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Guide to table relationships
When you define a relationship in Access, you relate data from one table to another. By doing this, you are linking your tables together so you can perform queries and extract specific data from multiple tables all at once. Access gives you three types of table relationships to use. Let's learn about these relationship types. Sometimes in Access, you'll need to view information from two or more tables from different database tables.
For this, you will need to create what's called a join. A join does as the name implies. It temporarily joins two tables together. Whenever you run a query to find data in tables that are related, Access will look for records on both sides of the relationship that have matching values. By learning what type of query join to use, you can determine what records will be displayed as the results.
When you do this, you can find the exact data that you need without sorting through data that you won't. Let's delve a little deeper so that this makes sense to you. An inner join is defined as a join that only selects records from tables that have matching values. You can select one or more fields to serve as the join fields. This is the default join type in Access.
You don't even need to do anything most of the time to create an inner join if relationships are already established. Access creates an inner join between related tables when you add them. When you create primary keys and foreign keys, have the same field in more than one table, Access creates that relationship for you and creates the inner join.
You can create an inner join by dragging a field from one table to another in query design view. There will be a line displayed to show that a join has been created. An outer join informs a query that the query needs to include all rows from one table, then rows from the other table too that share a value on both sides of the join. That said, outer joins can be left or right outer joins. Left outer joins include all rows from the first table, then rows from the other table that contains values that are the same in both tables.
A right join is vice versa with all rows from the second table included and only those rows with matching values included from the first table. You can click any join to see the Join Properties dialog box to see if it was a right or left join. Double-click on the line that illustrates the relationship in Design View. You can see the line below. Now that we have a join set up with two tables, we can create a query on these two tables.
This is often done to get data from more than one table and display it in one place. If you go to Design view, you can see the fields listed, as well as the two tables and the relationship between them that was created. Normalization sounds more difficult than it really is. It simply refers to the process of organizing data in your database efficiently.
Normalization has two goals: to eliminate redundant data storing the same data in more than one table and ensuring data dependencies make sense storing only related data in a table. By doing this, you reduce the amount of space a database takes up, and you insure that your data is logically stored. It's good to know this as it will help you understand why we create multiple tables and why we structure them in a certain way.
With that said, there's a series of guidelines that you follow to make sure your databases are normalized. These guidelines are called normal forms. They're numbered from one the lowest form that's called first normal form or 1NF through five fifth normal form of 5NF.
Sometimes you'll see a 4NF. However, 5NF's are rare. They're so rare, we're not even going to discuss them. Just remember that these are only guidelines. They're not requirements. You can use different variations to meet your own requirements. Reminder: The primary key of a relational table gives a unique identification to each record in the table. It is a normal attribute that is known to be unique, such as a social security number or account number.
You will not be allowed to enter duplicate primary keys within a database table. Reminder: A foreign key is a field in a relational table that is a match for a primary key of another table. It can be used to cross reference tables. For example, perhaps the customer's account number is used at the primary key in Table 1. Perhaps their phone number is used as the primary key in Table 2, but their account number is also used in that table.
It is a foreign key. A candidate key is a column or set of columns in a table that identify the records rows without referring to any other data. It means it's a "candidate" to become your primary key, so it's a possibility.
One of the candidate keys becomes your primary key. In a table, a determinant is an attribute that determines the values assigned to other attributes in the same row. For example, if you have the attributes employee ID, first name, last name, and job title, the attribute Employee ID would determine the values assigned to the other attributes. Your determinant may be your primary key. A multi-valued dependency happens when one or more rows in a table imply the presence of one more other rows in the same table.
If that doesn't make sense, think of it this way: if our example car dealership has a table that contains model name, color, and year of each car, there is a multi-valued dependency if you have a model name in blue, then another in red.
The record row of the blue car implies the presence of other rows that contain red cars, etc. You may or may not want to complete the fourth normalization form. It depends on the data that you want to store. Expressions in Access are used to do mathematical calculations, combine or extract text, or validate the data in your database.
Expressions use all or some of the following elements: functions, identifiers, operators, and constants. The plus and minus sign are operators, for example. However, before we move forward, let's talk briefly about the definitions of functions, identifiers, and constants before continuing. It's important that you understand what everything is before you begin to use it.
In this expression, Sum is the function, [Retail Price] is the identifier, the asterisk is the operator, and. A calculated field is defined as a column that contains an expression. Whenever you enter a record into the table, Access will use the expression to calculate what data will appear in that field.
To create a calculated field, open a table in your database. We've opened Books, as you can see in the snapshot below. Let's add a new field. We'll make it a calculated field. Click the arrow in the Click to Add field, then choose Calculated field from the dropdown menu, as shown below. Now you can choose what type of calculated field you want to add. Since your table contains a lot of text and not many numbers, we've chosen text. The Expression Builder then opens up. You can use the Expression Builder to create calculated fields or expressions for records.
Below you'll see a snapshot of the Expression Builder that we opened in the last section of this lesson. Let's learn how to use it in creating a calculated field, although you can use the Expression Builder for any expression you want to create anywhere in table, query, or other area. The upper section of the Expression Builder is called the Expression box.
This is where you put together your expression. The three columns below the Expression box are where you can go to construct the expression. If you want, you can type some or all of your expression directly into the Expression Box. If you go to the first of the three columns below, you can choose an expression element. If you'd like to see the month value for the publication date, you'd first expand the Functions group. Select a category, then select the value. We've selected Month.
Type in [Publication Date]. Access may suggest the field for you as you're typing. As we stated earlier, a function is a procedure that's used to determine a value. The concept is simple, but learning all the functions can be overwhelming and almost impossible.
Unless you're a programmer or familiar with programming language, using the Expression Box is the quickest and simplest way to do functions in Access In addition, MS Access explains what each function is at the bottom of the Expression Builder window. To see all functions for MS Access and what they do, as well as use them in your expressions, click on Functions in the first bottom column, then Built In Functions.
Select the field where you want to enter a calculated expression and select Properties. You'll see this window on the right hand side:.
Creating Relationships in Microsoft Access
It temporarily joins two tables together. Whenever you run a query to find data in tables that are related, Access will look for records on both sides of the relationship that have matching values. By learning what type of query join to use, you can determine what records will be displayed as the results. When you do this, you can find the exact data that you need without sorting through data that you won't. Let's delve a little deeper so that this makes sense to you.
An inner join is defined as a join that only selects records from tables that have matching values. You can select one or more fields to serve as the join fields. This is the default join type in Access. You don't even need to do anything most of the time to create an inner join if relationships are already established.
Access creates an inner join between related tables when you add them. When you create primary keys and foreign keys, have the same field in more than one table, Access creates that relationship for you and creates the inner join. You can create an inner join by dragging a field from one table to another in query design view.
There will be a line displayed to show that a join has been created. An outer join informs a query that the query needs to include all rows from one table, then rows from the other table too that share a value on both sides of the join. That said, outer joins can be left or right outer joins. Left outer joins include all rows from the first table, then rows from the other table that contains values that are the same in both tables.
A right join is vice versa with all rows from the second table included and only those rows with matching values included from the first table. You can click any join to see the Join Properties dialog box to see if it was a right or left join.
Double-click on the line that illustrates the relationship in Design View. You can see the line below. Now that we have a join set up with two tables, we can create a query on these two tables. This is often done to get data from more than one table and display it in one place.
If you go to Design view, you can see the fields listed, as well as the two tables and the relationship between them that was created. Normalization sounds more difficult than it really is. It simply refers to the process of organizing data in your database efficiently.
Normalization has two goals: to eliminate redundant data storing the same data in more than one table and ensuring data dependencies make sense storing only related data in a table. By doing this, you reduce the amount of space a database takes up, and you insure that your data is logically stored. It's good to know this as it will help you understand why we create multiple tables and why we structure them in a certain way.
With that said, there's a series of guidelines that you follow to make sure your databases are normalized. These guidelines are called normal forms. They're numbered from one the lowest form that's called first normal form or 1NF through five fifth normal form of 5NF.
Sometimes you'll see a 4NF. However, 5NF's are rare. They're so rare, we're not even going to discuss them. Just remember that these are only guidelines. They're not requirements. You can use different variations to meet your own requirements. Reminder: The primary key of a relational table gives a unique identification to each record in the table. It is a normal attribute that is known to be unique, such as a social security number or account number. Many web browsers, such as Internet Explorer 9, include a download manager.
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Download Microsoft Access Runtime from Official Microsoft Download Center Microsoft Premium Office apps, extra cloud storage, advanced security, and more—all in one convenient subscription For up to 6 people For 1 person. Microsoft Access Runtime. Select Language:. This relationship is called a many-to-many relationship. Note that to detect existing many-to-many relationships between your tables, it is important that you consider both sides of the relationship.
To represent a many-to-many relationship, you must create a third table, often called a junction table, that breaks down the many-to-many relationship into two one-to-many relationships. You insert the primary key from each of the two tables into the third table. As a result, the third table records each occurrence, or instance, of the relationship.
For example, the Orders table and the Products table have a many-to-many relationship that is defined by creating two one-to-many relationships to the Order Details table. One order can have many products, and each product can appear on many orders. In a one-to-one relationship, each record in the first table can have only one matching record in the second table, and each record in the second table can have only one matching record in the first table.
This relationship is not common because, most often, the information related in this way is stored in the same table.
You might use a one-to-one relationship to divide a table with many fields, to isolate part of a table for security reasons, or to store information that applies only to a subset of the main table. When you do identify such a relationship, both tables must share a common field.
You can create table relationships explicitly by using the Relationships window, or by dragging a field from the Field List pane. Access uses table relationships to decide how to join tables when you need to use them in a database object. There are several reasons why you should create table relationships before you create other database objects, such as forms, queries and reports.
To work with records from more than one table, you often must create a query that joins the tables. The query works by matching the values in the primary key field of the first table with a foreign key field in the second table.
For example, to return rows that list all of the orders for each customer, you construct a query that joins the Customers table with the Orders table based on the Customer ID field. In the Relationships window, you can manually specify the fields to join. But, if you already have a relationship defined between the tables, Access supplies the default join, based on the existing table relationship. In addition, if you use one of the query wizards, Access uses the information it gathers from the table relationships you have already defined to present you with informed choices and to prepopulate property settings with appropriate default values.
When you design a form or report, Access uses the information it gathers from the table relationships you have already defined to present you with informed choices and to prepopulate property settings with appropriate default values.
Table relationships are the foundation upon which you can enforce referential integrity to help prevent orphan records in your database. An orphan record is a record with a reference to another record that does not exist — for example, an order record that references a customer record that does not exist. When you design a database, you divide your information into tables, each of which has a primary key. You then add foreign keys to related tables that reference those primary keys.
These foreign key-primary key pairings form the basis for table relationships and multi-table queries. Referential integrity, which is dependent on table relationships, helps ensure that references stay synchronized. When you design a database, you divide your database information into many subject-based tables to minimize data redundancy.
You then give Access a way to bring the data back together by placing common fields into related tables. For example, to represent a one-to-many relationship you take the primary key from the "one" table and add it as an additional field to the "many" table. Because matching and bringing together data from more than one table is something you will do frequently in all but the most simple databases, setting defaults by creating relationships can be time saving and beneficial.
A multiple table query combines information from more than one table by matching the values in common fields. The operation that does the matching and combining is called a join. For instance, suppose you want to display customer orders. The query result contains customer information and order information for only those rows where a corresponding match was found. One of the values you can specify for each relationship is the join type.
The join type tells Access which records to include in a query result. For example, consider again a query that joins the Customers table and the Orders table on the common fields that represents the Customer ID. Using the default join type called an inner join , the query returns only the Customer rows and the Order rows where the common fields also called the joined fields are equal. However, suppose you want to include all Customers — even those who have not yet placed any orders.
To accomplish this, you have to change the join type from an inner join to what is known as a left outer join. A left outer join returns all of the rows in the table on the left side of the relationship and only those that match in the table on the right. A right outer join returns all of the rows on the right and only those that match on the left.
Note: In this case, "left" and "right" refer to the position of the tables in the Edit Relationships dialog box, not the Relationships window. You should think about the result you will most often want from a query that joins the tables in this relationship, and then set the join type accordingly.
The following table using the Customers and Orders tables shows the three choices that are displayed in the Join Properties dialog box, the type of join they use, and whether all rows or matching rows are returned for each table.
Include ALL records from 'Customers' and only those records from 'Orders' where the joined fields are equal. Include ALL records from 'Orders' and only those records from 'Customers' where the joined fields are equal.
When you choose option 2 or option 3, an arrow is shown on the relationship line. This arrow points to the side of the relationship that shows only matching rows. All tables that have relationships are displayed, showing relationship lines. Double-click the relationship line.
In the Join Properties dialog box, click an option, and then click OK. The purpose of referential integrity is to prevent orphan records — records that reference other records that no longer exist.
You enforce referential integrity by enabling it for a table relationship. Once enforced, Access rejects any operation that would violate referential integrity for that table relationship.
This means that Access will reject both updates that change the target of a reference and deletions that remove the target of a reference. To have Access propagate referential updates and deletions so that all related rows are changed accordingly, see the Set the cascade options section. You cannot enter a value in the foreign key field of a related table if that value doesn't exist in the primary key field of the primary table — doing so creates orphan records.
You cannot delete a record from a primary table if matching records exist in a related table. For example, you cannot delete an employee record from the Employees table if there are orders assigned to that employee in the Orders table. You can, however, choose to delete a primary record and all related records in one operation by selecting the Cascade Delete Related Records check box. You cannot change a primary key value in the primary table if doing so would create orphan records.
For example, you cannot change an order number in the Orders table if there are line items assigned to that order in the Order Details table. You can, however, choose to update a primary record and all related records in one operation by selecting the Cascade Update Related Fields check box. Notes: If you have difficulty enabling referential integrity, note that the following conditions are required to enforce referential integrity:. Both tables exist in the same Access database. Referential integrity cannot be enforced on linked tables.
However, if the source tables are in Access format, you can open the database in which they are stored and enable referential integrity in that database. You might encounter a situation where you have a valid need to change the value on the "one" side of a relationship.
In such a case, you need Access to automatically update all of the affected rows as part of a single operation. That way the update is completed in full so that your database is not left in an inconsistent state — with some rows updated and some not.
Access helps you avoid this problem by supporting the Cascade Update Related Fields option. When you enforce referential integrity and choose the Cascade Update Related Fields option, and you then update a primary key, Access automatically updates all fields that reference the primary key.
You might also need to delete a row and all related records — for instance, a Shipper record and all related orders for that shipper. When you enforce referential integrity and select the Cascade Delete Related Records check box, Access automatically deletes all records that reference the primary key when you delete the record that contains the primary key.
Note: If the primary key is an AutoNumber field, selecting the Cascade Update Related Fields check box will have no effect, because you cannot change the value in an AutoNumber field. Important: When you remove a relationship, you also remove referential integrity support for that relationship, if it is enabled. As a result, Access will no longer automatically prevent the creation of orphan records on the "many" side of a relationship.
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