Calculated fields
Card calculated fields allow you to obtain results by combining one or more fields resulting from the queries made.

## Arithmetic operations

Basic calculated fields can be composed using basic arithmetic operators, such as addition (+), subtraction (-), multiplication (*), or division (/), between one or more fields.
Here are some examples:
• Calculation of amount with VAT: metric_field * 1.21
• Difference between two fields: metric_field1 - metric_field2
• Application of a fixed margin to an amount: metric_field + 100
• Calculation of a ratio between 2 fields: metric_field1 / metric_field2
Note that when including a field in a calculated field expression, it must be enclosed in curly braces { }. Following the previous examples, it would be {metric_field} * 1.21

## Comparisons with previous periods

It is often interesting to be able to have a comparison for a metric with respect to past periods. For example, if we are visualizing the amount of the sale monthly, and the business has a strong dependence on the calendar, we may be interested in comparing it with the amounts of the sale produced the previous year.
Biuwer includes a set of functions to compare a metric with previous periods, which you can use when composing card calculated fields:
• PREV_YEAR: Compares the specified metric to the previous year.
• PREV_QUARTER: Compares the specified metric to the previous quarter.
• PREV_MONTH: Compares the specified metric to the previous month.
• PREV_WEEK: Compares the specified metric to the previous week.
• PREV_DAY: Compares the specified metric to the previous day.
All these functions are called in the same way, and always taking into account a date field as a reference to make the comparison.
The syntax is as follows, using a metric field and a date field as a time reference. Remember that when including the fields they must be enclosed in braces { }. We use PREV_YEAR as an example:
General syntax:
PREV_YEAR(metric_field, date_field)
A real example:
PREV_YEAR({f_sales_invoice.total_amount}, {f_sales_invoice.issue_date})
Comparisons with previous periods are very useful if they are configured correctly with the appropriate date levels, but may not be useful in other cases.
For example, if we configure a chart with a YEAR date level and configure a time comparison with respect to the previous DAY, the result obtained will not make much sense.