Daily DAX : Day 74 FIRSTDATE

 The FIRSTDATE function in Power BI's Data Analysis Expressions (DAX) is used to return the first date in the specified column of dates. Here's a detailed explanation:

Syntax:

dax

FIRSTDATE(<dates>)

    <dates>: This is a column that contains dates.

Description:

    FIRSTDATE scans through the specified date column and returns the earliest date it finds.

    If there are no dates in the column or if the column is empty, it returns a blank.

Typical Use Cases:

    Date Dimension Analysis:

        When working with time intelligence in Power BI, you often need to know the first date of a period for calculations like year-to-date, month-to-date, etc. For example, to calculate sales from the start of the year:

        dax

    SalesYTD = CALCULATE(SUM(Sales[Amount]), DATESYTD(Dates[Date]))

    Here, FIRSTDATE could be used within DATESYTD to ensure you're starting from the first date of the year.

Cohort Analysis:

    If you're looking at customer behavior over time, you might want to find the first date a customer made a purchase to define cohorts:

    dax

    FirstPurchaseDate = FIRSTDATE(CustomerPurchases[PurchaseDate])

Filtering Data Based on Date Conditions:

    You might want to filter data to include only records from the first date of a period:

    dax

    FirstDaySales = CALCULATE(SUM(Sales[Amount]), DATESBETWEEN(Dates[Date], FIRSTDATE(Dates[Date]), FIRSTDATE(Dates[Date])))

Setting Up Time-Based Metrics:

    If you're creating metrics like "time since first purchase" or similar, FIRSTDATE helps in anchoring your calculations:

    dax

        DaysSinceFirstPurchase = DATEDIFF(FIRSTDATE(CustomerPurchases[PurchaseDate]), TODAY(), DAY)

Points to Consider:

    Performance: Using FIRSTDATE in larger datasets or complex models might impact performance since it scans through all dates in the column each time it's called. 

    Date Context: FIRSTDATE respects the filter context, so if you apply filters elsewhere in your model, it will return the first date within that filtered context.

By understanding and applying FIRSTDATE effectively, you can enhance your time-based analysis in Power BI, making your reports more insightful and dynamic.

https://dax.guide/firstdate/