To ensure every date in a time period is represented when querying transactions, which type of join should you use?

When ensuring that every date in a specified time period is represented in your query results, an outer join is the most suitable choice. This type of join combines records from both tables involved (the transactions table and the date table), including all records from both tables regardless of whether there is a match.

In the context of this question, using an outer join ensures that even if there are dates in the date table that do not have corresponding transactions in the transactions table, those dates will still be included in the result set. This is particularly crucial when analyzing data over a time period, as it allows for a complete view of all days, including those without transactions, making it possible to observe trends or gaps in activity.

This is in contrast to inner joins, which only include records that have matching entries in both tables, potentially omitting dates with no transactions. Similarly, left and right joins only focus on the records from one table or the other, specifically the left or right side, which may not provide the comprehensive overview of all dates in the time period as needed. Thus, using an outer join provides the most robust approach to ensure all dates are represented in your query results.