In MS Access, a calculated field is a field that displays the result of an expression. An expression is a combination of operators, constants, functions, and field names that evaluates to a single value. You can use calculated fields to perform calculations or manipulate data without changing the underlying data in your tables.
One of the advantages of calculated fields is that you can use fields from other tables or queries in your expressions, as long as there is a relationship between the tables or queries. This allows you to create more complex and dynamic calculations that depend on data from multiple sources.
In this article, we will show you how to create a calculated field that uses a field from another table in MS Access, and explain the basic steps and concepts involved.
Example scenario
Suppose you have two tables in your database: Products and Orders. The Products table contains information about the products you sell, such as product ID, name, price, and category. The Orders table contains information about the orders placed by your customers, such as order ID, date, customer ID, product ID, quantity, and total amount.
You want to create a calculated field in the Orders table that shows the profit margin for each order, which is the difference between the total amount and the cost of the products ordered. To calculate the profit margin, you need to use the price field from the Products table, which is related to the product ID field in the Orders table.
Here is a sample of the data in the Products table:
| Product ID | Name | Price | Category |
|---|---|---|---|
| 1 | Book | 10 | Literature |
| 2 | Pen | 2 | Stationery |
| 3 | Notebook | 5 | Stationery |
| 4 | DVD | 15 | Entertainment |
Here is a sample of the data in the Orders table:
| Order ID | Date | Customer ID | Product ID | Quantity | Total Amount |
|---|---|---|---|---|---|
| 1 | 01/01/2024 | 101 | 1 | 2 | 20 |
| 2 | 01/02/2024 | 102 | 2 | 5 | 10 |
| 3 | 01/03/2024 | 103 | 3 | 3 | 15 |
| 4 | 01/04/2024 | 104 | 4 | 1 | 15 |
Steps to create a calculated field using a field from another table
To create a calculated field using a field from another table in MS Access, follow these steps:
- Open the table where you want to create the calculated field in Design View.
- In the Field Name column, type a name for the calculated field, such as Profit Margin.
- In the Data Type column, select Calculated from the drop-down list.
- In the Expression Builder dialog box that appears, type or select the expression for the calculated field. In our example, the expression is:
[Total Amount] - ([Quantity] * DLookup("Price", "Products", "Product ID = " & [Product ID]))This expression subtracts the product of the quantity and the price of the product from the total amount. The DLookup function is used to look up the price of the product from the Products table, based on the product ID. The DLookup function has three arguments: the name of the field to look up, the name of the table or query to look up from, and the criteria to match the records. The criteria is a string that specifies the condition for the lookup, such as “Product ID = ” & [Product ID], which means the product ID in the Products table must match the product ID in the Orders table.
- Click OK to close the Expression Builder dialog box.
- In the Format column, select a format for the calculated field, such as Currency or Percent.
- Save the table and switch to Datasheet View to see the calculated field in action.
Here is how the Orders table looks like after adding the calculated field:
| Order ID | Date | Customer ID | Product ID | Quantity | Total Amount | Profit Margin |
|---|---|---|---|---|---|---|
| 1 | 01/01/2024 | 101 | 1 | 2 | 20 | 0 |
| 2 | 01/02/2024 | 102 | 2 | 5 | 10 | 0 |
| 3 | 01/03/2024 | 103 | 3 | 3 | 15 | 0 |
| 4 | 01/04/2024 | 104 | 4 | 1 | 15 | 0 |
As you can see, the profit margin for each order is zero, because the total amount is equal to the cost of the products. To make the example more realistic, you can change the price of the products in the Products table and see how the profit margin changes accordingly.
Tips and considerations
- You can use calculated fields to perform various calculations and manipulations on your data, such as arithmetic operations, string functions, date functions, logical functions, and more. You can also use parameters, variables, and constants in your expressions. For more information on the syntax and usage of expressions.
- You can use fields from other tables or queries in your calculated fields, as long as there is a relationship between the tables or queries. You can use the DLookup, DSum, DAvg, DCount, DMax, DMin, and DStDev functions to look up or aggregate values from other tables or queries. For more information on these functions.
- You can also create calculated fields in queries, forms, and reports, using the same or similar steps as described above. However, there are some differences and limitations depending on the object type and the data source. For more information on creating calculated fields in different objects.
- Calculated fields are read-only, meaning you cannot edit or update their values directly. If you want to change the values of calculated fields, you need to change the values of the fields or the expressions that are used in the calculation.
- Calculated fields are stored in the database, but they are not indexed. This means that you cannot use calculated fields as primary keys, foreign keys, or in relationships. It also means that calculated fields may affect the performance of your database, especially if you use complex expressions or large amounts of data. To improve the performance of calculated fields, you can use indexes on the fields that are used in the calculation, or use queries to filter or sort the data before applying the calculation.
