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What can I put in my table to validate so my 'DateFrom' is always before my 'DateTo' so that I can get a popup box stating date from cannot be after date to? Also supply your own Validation Text message if you want something other than the error text Access displays when the rule is violated. Not the answer you're looking for?Browse other questions tagged validation datetime ms-access or ask your own question.
If time travel is possible in the future, no matter how distant, why haven't they come back to tell us? Note: It is commonly thought that the word relational in the relational model comes from the fact that you relate together tables in a relational database. The relational model can be applied to both databases and database management systems (DBMS) themselves. Fortunately, you don't have to wait until Microsoft Access is perfect in a relational sense before you can benefit from the relational model. When designing a database, you have to make decisions regarding how best to take some system in the real world and model it in a database. The benefits of a database that has been designed according to the relational model are numerous. Since much of the information is stored in the database rather than in the application, the database is somewhat self-documenting. The goal of this article is to explain the basic principles behind relational database design and demonstrate how to apply these principles when designing a database using Microsoft Access. The decision as to which candidate key is the primary one rests in your hands—there's no absolute rule as to which candidate key is best.
Counter columns in Microsoft Access make good primary keys, especially when you're having trouble coming up with good candidate keys, and no existing arbitrary identification number is already in place. Although primary keys are a function of individual tables, if you created databases that consisted of only independent and unrelated tables, you'd have little need for them. Continuing the example presented in the last section, let's say that you choose CustomerId as the primary key for tblCustomer.
It is important that both foreign keys and the primary keys that are used to reference share a common meaning and draw their values from the same domain. Two tables are related in a one-to-one (1—1) relationship if, for every row in the first table, there is at most one row in the second table.
Tables that are related in a one-to-one relationship should always have the same primary key, which will serve as the join column. Two tables are related in a one-to-many (1—M) relationship if for every row in the first table, there can be zero, one, or many rows in the second table, but for every row in the second table there is exactly one row in the first table. One-to-many relationships are also used to link base tables to information stored in lookup tables. Two tables are related in a many-to-many (M—M) relationship when for every row in the first table, there can be many rows in the second table, and for every row in the second table, there can be many rows in the first table. As mentioned earlier in this article, when designing databases you are faced with a series of choices. Normalization theory gives us the concept of normal forms to assist in achieving the optimum structure.
Microsoft Access doesn't require you to define a primary key for each and every table, but it strongly recommends it. For all practical purposes the terms table and relation are interchangeable, and I will use the term table in the remainder of this chapter.
You'd have a difficult time retrieving information from this table, because too much information is being stored in the Items field. 1NF also prohibits the presence of repeating groups, even if they are stored in composite (multiple) columns. While this design has divided the information into multiple fields, it's still problematic. A table is said to be in Second Normal Form (2NF), if it is in 1NF and every non-key column is fully dependent on the (entire) primary key.
You decompose the tables in such a way as to allow them to be put back together again using queries. A table is said to be in Third Normal Form (3NF), if it is in 2NF and if all non-key columns are mutually independent.
The relational model defines several integrity rules that, while not part of the definition of the Normal Forms are nonetheless a necessary part of any relational database. The referential integrity rule says that the database must not contain any unmatched foreign key values. A row may not be added to a table with a foreign key unless the referenced value exists in the referenced table.
In general, there are three options available when a referenced primary key value changes or a row is deleted. All integrity constraints that do not fall under entity integrity or referential integrity are termed database-specific rules or business rules.
Without the specification and enforcement of business rules, bad data will get in the database. Microsoft Access 2.0 supports the specification of validation rules for each column in a table. Microsoft Access 2.0 also supports the specification of a global rule that applies to the entire table. Although Microsoft Access business-rule support is better than most other desktop DBMS programs, it is still limited (especially the limitation of one global table rule), so you will typically build additional business rule logic into applications, usually in the data entry forms. If this system was never before computerized, take the existing paper-based system and rough out the table design based on these forms.
If the database will be converted from an existing computerized system, use its tables as a starting point. Take the roughed-out reports from step 5 and make sure that the tables from step 4 include this data.
This list doesn't cover every facet of the design process, but it's useful as a framework for the process.
Sometimes it's necessary to break the rules of normalization and create a database that is deliberately less normal than it otherwise could be.
If you're considering denormalizing for performance reasons, don't always assume that the denormalized approach is the best.
If you're considering denormalizing because your users think they need it, investigate why.
A database designed according to the relational model will be efficient, predictable, well performing, self-documenting and easy to modify. You can establish three kinds of relationships between tables in a relational database: one-to-one, one-to-many or many-to-many. Normalization is the process of simplifying the design of a database so that it achieves the optimum structure. Many others attempt to learn it, but give up because of the dry, academic treatment it is usually given by most authors and teachers.

The relational fidelity of database programs can be compared using Codd's 12 rules (since Codd's seminal paper on the relational model, the number of rules has been expanded to 300) for determining how DBMS products conform to the relational model. The relational model can also be applied to the design of databases, which is the subject of the remainder of this article. This consists of deciding which tables to create, what columns they will contain, as well as the relationships between the tables.
If you allow duplicate rows in a table, then there's no way to uniquely address a given row via programming. Following Pascal's guidelines, you would rule out the last three candidates because addresses and phone numbers can change fairly frequently.
Primary keys become essential, however, when you start to create relationships that join together multiple tables in a database.
Relationships between real-world entities can be quite complex, involving numerous entities each having multiple relationships with each other. Many-to-many relationships can't be directly modeled in relational database programs, including Microsoft Access.
The normal forms are a linear progression of rules that you apply to your database, with each higher normal form achieving a better, more efficient design.
It's important to note, however, that when I use the term table, I actually mean a table that also meets the definition of a relation.
1NF dictates that, for every row-by-column position in a given table, there exists only one value, not an array or list of values. For example, the same table might be improved upon by replacing the single Items column with six columns: Quant1, Item1, Quant2, Item2, Quant3, Item3 (see Figure 7).
For example, how would you go about determining the quantity of hammers ordered by all customers during a particular month? The table in Figure 8, tblOrder3, is 1NF since each column contains one value and there are no repeating groups of columns. Since tblOrder3 is in First Normal Form, you can easily construct a Totals query to determine the total number of hammers ordered by customers. The process of breaking a non-normalized table into its normalized parts is called decomposition. In fact, this form of decomposition is termed non-loss decomposition because no information is sacrificed to the normalization process.
For example, say you need to add 100 detail records, each of which involves the purchase of screwdrivers. For example, the insertion of a new row causes a calculated total field stored in another table to report the wrong total. For example, the deletion of a row in the database deletes more information than you wished to delete. For example, updating a description column for a single part in an inventory database requires you to make a change to thousands of rows.
While it's relatively easy to work through the examples in this article, the process gets more difficult when you are presented with a business problem (or another scenario) that needs to be computerized (or downsized). Otherwise, Microsoft Access will allow your users to enter a Null foreign key value, thus violating strict referential integrity.
These type of rules are specific to each database and come from the rules of the business being modeled by the database. Because of this change, however, column rules can no longer reference other columns or use domain, aggregate, or user-defined functions.
This is useful for creating rules that cross-reference columns as the example in Figure 15 demonstrates.
A table validation rule has been created to require that deliveries be made on or after the date the pizza was ordered. This logic should be layered on top of any table-based rules and can be built into the application using combo boxes, list-boxes and option groups that limit available choices, form-level and field-level validation rules, and event procedures.
While it's true that a properly designed database should follow the normal forms and the relational model, you still have to come up with a design that reflects the business you are trying to model. This will usually involve sitting down and meeting with the people who will be using the system and asking them lots of questions.
If normalization doesn't come naturally (or from experience), you can start by creating one huge, non-normalized table per form that you will later normalize.
First, identify candidate keys for every table and using the candidates, choose the primary key.
You'll usually do this for performance reasons or because the users of the database demand it. In most cases, when you denormalize, you will be required to create additional application code to avoid insertion, update, and deletion anomalies that a more normalized design would avoid. Instead, I suggest you first fully normalize the database (to Third Normal Form or higher) and then denormalize only if it becomes necessary for reasons of performance.
Often they will be concerned about simplifying data entry, which you can usually accomplish by basing forms on queries while keeping your base tables fully normalized. This violates 3NF because the two non-key columns are mutually dependent, but it significantly improves the performance of certain commonly run reports.
But if creating databases is part of your job, then you're treading on thin ice if you don't have a good solid understanding of relational database design theory. Instead, the word relational has its roots in the terminology that Codd used to define the relational model. When compared with other database management programs, Microsoft Access fares quite well in terms of relational fidelity. While it would be nice if this process was totally intuitive and obvious, or even better automated, this is simply not the case. Many books have been written on database design theory; in fact, many careers have been devoted to its study. The choice among CustomerId and the name composite key is less obvious and would involve tradeoffs.
Also, counter columns only make sense for tables on the one side of a one-to-many relationship (see the discussion of relationships in the next section).
For example, CustomerId is of the domain of valid customer ID #'s, which in this case might be Long Integers ranging between 1 and 50,000. On the other hand, Microsoft Access will not prevent you from joining the Integer column EmployeeAge from one table to the Integer column YearsWorked from a second table, even though these two columns are obviously from different domains. For example, a family has multiple relationships between multiple people—all at the same time.
This type of relationship is often created to get around some limitation of the database management software rather than to model a real-world situation. Any query would have to search all three Item columns to determine if a hammer was purchased and then sum over the three quantity columns. It's better to leave this column out of the table and make the calculation in a query or on a form or a report instead.
I have outlined an approach to take later in this article, but first the subject of integrity rules will be discussed.

It is important to note that the enforcement of business rules is as important as the enforcement of the general integrity rules discussed in the previous section. These application-based rules, however, should be used only when the table-based rules cannot do the job.
Relational database design theory can usually tell you what not to do, but it won't tell you where to start or how to manage your business. If you're comfortable with normalization theory, try and keep it in mind as you create your tables, remembering that each table should describe a single entity.
While this won't get you any points with database design purists, ultimately you have to deliver a solution that satisfies your users. For example, if you decide to store a calculation in a table, you'll need to create extra event procedure code and attach it to the appropriate event properties of forms that are used to update the data on which the calculation is based. It's an indexed column (with duplicates allowed) and is calculated using a user-defined function. Once again, this is done to avoid a join to the tblEmployee table, but introduces redundancies and adds the risk of update anomalies.
The table in Codd's writings was actually referred to as a relation (a related set of information).
Instead, this article is meant as an informal introduction to database design theory for the database developer.
You guarantee uniqueness for a table by designating a primary key—a column that contains unique values for a table. Say that a company has a table of customers called tblCustomer, which looks like the table shown in Figure 1. In a relational database such as Microsoft Access, however, you consider only relationships between pairs of tables. In Microsoft Access, one-to-one relationships may be necessary in a database when you have to split a table into two or more tables because of security or performance concerns or because of the limit of 255 columns per table. The one-to-many relationship is also referred to as a parent-child or master-detail relationship. For example, a patient may be covered by multiple insurance plans and a given insurance company covers multiple patients. That is, with a few exceptions, the manipulation of tables in Microsoft Access doesn't depend upon a specific ordering of columns or rows. If lists of values are stored in a single column, there is no simple way to manipulate those values. In other words, does the value of OrderId and OrderItem# for a given record imply the value of every other column in the table?
Simply put everything that applies to each order in one table and everything that applies to each order item in a second table.
You can't uniquely identify or reference a row in a table, if the primary key of that table can be null.
Deletions of rows in tblCustomer will be disallowed if rows in tblOrders would be orphaned. The more you can build business rules in at the table level, the better, because these rules will always be enforced and will require less maintenance. This is where it helps to understand the business (or other scenario) you are trying to model. Print out the existing schema, table by table, and the existing data entry forms to use in the design process. If you wish to perform searches on the Soundex column with any but the smallest tables, you'll find a significant performance advantage to storing the Soundex column in the table and indexing this computed column. Since you often create reports that need to include the total order cost, but not the cost of individual items, you've broken 2NF to avoid having to join these two tables every time this report needs to be generated.
The basic idea behind the relational model is that a database consists of a series of unordered tables (or relations) that can be manipulated using non-procedural operations that return tables.
Each table can have only one primary key, even though several columns or combination of columns may contain unique values. Domains can be thought of as user-defined column types whose definition implies certain rules that the columns must follow and certain operations that you can perform on those columns.
These tables can be related in one of three different ways: one-to-one, one-to-many or many-to-many.
Thus, the tblPatient table in a medical database would be related to the tblInsurer table in a many-to-many relationship. Make sure that the primary key will guard against all present and future duplicate entries. You'd likely use an event procedure attached to a form to perform the Soundex calculation and store the result in the Soundex column. This model was in vast contrast to the more traditional database theories of the time that were much more complicated, less flexible and dependent on the physical storage methods of the data. All columns (or combination of columns) in a table with unique values are referred to as candidate keys, from which the primary key must be drawn. Given the OrderId, you know the customer and date of the order, without having to know the OrderItem#. 2 Phillips-head screwdriver" at some later time, you will have to update all 100 records.
If the table has a composite primary key, then the decomposition should, in general, be guided by breaking the key apart and putting all columns pertaining to each component of the primary key in their own tables. To avoid update anomalies, you'll want to ensure that this column cannot be updated by the user and that it is updated every time LastName changes. As a second example, perhaps you need to transfer only a portion of a large table to some other application on a regular basis.
Say that you decided that a customer would never order more than twenty-five items in any one order and designed the table accordingly.
Thus, these two columns are not dependent on the entire primary key which is composed of both OrderId and OrderItem#. Another problem arises when you wish to delete all of the 1994 screwdriver purchase records at the end of the year. There's no right answer, but most developers favor numeric primary keys because names do sometimes change and because searches and sorts of numeric columns are more efficient than of text columns in Microsoft Access (and most other databases).
You can split the table into the transferred and the non-transferred pieces, and join them in a one-to-one relationship. That means you would be using 50 columns to store the item and quantity information per record, even for orders that only involved one or two items. A simple key is a key made up of one column, whereas a composite key is made up of two or more columns.

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