Daily DAX : Day 247 PREVOIUS

 The PREVIOUS function in Power BI's DAX (Data Analysis Expressions) language is used to reference the value of a column in the previous row within a table, based on the current context. It is primarily used in calculated columns to compare or perform calculations with data from the preceding row in a specified order.

Syntax

dax

PREVIOUS ( <ColumnName> )

    ColumnName: The name of the column from which to retrieve the value from the previous row.

Key Points

    The PREVIOUS function is only available in calculated columns, not in measures.

    It requires a well-defined sort order in the table to determine what "previous" means. You typically use a column like a date, index, or any column that defines the sequence of rows.

    If there is no previous row (e.g., for the first row in the table), the function returns BLANK().

    It respects the current filter context, meaning it only considers rows that are visible in the current context.

Use Case

The PREVIOUS function is commonly used for sequential calculations where you need to compare the current row's value with the previous row's value. Common scenarios include:

    Calculating differences between consecutive rows (e.g., day-over-day changes in sales).

    Computing running totals or cumulative differences.

    Analyzing trends or changes over time, such as stock price changes or inventory levels.

Example

Suppose you have a table named Sales with the following columns: Date, SalesAmount. You want to calculate the difference in sales between consecutive days.

    Table Structure:

    Date       | SalesAmount

    2023-01-01 | 100

    2023-01-02 | 150

    2023-01-03 | 200

    Create a Calculated Column:

    Add a calculated column in Power BI to compute the difference in sales from the previous day:

    dax

    SalesDifference = 

    Sales[SalesAmount] - PREVIOUS(Sales[SalesAmount])

    Result:

    Date       | SalesAmount | SalesDifference

    2023-01-01 | 100         | BLANK()

    2023-01-02 | 150         | 50

    2023-01-03 | 200         | 50

        For 2023-01-01, there is no previous row, so SalesDifference is BLANK().

        For 2023-01-02, the previous row's SalesAmount is 100, so 150 - 100 = 50.

        For 2023-01-03, the previous row's SalesAmount is 150, so 200 - 150 = 50.

Sorting Requirement

To ensure the PREVIOUS function works correctly, the table must be sorted appropriately (e.g., by Date in ascending order). You can enforce this by:

    Sorting the table in the data model using the Sort By Column feature in Power BI.

    Ensuring the column used for ordering (e.g., Date or an index) has unique values to avoid ambiguity.

Limitations

    Not for Measures: The PREVIOUS function cannot be used in measures because it relies on row context, which is only available in calculated columns.

    Single Column: It only retrieves the value from the specified column in the previous row, not multiple columns.

    Performance: For large datasets, using PREVIOUS in calculated columns can impact performance, as it processes row by row.

Alternative Approaches

For more complex scenarios or when working with measures, you might use other DAX functions like:

    EARLIER: To reference earlier row contexts in nested calculations.

    FILTER and TOPN: To dynamically identify the previous row based on a condition.

    INDEX or RANKX: To create an index or rank column for ordering, then use it with other functions to mimic PREVIOUS.

Practical Example

Scenario: A retail company wants to analyze daily sales growth.

    Table: SalesData with columns Date, Store, Sales.

    Goal: Calculate the percentage change in sales from the previous day for each store.

Steps:

    Ensure the table is sorted by Date within each Store.

    Create a calculated column:

    dax

    SalesGrowth = 

    VAR PrevSales = PREVIOUS(SalesData[Sales])

    RETURN

    IF(

        NOT ISBLANK(PrevSales),

        (SalesData[Sales] - PrevSales) / PrevSales,

        BLANK()

    )

    This column will show the percentage growth in sales compared to the previous day for each store.

Summary

The PREVIOUS function is a straightforward way to access the value of a column in the prior row within a calculated column. It’s particularly useful for sequential data analysis, such as time-series or ordered datasets, but requires careful sorting and is limited to calculated columns. For more complex scenarios, combining it with other DAX functions or using alternative approaches may be necessary.