The nested option allows you to group criteria together across multiple fields to force correct order of operation. For example, let's say you have three fields: department, location and salary and they are added to Yurbi in this order. We want to return results for a particular dept and/or a particular location and a salary above a certain limit. If we add criteria without the nested option we will get:
dept = 1 OR loc = 2 AND salary < 3000
Now if we check the nested option on the department field and the location field we will get:
(dept = 1 OR loc = 2) and salary < 3000
The logical AND/OR on each criterion is applied to the previous criterion even if it belongs to a preceding field. So the first field that has criteria, the first criterion's logical operator will not be used. The last criterion is used to AND/OR it with the previous criterion.
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