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:.
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Microsoft access 2016 relationships free.Access 2016: Create a Relationship
In fact, with referential integrity you cannot enter a record in tblOrders without there being an existing record in tblCustomers. Once enforced, Access rejects any operation that would violate referential integrity for that table relationship. This relationship is called a many-to-many relationship. If you enforce referential integrity for this relationship, the line appears thicker at each end. Also this is the second in a series of Access tutorials. Note: Do not select Allow Multiple Values in the Lookup Wizard when you are using the wizard to create a relationship. The purpose of referential integrity is to prevent orphans and keep references in sync so that this hypothetical situation never occurs.
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5 comment
Спасибо. Джабба выдавил из себя смешок и попытался обратить все в шутку. - Если только Стратмор не придумал что-то особенное и не обошел мои фильтры.