DAX, or Data Analysis Expressions, is an essential part of the DP-500 Designing and Implementing Enterprise-Scale Analytics Solutions Using Microsoft Azure and Microsoft Power BI exam. DAX is used in Power BI, Analysis Services, and Power Pivot in Excel. It’s a collection of formulas or expressions that are utilized to define custom calculations in Power BI, Analysis Services, and Power Pivot.
Handling Blanks or Errors Using DAX
In DAX, we have special functions to manage blanks or errors. For example, the ISBLANK()
function can be used to find out if a cell in a column is blank or not. It returns TRUE if the value is blank, and FALSE if not.
Here’s an example:
DAX
Measure =
IF (
ISBLANK ( [MyColumn] ),
"Blank Value",
"Non-Blank Value"
)
The code above will return the string “Blank Value” if the value in the column MyColumn
is blank, otherwise, it will return “Non-Blank Value.”
ISERROR
is another useful function that can check whether a certain operation leads to an error.
Creating Virtual Relationships
For relational data modelling, DAX enables the creation of virtual relationships by using USERELATIONSHIP
function. This function helps in modifying context transition, activating a relationship that is not active.
Consider two tables: Sales
and Budget
. Both tables have a common field Year
.
DAX
Measure =
CALCULATE (
SUM ( 'Sales'[Amount] ),
USERELATIONSHIP ( 'Sales'[Year], 'Budget'[Year] )
)
In the DAX expression above, USERELATIONSHIP instructs Power BI to use the relationship defined between Sales[Year]
and Budget[Year]
for this specific calculation.
Working with Iterators
Iterators are a special kind of DAX function that will apply a given expression or operation to each row of a table and then aggregate the result. Some common DAX iterators include SUMX
, AVERAGEX
, MINX
, MAXX
, etc.
For example, consider a Sales
table having data about Quantity
sold and Price
per unit. To calculate the total sale, we use the SUMX
iterator as follow:
DAX
Measure = SUMX ( 'Sales', 'Sales'[Quantity] * 'Sales'[Price] )
The SUMX
function in the expression above iterates over each row in the Sales
table, calculates the product of Quantity
and Price
for each row, and then sums up those results.
Understanding and mastering these calculations using DAX functions and variables significantly contributes to the success of passing the DP-500 exam. Practicing the usage of these commands in various scenarios can help deepen the understanding of DAX and how it functions in a real-world environment, each with their unique requirements and constraints.
Practice Test
True or False: The DAX formula language contains functions that can handle blanks or errors in your data.
- True
Answer: True
Explanation: DAX does indeed include functions like ISBLANK, IFERROR, and COALESCE that are designed to handle blanks and errors in your data.
Which of these is not a built-in DAX function?
- A. SUMX
- B. COUNTX
- C. DIVIDE
- D. RESOLVE
Answer: D. RESOLVE
Explanation: RESOLVE is not a built-in DAX function. SUMX, COUNTX, and DIVIDE are all valid DAX functions.
True or False: DAX variables cannot be used in iterative functions.
- False
Answer: False
Explanation: DAX variables can be used with iterators. They help to simplify complex formulas.
Which DAX function is used to create virtual relationships between tables?
- A. RELATIONSHIP
- B. RELATED
- C. TREATAS
- D. LINKED
Answer: C. TREATAS
Explanation: The TREATAS function in DAX is able to define a virtual relationship between tables in the absence of a physical one.
What is the correct DAX function to return the total rows in a table?
- A. ROWS
- B. COUNTROWS
- C. COUNTAX
- D. ROWCOUNTER
Answer: B. COUNTROWS
Explanation: The COUNTROWS function is used to count the number of rows in a table.
In DAX, which function returns a blank value if there is a division by zero error?
- A. ISERROR
- B. DIVIDE
- C. COALESCE
- D. BLANK
Answer: B. DIVIDE
Explanation: The DIVIDE function in DAX returns a blank value when a division by zero error occurs.
True or False: DAX variables can only be defined once and are fixed in their value.
- True
Answer: True
Explanation: DAX variables are immutable. They cannot be changed once they’re defined.
What does the RELATED function do in DAX?
- A. Creates a virtual relationship between tables
- B. Returns a related value from another table
- C. Merges two tables into one
- D. None of the above
Answer: B. Returns a related value from another table
Explanation: RELATED function in DAX is used to get data from related tables.
Which DAX function is used to handle errors?
- A. HANDLED
- B. MANAGED
- C. IFERROR
- D. ERRORHANDLE
Answer: C. IFERROR
Explanation: IFERROR function in DAX is used to handle errors in data or calculations.
True or False: It is possible to write calculations in DAX that take advantage of variables.
- True
Answer: True
Explanation: Variables can be defined and used within DAX calculations to help simplify and improve the performance of complex formulas.
In DAX, which function can be used to substitute all occurrences of a specific string value with another string?
- A. SUBSTITUTE
- B. REPLACE
- C. EXCHANGE
- D. SWAP
Answer: A. SUBSTITUTE
Explanation: The SUBSTITUTE function in DAX replaces all occurrences of a string with another string.
What is the role of iterators in DAX?
- A. They automatically repeat a calculation for each row in a table
- B. They create virtual relationships between tables
- C. They handle blanks or errors in the data
- D. None of the above
Answer: A. They automatically repeat a calculation for each row in a table
Explanation: In DAX, iterators perform calculations on a row-by-row basis for the entirety of a table.
Which of the following is not a valid Iterator function in DAX?
- A. SUMX
- B. MINX
- C. MAXX
- D. DIVIDEX
Answer: D. DIVIDEX
Explanation: DIVIDEX is not a valid Iterator function in DAX. SUMX, MINX, and MAXX are examples of iterator functions.
True or False: In DAX, you can use the IF function to return different results based on the outcome of a logical test.
- True
Answer: True
Explanation: The IF function in DAX is used for conditional tests. Depending on the result, it returns one value if the condition is TRUE, and another value if the condition is FALSE.
Which DAX function can be used to ignore any filters that might be applied to a column in a table?
- A. IGNORE
- B. ALL
- C. CLEAR
- D. RESET
Answer: B. ALL
Explanation: The ALL function in DAX is used to clear any filters that might have been applied to a column in a table.
Interview Questions
What is the function of DAX?
The Data Analysis Expressions (DAX) is a formula language that allows the creation of custom calculated fields in Power Pivot tables and custom measures in Analysis Services Tabular models and Power BI.
How can you handle blanks in DAX?
The function to handle blanks in DAX is called BLANK(). This function can be used to represent database nulls or other missing values.
What is the purpose of VAR in DAX?
VAR in DAX is used to declare a variable that stores the result of a calculation or function, which can then be reused multiple times throughout the expression.
Can you name one function that is used to handle errors in DAX?
The function to handle errors in DAX is called ERROR(). The ERROR function returns an error value that can be caught by the error handling framework around DAX.
What are virtual relationships in DAX?
Virtual relationships in DAX are temporary relationships between tables that are created as part of a DAX formula and are not part of the model schema.
How can you create a virtual relationship in DAX?
The function to create a virtual relationship in DAX is called TREATAS(). The TREATAS function applies the result of an expression as a table of values in the context, even if the table is not related to the current table.
How do iterators work in DAX?
In DAX, iterators are functions that return a table and iterate row by row through that table, performing calculations for each row.
Can you give an example of an iterator function in DAX?
An example of an iterator function in DAX is SUMX, which finds the sum of an expression evaluated for each row in a table.
What does the BLANK() function in DAX do?
In DAX, the BLANK() function is used to indicate and handle missing or undefined values. When used in a mathematical expression, BLANK will yield another blank.
How can DAX handle errors while evaluating an expression?
DAX provides the function TRY to handle errors. This function tries to evaluate its first argument; if this results in an error, it returns the second argument. If no error occurs, it returns the result of the first argument.
Why is the VAR function useful in DAX calculations?
The VAR function in DAX allows you to store the result of an expression as a named variable, which can be then reused throughout the DAX formula. This significantly simplifies writing complex DAX formulas and enhances their readability.
How does the TREATAS function work in DAX?
The TREATAS function in DAX creates a virtual relationship between tables by applying the result of a table expression as a filter context to another table, whether related or not.
What is the difference between SUM and SUMX in DAX?
SUM is a simple aggregation function that adds up the values of a column. SUMX, on the other hand, is an iterator function that evaluates an expression for every row of a table and then sums up the results.
What happens if BLANK() is used within a conditional statement in DAX?
In DAX, if BLANK() is used within a conditional statement, such as in an IF function, the condition is treated as FALSE if the result is a blank.
How do you use TRY function in DAX to handle errors?
In DAX, TRY function is used in the following way: TRY(<expression>, <value_if_error>). The function attempts to evaluate the expression, and if an error occurs, it returns the value_if_error. If the expression is successfully evaluated, it returns the result of the expression.