How To Open Up The Entity Relationship Diagram In Access | Access All In One
Note: This article doesn't apply to Access web apps – the kind of database you . The box in the lower portion of the form diagram represents the subform. To create a relationship in Access or While viewing a table in Design view, and ensuring that the DESIGN tab is selected, click. MS Access Lab 3. Topic: Relationships in ER Diagram and Relationships in MS Access from five tables: Fig. 1: A Form Using Information from Five Tables.
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. A one-to-one relationship 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. Top of Page Why create table relationships? 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.
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.
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.
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.
Relationship Report with extended field information
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. The Default Value is less important than other properties, but we include it here as there is a bug in Access where it will wrongly assigns the Default Value to a lookup table field in a new record in a multi-table query, causing the insert for the main table to fail.
- Create a relationship
- Guide to table relationships
- Create a form that contains a subform (a one-to-many form)
How it works You can use the code without having to understand how it works. Read this section in conjunction with the VBA code. The RowSource of each list box is a value list - just a concatenated list of field names. This utility triggers the built-in relationships report, switches it to design view, and replaces the RowSource string with a more detailed list. The main entry point is the RelReport function. It calls OpenRelReport to get Access to create the relationships report.
In Access and later, this is just a matter of opening the relationships window, asking it to print the relationships and then switching to design view with just these three lines: As always, SendKeys is a kludge, and it does not work unless the Relationships window has focus.
If it was already open at the time our code called RunCommand acCmdRelationships, the already open window does not take focus, and we receive error The error handling branches for Accessand asks the user to close the Relationships window so that it will have focus when this routine opens it. That's the reason you cannot use the toolbar button on the Relationships toolbar in the old version. OpenRelReport then returns the name of the last report that was opened, i.
Back in our main function, we can now loop through the controls on that report, and find the list boxes. The attached label of the list box has the name of the table in its Caption.
The attached label is Controls 0so we will set a TableDef to the name in that caption, and we can get all the field information we need. Unfortunately, this fails if the name of the table in the Relationships window is an alias.
This happens when you have multiple copies of a table in the window - for example so you can join a table to itself, or create multiple joins between a pair of tables.
Create a form that contains a subform (a one-to-many form) - Access
If we strike an alias, we are happy to just leave it displaying the default information, since there is probably another copy of the table that will have our extended details anyway.
Since we anticipate errors on setting the TableDef, we call a separate routine to run that line. We pass TdfSetOk the current database more efficient than reopening it every callan uninitialized TableDef variable so the subroutine can set it and pass it back, the list box so it can get the caption from the attached label, which could theoretically error as well if it had no attached labeland a string to append any error message to.
If the function succeeds, it returns True, so our main routine tests that before operating on the returned TableDef variable.
Back in the main routine, we now pass the initialized TableDef variable to DescribeFields. This function will return a string of delimited values, suitable to use as the RowSource of the list box.