A many-to-many relationships in a relational database exists when one record in table A can be referenced by one or more records in table B and one record in table B can be referenced by one or more records in table A. In this tutorial you will learn how to create a many-to-many relationship in Access 2010.
- How To Create Foreign Key In Ms Access Code
- Create Foreign Key Mysql
- How To Create Foreign Key In Ms Access Code
A many-to-many relationship consists of three tables. Below is a schematic view of the many-to-many relationship.
Schematic representation of a many-to-many relationship.The table in the middle is called a
Create a many-to-many relationship in Access
I will illustrate the creation of a many-to-many relationship with an example, including a Product table, an Order table and a junction table called Order2Product. To create a many-to-many relationship in Access follow these steps.
Create the two tables you want to link in a many-to-many relationship
In this example I created the Order table and the Product table below. Both tables have an Autonumber primary key field. These primary key fields will later be used to create a many-to-many relationship between these tables. If you want to know how to create a table in design view, have a look at this article: how to create a table in Access 2010.
The product table
Creating the junction table
In this case there are two ways of linked products to orders. We can create a junction table with an surrogate primary key field, like the ID Autonumber field Access automatically provides.
We can also create our own composite primary key that consists of the two foreign key field in the junction table. I will show you both ways and discuss the implications of each solution.
How To Create Foreign Key In Ms Access Code
Create the junction table using a surrogate key
The junction table with surrogate primary key will have three fields, including the ID primary key field that Access automatically creates for you. The two non-primary key fields are foreign keys. They will reference the primary keys of the Order and Product tables (or perhaps some other tables in your case).
- Create a new table and name it something like LeftTable2RightTable. In my example I named it Order2Product. This notation is often chosen for junction tables.
- Go to the design view of your new table (Read how to create a table in Access 2010 if you need more info on creating tables in design view).
- Leave the automatically supplied ID primary key field in tact. Add two new fields. In my example I created order and product fields. The order field will reference the primary key of the Order table. The product field will reference the primary key of the Product table.
- Make sure your fields are of the same data type as the primary key fields they will reference.
The example below is a finished junction table with some data. The order and product fields are both of the Number type, because they both refer to an Autonumber primary key field, which is in fact a Number field that is managed by the database.
The junction table has a primary key ID field and two foreign key fields. Each foreign key field references one side of the many-to-many relationship.
Implications
Now, let's look at the implications of this solution. Each record in the junction table ties a product to an order. Order 1 consists of just one product (4). Order 2 consists of 4 products, or does it? There are indeed 4 products in the order. That is, the quantity is 4. But, there are only 2 unique products in the order, because product 5 (the Nirvana Nevermind album) was added 3 times.
That is the implication of this solution: when a product is added to an order twice, this creates two records in the Order2Product table. This is fine. It is just good to be aware of this implication, because there are other solutions.
Often junction tables are created with a primary key that consists of the wor foreign keys (product and order). Let's look at a junction table that uses a composite primary key and a quantity field.
Create the junction table using a composite primary key
Let's create a junction table with a composite primary key in order to accomplish a many-to-many relationship.
- Create a new table and call it LeftTable2RightTable, replacing LeftTable and RightTable with the names of your tables.
- Go to the design view of your new table.
- Remove the ID Autonumber primary key field that Access automaticcaly provides by selecting it and clicking the Delete rows button on the ribbon.
- Add two new fields to reference the tables on each end of the many-to-many relationship.
- Make sure each of your two fields have the same data type of the primary key fields they will reference.
- Now, add a primary key including both these fields by following the steps below.
- Select both fields by placing the mouse pointer over the gray vertical bar on the left of the field definitions and dragging down, while holding the left mouse button down.
- While the fields still selected click the Primary key button on the ribbon.
This will create a primary key that consists of the two selected fields.
- Select both fields by placing the mouse pointer over the gray vertical bar on the left of the field definitions and dragging down, while holding the left mouse button down.
Now you should have a junction table that is made of two fields that are both included in the primary key, like the one below.
Implications
There is an important implication of this solution. As you may know, primary keys must be unique accross all rows in a table. This means that it is now impossible to add the same product to an order twice. We can't have a (1,2) record twice for example, so we can only add product 2 to order 1 once with this solution.
The solution to this problem (or shall we call it a feature?) is to add a quantity field to the junction table, like this.
Create Foreign Key Mysql
Now, when you want to add product 2 to order 1 twice, you just set the quantity field to 2.
This is how you would add multiple product to an order with the quantity field solution.
Create the actual relationships
The final steps in creating the many-to-many relationship is creating the actual relationships in the Relationships view. Creating a many-to-many relationship is a matter of creating two one-to-many relationships. Here is how you do it.
- Open the Relationship view by going to the Database Tools tab on the ribbon and selecting the Relationships button.
- In the Relationships screen, add the three tables you want to create the many-to-many relationship with to the screen. You can do this by dragging tables from the left onto the screen or by using the Show Table window, which is accessible via the Show Table button on the ribbon.
- When your tables are on the Relationship screen drag the primary key fields of the entity tables (Order and Product in this example) to their counterparts in the junction table.
- When you drag and drop a primary key field onto a field in another table, Access knows you want to create a relationship and it brings up the Edit Relationships window. This is where you create the actual relationship. In order to create a many-to-many relationship you must create two one-to-many relationships.
- Create a one-to-many relationship between both your entity tables (Product and Order in this example) and the junction table. Select all three integrity options (Note: the integrity options are discussed in the article Create a one-to-many relationship in Access).
How To Create Foreign Key In Ms Access Code
When both one-to-many relationships are created you have really created a many-to-many relationship, because a many-to-many relationship is made of two one-to-many relationships. The end result should look like this:
About this tutorial:
Video duration: 5:17
This is part 6 of a multiple part series of the Access 2016 tutorial. This session focuses on concepts related to creating tables by importing data from Excel and .csv files and understanding the primary and foreign keys. Learn more at
This is part 6 of a multiple part series of the Access 2016 tutorial. This session focuses on concepts related to creating tables by importing data from Excel and .csv files and understanding the primary and foreign keys. Learn more at
In more detail, this session we will focus on:
– How to create a table by importing the data from a .csv file in Access 2016
– Learn about setting the primary key in a table in Access 2016.
– Learn the concepts related to the foreign keys keys in a table.
– How to create a table by importing the data from a .csv file in Access 2016
– Learn about setting the primary key in a table in Access 2016.
– Learn the concepts related to the foreign keys keys in a table.
If you like the video, please “Like,” “Favorite,” and “Share” it with our…
MS-Access / Getting StartedThe PRIMARY KEY constraint is used to uniquely identify every record in a table. The specification of a primary keyensures that there are no duplicate values in a column. Additionally, primary key fields are stored in ascending order and default to NOT NULL.
In the Create Toys script, the ToyID column contains a PRIMARY KEY constraint. The CONSTRAINT and PRIMARYKEY keywords are used to define the primary key. The name of the constraint (ToyPk) follows the CONSTRAINT keyword.Primary keys can also be defined using only the PRIMARY KEY keywords; however, this method does not enable you toassign a name to your primary key constraint. Assigning a name to your PRIMARY KEY constraint is vital because itmakes it easier for you to update the constraint if necessary. To view the new Toys table, type the following script:
This script uses a SELECT statement to retrieve records from a table. The SELECT keyword combined with an asterisk (*)instruct Microsoft Access to retrieve all the columns from a table. The FROM keyword instructs Microsoft Access toretrieve the records from the Toys table.
Example-C
You want to link the Toys table in Example-B to a new table named Manufacturers. Additionally, you want to ensure that allphone numbers entered into the PhoneNumber column in the Manufacturers table are unique and that all updates and deletionsmade to the Manufacturers table affect corresponding records in the Toys table. Take a look at the following script:
Note: Make sure you have created the Toys table in Example-B (in previous section) before you create the Manufacturers table since it containsthe foreign key reference to the Toy table.
The preceding SQL script creates a table named Manufacturers with nine columns (ManufacturerID, ToyID, CompanyName,Address, City, State, PostalCode, AreaCode, PhoneNumber). A PRIMARY KEY constraint is defined for the ManufacturerIDcolumn, and the NOT NULL constraint is defined for all other columns. The PhoneNumber column contains a UNIQUE constraintand the ToyID column contains a FOREIGN KEY constraint. Following is an explanation of the ON UPDATE CASCADE and ON DELETE CASCADE keywords and theUNIQUE and FOREIGN KEY constraints used in Example-C.
In this tutorial:
This is part 6 of a multiple part series of the Access 2016 tutorial. This session focuses on concepts related to creating tables by importing data from Excel and .csv files and understanding the primary and foreign keys. Learn more at http://learn.kaceli.com In more detail, this session we will focus on: – How to create a table by importing the data from a .csv file in Access 2016 – Learn about setting the primary key in a table in Access 2016. – Learn the concepts related to the foreign keys keys in a table. If you like the video, please “Like,” “Favorite,” and “Share” it with our friends to show your support on this work. Subscribe to this channel so that you do not miss out on the new videos on this series. Video URL: https://youtu.be/1jNnEdrFydE. The full tutorial is coming s
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by Luke Chung, FMS President
Most databases, whether it's Microsoft Access, SQL Server, Oracle, etc., let you create tables and specify any field or fields as the primary key. Unfortunately, this makes it easy to create primary keys that are not optimal which can cause performance problems and even worse, architectural problems that become expensive to fix later.
Every table in a relational database should be keyed. Records in a relational database can be sorted and physically stored in any order, but the key field (or fields) define uniqueness for that record. This makes it easy to link this table to data in other tables. The primary key is also important for secondary indexes.
Secondary indexes can be one or more fields that are defined to speed up searches by maintaining a pre-sorted list. For instance, a customer table may have a secondary index on Zip Code to allow quicker searches on zip code values.
Here are some tips for creating primary keys. As with all rules, there are specific situations where one may deviate from them. Additionally, if your tables are small, adjusting the primary key may have no perceivable impact, so these tips may not matter. However, for larger tables and to support scalability over time, these tips can have a huge impact. The hope is to know these reasons and why you may intentionally deviate from these best practices.
Primary Key Fields Come First
The primary key should be the first field (or fields) in your table design. While most databases allow you to define primary keys on any field in your table, the common convention and what the next developer will expect, is the primary key field(s) coming first.
Use a Meaningless Primary Key
The values in a table's primary key should never change over time. For instance, a numeric customer ID can point to a customer, and regardless of whether they change their name, address, phone number, etc., we can track the data properly without updating more than one table.
If a primary key value has meaning, it can change which requires updating the tables that reference this value. That may be automatic through cascading referential integrity, but it adds overhead, slows performance, and may create locking conflicts as the data is updated.
Therefore, it's critical to avoid using things like people's names as primary keys. Ps3 spiele need for speed. Fields with meaning can also result in duplicates, which is a problem when the primary key is supposed to define each record uniquely.
Fortunately, it's easy to define a unique primary key. An auto generated value such as an Identity column in SQL Server or an AutoNumber field in Microsoft Access does this. It doesn't matter if you delete a record and the key values are no longer consecutive. The primary key should have no meaning other than the permanent tag to that record.
Exception
A reasonable exception to this is geographic lists such as Countries, States, and Zip Codes, where the name is keyed rather than using a separate ID. These data are relatively stable and small.
Only Use One Numeric Field as the Primary Key
Uniquely tagging a record can be done with a number (long integer). There's no need to have more than one field defining its uniqueness. Text fields require more bytes than numeric fields, so using a number saves considerable space.
Making the primary key as small as possible reduces the memory required to open the primary key when links or searches are performed.
Impact on Secondary Indexes
Primary key size also impacts secondary indexes. A secondary index contains the sorted information on the field or fields in the index, plus a reference back to the primary key. Searching on the secondary index is fast because it uses a presorted list, but still points back to the record using the primary key to retrieve all of the record's other fields.
The smaller the primary key, the smaller the secondary index, and the faster the links and searches are. If a table has lots of secondary indexes and lots of records, the difference can be very significant if you are not using a single numeric field as your primary key.
Don't use Primary Keys to Prevent Duplicate Records
Sometimes people justify composite (multi-field) primary keys to avoid duplicates. This is the wrong approach. To prevent duplicates, create a secondary index for those fields and define it as unique. That lets the secondary index handle that job while the single field primary index retains its efficiencies.
Additional Resources
Our Total Access Analyzer add-in performs detailed analysis of your MS Access database objects and detects tables with non-optimal primary keys, field type inconsistencies, plus hundreds of others tips to improve your database designs.