table also uses a combination of these two columns as the primary key. This means that each student-classroom pair is unique.
What is a junction table in DBMS?
A junction table is a table that enables to create a many to many relation between two tables. You create a table that contains a column for each of the primary columns of your master tables and any extra columns that are specific for that particular many to many relation.
Why does the junction table have a foreign key?
You should also note that the columns in the junction table are setup as foreign keys to the Students and Classrooms tables. This is important as it keeps us from adding students to a classroom that doesn’t exist or deleting a classroom from the database if there are still students belonging to it.