3. Data, Tables, and Database Design - Fixing Access Annoyances [Book]
1: You select [Date], and this is not a built in function in my version of access. WHERE (((Format([Date],"mm/dd/yyyy")) _ BETWEEN [BeginningDate] AND. Describes how to define relationships in a database in Access , or one-to- one relationship; How to define a many-to-many relationship. When you include multiple data sources in an Access query, you use joins to limit is already a relationship between two data sources that you use in a query.
One-to-many relationships A one-to-many relationship is the most common kind of relationship. In this kind of relationship, a row in table A can have many matching rows in table B.
But a row in table B can have only one matching row in table A. For example, the "Publishers" and "Titles" tables have a one-to-many relationship.
That is, each publisher produces many titles. But each title comes from only one publisher. A one-to-many relationship is created if only one of the related columns is a primary key or has a unique constraint. In the relationship window in Access, the primary key side of a one-to-many relationship is denoted by a number 1.
The foreign key side of a relationship is denoted by an infinity symbol.
Many-to-many relationships In a many-to-many relationship, a row in table A can have many matching rows in table B, and vice versa. You create such a relationship by defining a third table that is called a junction table.
The primary key of the junction table consists of the foreign keys from both table A and table B. For example, the "Authors" table and the "Titles" table have a many-to-many relationship that is defined by a one-to-many relationship from each of these tables to the "TitleAuthors" table.
One-to-one relationships In a one-to-one relationship, a row in table A can have no more than one matching row in table B, and vice versa. A one-to-one relationship is created if both of the related columns are primary keys or have unique constraints. This kind of relationship is not common, because most information that is related in this manner would be in one table. You might use a one-to-one relationship to take the following actions: Divide a table with many columns.
Isolate part of a table for security reasons. Store data that is short-lived and could be easily deleted by deleting the table. Store information that applies only to a subset of the main table.
Join tables and queries - Access
In Access, the primary key side of a one-to-one relationship is denoted by a key symbol. The foreign key side is also denoted by a key symbol. How to define relationships between tables When you create a relationship between tables, the related fields do not have to have the same names.
However, related fields must have the same data type unless the primary key field is an AutoNumber field. You can match an AutoNumber field with a Number field only if the FieldSize property of both of the matching fields is the same. Even when both matching fields are Number fields, they must have the same FieldSize property setting. How to define a one-to-many or one-to-one relationship To create a one-to-many or a one-to-one relationship, follow these steps: You cannot create or change relationships between open tables.
In Access or Accessfollow these steps: Press F11 to switch to the Database window. On the Tools menu, click Relationships. If you have not yet defined any relationships in your database, the Show Table dialog box is automatically displayed. To create a relationship between a table and itself, add that table two times.
Drag the field that you want to relate from one table to the related field in the other table. To drag multiple fields, press Ctrl, click each field, and then drag them.
In most cases, you drag the primary key field this field is displayed in bold text from one table to a similar field this field frequently has the same name that is called the foreign key in the other table. Make sure that the field names that are displayed in the two columns are correct.
Guide to table relationships
You can change the names if it is necessary. Set the relationship options if it is necessary. These options will be explained in detail later in this article. Click Create to create the relationship.
- How to define relationships between tables in an Access database
- Join tables and queries
- Fixing Access Annoyances by Phil Mitchell, Evan Callahan
Repeat steps 4 through 7 for each pair of tables that you want to relate. Whether you save the layout or do not save the layout, the relationships that you create are saved in the database. However, referential integrity is not enforced with queries. How to define a many-to-many relationship To create a many-to-many relationship, follow these steps: Create the two tables that will have a many-to-many relationship. Create a third table. This is the junction table. In the junction table, add new fields that have the same definitions as the primary key fields from each table that you created in step 1.
In the junction table, the primary key fields function as foreign keys. You can add other fields to the junction table, just as you can to any other table.
In a query, a relationship is represented by a join. When you add tables to a query, Access creates joins that are based on relationships that have been defined between the tables. You can manually create joins in queries, even if they do not represent relationships that have already been defined.
If you use other queries instead of or in addition to tables as sources of data for a query, you can create joins between the source queries, and also between those queries and any tables that you use as sources of data.
Joins behave similarly to query criteria in that they establish rules that the data must match to be included in the query operations. Unlike criteria, joins also specify that each pair of rows that satisfy the join conditions will be combined in the recordset to form a single row.
There are four basic types of joins: This article explores each type of join you can use, why you use each type, and how to create the joins. Joins are to queries what relationships are to tables: This line between the tables represents the join.
Double-click a join to open the Join Properties dialog box depicted and review or change the join. Joins are sometimes directional. This area of the dialog box shows you which table is which in the join, and which fields are used to join the tables. This area determines the type of join: Fields from both tables can be used, and data that pertains to a given task is displayed from each. In an inner join, no other data is included.
In an outer join, unrelated records from one table are also included in the query results. Top of Page Types of joins There are four basic types of joins: Cross joins and unequal joins are advanced join types and are rarely used, but you should know about them to have a full understanding of how joins work.
Most of the time, you will use inner joins. Outer joins are directional: