Create a relationship - Access
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 . Objectives of database design. 1. Process of database design. 1. Creating a New Calculated Field Properties (Access only) . relationships between your tables so that Access can bring related . capture below shows a table with two fields selected. .. The most common relationship is a One-to-Many relationship. Learn how to create a query in Access that uses multiple tables as its data source . Access for Office Access Access Access Access Access If you do not find a relationship between the two tables, create one by This will often be the case when you are creating a one-to-many relationship.
There are several reasons why you should create table relationships before you create other database objects, such as forms, queries and reports. Table relationships inform your query designs 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.
Table relationships inform your form and report designs 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. 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. Top of Page Understanding referential integrity 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.
Create a query based on multiple tables
To bring the data back together, Access takes the value in the "many" table and looks up the corresponding value in the "one" table. In this way the values in the "many" table reference the corresponding values in the "one" table.
Suppose you have a one-to-many relationship between Shippers and Orders and you want to delete a Shipper. If the shipper you want to delete has orders in the Orders table, those orders will become "orphans" when you delete the Shipper record.
The orders will still contain a shipper ID, but the ID will no longer be valid, because the record that it references no longer exists. The purpose of referential integrity is to prevent orphans and keep references in sync so that this hypothetical situation never occurs. You enforce referential integrity by enabling it for a table relationship see Enforce referential integrity for step-by-step instructions. Once enforced, Access rejects any operation that violates referential integrity for that table relationship.
This means Access will reject both updates that change the target of a reference, and deletions that remove the target of a reference. For such cases, what you really need is for Access to automatically update all the effected rows as part of a single operation.
That way, Access ensures that the update is completed in full so that your database is not left in an inconsistent state, with some rows updated and some not.
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.
When you enforce referential integrity and choose the Cascade Delete Related Records option, and you then delete a record on the primary key side of the relationship, Access automatically deletes all records that reference the primary key. The Relationships window opens and displays any existing relationships.
If no table relationships have been defined and you are opening the Relationships window for the first time, Access prompts you to add a table or query to the window.
Open the Relationships window Click File, and then click Open. Select and open the database. On the Database Tools tab, in the Relationships group, click Relationships. If the database contains relationships, the Relationships window appears. If the database does not contain any relationships and you are opening the Relationships window for the first time, the Show Table dialog box appears. Click Close to close the dialog box. On the Design tab, in the Relationships group, click All Relationships.
This displays all of the defined relationships in your database. Note that hidden tables tables for which the Hidden check box in the table's Properties dialog box is selected and their relationships will not be shown unless the Show Hidden Objects check box is selected in the Navigation Options dialog box.
A table relationship is represented by a relationship line drawn between tables in the Relationships window. A relationship that does not enforce referential integrity appears as a thin line between the common fields supporting the relationship. When you select the relationship by clicking its line, the line thickens to indicate it is selected.
If you enforce referential integrity for this relationship, the line appears thicker at each end. When the Relationships window is active, you can select from the following commands on the ribbon: On the Design tab, in the Tools group: When you select a relationship line, you can click Edit Relationships to change the table relationship.
Create a relationship
You can also double-click the relationship line. The report shows only the tables and relationships that are not hidden in the Relationships window. On the Design tab, in the Relationships group: Note that hidden tables tables for which the Hidden check box in the table's Properties dialog box is selected and their relationships will not be shown unless Show Hidden Objects is selected in the Navigation Options dialog box.
If you made any changes to the layout of the Relationships window, you are asked whether to save those changes. Top of Page Create a table relationship You can create a table relationship by using the Relationships window, or by dragging a field onto a datasheet from the Field List pane.
When you create a relationship between tables, the common fields are not required to have the same names, although it is often the case that they do. Rather, those fields must have the same data type. If the primary key field is an AutoNumber field, however, the foreign key field can be a Number field if the FieldSize property of both fields is the same.
When both common fields are Number fields, they must have the same FieldSize property setting. Create a table relationship by using the Relationships window Click File, and then click Open.
If you have not yet defined any relationships, the Show Table dialog box automatically appears. If it does not appear, on the Design tab, in the Relationships group, click Show Table. The Show Table dialog box displays all of the tables and queries in the database.
To see only tables, click Tables. To see only queries, click Queries. To see both tables and queries, click Both. Select one or more tables or queries and then click Add. When you have finished adding tables and queries to the Relationships window, click Close. Drag a field typically the primary key from one table to the common field the foreign key in the other table.
To drag multiple fields, press the CTRL key, click each field, and then drag them. The Edit Relationships dialog box appears.
Verify that the field names shown are the common fields for the relationship. If a field name is incorrect, click the field name and select a new field from the list. To enforce referential integrity for this relationship, select the Enforce Referential Integrity check box. For more information about referential integrity, see the Understanding Referential Integrity and the Enforce Referential Integrity sections.
The relationship line is drawn between the two tables. If you selected the Enforce Referential Integrity check box, the line appears thicker at each end. This means the Indexed property for these fields should be set to Yes No Duplicates. If both fields have a unique index, Access creates a one-to-one relationship. This means 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.
When one field has a unique index and the other does not, Access creates a one-to-many relationship. Create a table relationship by using the Field List pane You can add a field to an existing table that is open in Datasheet view by dragging it from the Field List pane. The Field List pane shows fields available in related tables and also fields available in other tables.
- Guide to table relationships
When you drag a field from an "other" unrelated table and then complete the Lookup Wizard, a new one-to-many relationship is automatically created between the table in the Field List pane and the table to which you dragged the field. This relationship, created by Access, does not enforce referential integrity by default.
To enforce referential integrity, you must edit the relationship. You do this by creating a third table, called a junction table or a relationship table, that has a primary key and a foreign key for each of the other tables.
A one-to-many relationship is then created between each foreign key in the junction table and the corresponding primary key of one of the other tables. In such cases, you need to include all three tables in your query, even if you want to retrieve data from only two of them.
Build a select query by using tables with a many-to-many relationship On the Create tab, in the Queries group, click Query Design. The Show Table dialog box opens. In the Show Table dialog box, double-click the two tables that contain the data you want to include in your query and also the junction table that links them, and then click Close.
All three tables appear in the query design workspace, joined on the appropriate fields. Double-click each of the fields that you want to use in your query results. Each field then appears in the query design grid.
In the query design grid, use the Criteria row to enter field criteria. To use a field criterion without displaying the field in the query results, clear the check box in the Show row for that field. To sort the results based on the values in a field, in the query design grid, click Ascending or Descending depending on which way you want to sort the records in the Sort row for that field. On the Design tab, in the Results group, click Run. Access displays the query output in Datasheet view.
An example that uses the Northwind sample database Note: You may want to make a backup copy of the Northwind sample database, and then follow this example by using the backup copy. Suppose you have a new opportunity: They supply every category of food product that you broker. They are a fairly big business, and want your assurances that you can provide them access to enough potential sales to make it worthwhile: Can you provide them with the market they require?
The data you need to answer this question is found in two places: These tables are linked to each other by an Orders table. Relationships between the tables have already been defined. In the Orders table, each order can have only one customer, related to the Customers table on the CustomerID field. Thus, a given customer can have many orders, each of which has many order details.
Build the query in Design view Open the Northwind database. On the Create tab, in the Queries group, click Query Design. All three tables appear in the query design workspace. In the Customers table, double-click the City field to add it to the query design grid. This causes only those records where the customer is in one of these two cities to be included in the query.
The fields are added to the query design grid. In the ShippedDate column in the query design grid, select the Field row.
Replace [ShippedDate] with Year: This creates a field alias, Year, that allows you to use just the year portion of the value in the ShippedDate field. In the UnitPrice column in the query design grid, select the Field row. Replace [UnitPrice] with Sales: This creates a field alias, Sales, that calculates the sales for each record. On the Design tab, in the Query Type group, click Crosstab. Two new rows, Total and Crosstab, appear in the query design grid.
In the City column in the query design grid, click the Crosstab row, and then click Row Heading. This makes city values appear as the row headings that is, the query returns one row for each city.
In the Year column, click the Crosstab row, and then click Column Heading. This makes year values appear as the column headings that is, the query returns one column for each year. In the Sales column, click the Crosstab row, and then click Value. This makes sales values appear at the intersection of rows and column that is, the query returns one sales value for each combination of city and year.
Guide to table relationships - Access
In the Sales column, click the Totals row, and then click Sum. This causes the query to sum the values in this column. You can leave the Totals row for the other two columns at the default value of Group By, because you want to see each value for these columns, not aggregate values. Top of Page View all the records from two similar tables Sometimes, you will want to combine data from two tables that are identical in structure, but one of them is located in another database.
Consider the following scenario. Suppose you are an analyst working with student data. You are embarking on a data sharing initiative between your school and another school, so that both schools can improve their curricula. For some of the questions you want to explore, it would be better to look at all records from both schools together, rather than each school's records separately. You could import the other school's data into new tables in your database, but then any changes to the other school's data would not be reflected in your database.
A better solution would be to link to the other school's tables, and then create queries that combined the data when you run them. You would be able to analyze the data as a single set, rather than performing two analyses and trying to interpret them as if they were one.
To view all the records from two tables with identical structure, you use a union query.