How can you create a custom duration field for a created dimension group of type: time?

To create a custom duration field for a created dimension group of type time, specifying a reference type for the dimensions in sql_start and sql_end is the most appropriate approach. By defining these two dimensions, you establish the beginning and ending points of the time frame you want to measure the duration for. This allows Looker to calculate the difference between the two timestamps effectively.

When you set the reference types correctly for both sql_start and sql_end, Looker understands how to compute the duration by comparing these timestamps. This mechanism is essential for creating accurate time-based calculations, enabling users to derive insights related to time metrics, such as the length of an event or the duration between two dates.

In contrast, other approaches like using standard SQL commands may not integrate seamlessly within Looker's LookML structure. Similarly, specifying a ::string reference type is not suitable for time calculations, as it would not allow Looker to perform the necessary date arithmetic. Defining the duration in terms of weeks, while potentially useful in specific contexts, does not encompass the broader and more flexible requirement of specifying the start and end references necessary for general duration calculations. Hence, focusing on sql_start and sql_end with appropriate reference types is key to achieving a precise and functional custom duration field.