How to: Summarize or Aggregate Values Using Custom Expressions
In addition to using aggregate functions to aggregate data, you can create custom expressions to produce aggregate values. You can use custom expressions in place of aggregate functions anywhere in an aggregate query.
For example, in the titles table you might want to create a query that shows not just the average price, but what the average price would be if it were discounted.
You cannot include an expression that is based on calculations involving only individual rows in the table; the expression must be based on an aggregate value, because only the aggregate values are available at the time the expression is calculated.
Note
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To specify a custom expression for a summary value
Specify the groups for your query. For details, see How to: Group Rows in Query Results.
Move to a blank row of the Criteria pane, and then type the expression in the Columns column.
The Query and View Designer automatically assigns a column alias to the expression to create a useful column heading in query output. For more details, see How to: Create Column Aliases.
In the Group By column for the expression, select Expression.
Run the query.