What is DAX

What is DAX (DATA ANALYSIS EXPRESSIONS)

Data Analysis Expression language or DAX, was originally released with Power Pivot in Excel in 2010 by Microsoft. It’s a functional language that is very similar to excel, as you would expect as it was part of the excel tools.  Its initial basic function was to allow excel users solve more complex business problems.

Now, not only is DAX used in Power Pivot but it is also the language used in Analysis Services tabular and Power BI.

So I mentioned DAX is similar to Excel but there are some major differences.  Excel works on cell references and ranges of cells.  DAX does not. DAX works on tables and subsets of tables of data.  

In addition to the suite of Excel functions that work in DAX, DAX also has many additional and more powerful functions that are not available in Excel. Functions or Expressions that make the DAX language so much more exciting than Excel.

When we say DAX is a functional language we mean that the language makes a call to a function, which is known as an expression in DAX. The result from this expression can then be used in a call to another function.  If you are familiar with nesting in Excel, commonly used with IF statements, well you could say it’s something similar to that.

Data Types in DAX

DAX supports a number of different data types; a few more than excel.

Integer            Which is just a whole number

Decimal           Which is just a fraction of a number 

Currency         is a combination of an integer and decimal

Date and Time   is also a combination of an integer and decimal.  Very much like in Excel DATE and time are just a number and if you are familiar with date and time in Excel then you know that data is represented by a whole number and time is represented by a decimal number.

Boolean           this is a true or false result

String               is text

Binary              is an object such as a file or an image

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DAX Syntax

DAX syntax is not that difficult, but it does differ to Excel.  We mentioned earlier that Excel works with cell references or ranges, but DAX works with tables and columns in a table

Let’s look at a simple DAX expression

               = DISTINCTCOUNT (

                    ‘Sales’[customer_id]

          )

Like Excel, DAX always starts with an equals sign (=)

After the equal sign DAX then calls an Expression, which is a function or a formula.  In this example we have called the DISTINCT expression which will count the distinct values of a selected column.

The table name is shown in the quotes (‘Sales’).  Now these quotes can be left out if the table name does not contain any special characters.  After this we reference the column in the table contained within [].  In this example we have selected the customer id column.  The result for this DAX expression will give us the number of unique customers that we have sold to.

You Need to KNOW...

There are two types of DAX calculations.  A calculated column and a measure.  Both have their uses and their benefits.

It is also worth bringing up at an early stage that DAX calculations are carried out in two contexts.

  1. A row context, which is the rows or selected rows of a table
  2. A filter context which is defined by the filters on a pivot table or visualisation.

Understanding the Row and Filter context is very important when writing DAX calculations. If you do not take these into consideration, you may not get the value or result that you expected.  This becomes especially apparent when your data structure contains multiple tables and relationships. As we work through the course this is something we will continue to build upon from an early stage.

Calculated Columns and Measures 

There are two type of calculations in DAX.  There is a calculated column and there is a measure.  Both of these have different behaviors and different uses so it is worth talking about them a little.

Calculated Columns

Calculated columns are calculated at the time of input and are refreshed only when the model is refreshed.  One drawback is that calculated columns use RAM. If you have a few billion rows, it will use a lot of RAM, so only use them when you have to.  When you carry out a calculated column, it is calculated row by row, at row level.  Understanding row level and filter level are important when you get into more complex DAX.

A calculated column can be used as a row header or a slicer to analysed data in pivot tables and visualisations.  It can also be used as a value within a pivot table or a visualisation.

Measures

Measures, known as calculated fields in Power Pivot 2013 are calculated at report level, they are not stored in the data model.  They are also calculated at report level on an aggregate value.  That means that measures are not calculated at row level like calculated columns. But when they are used in a pivot table or visualization.  Measures are used as valued fields. They cannot be used to slice and dice data with because only fields (columns) from your data set can be used.

In a measure you need to define the row. The row context is not automatically defined when creating a measure.  To define the rows in a measures you will need to add an aggregate such as SUM, around the columns in the expression. The most common used aggregate values used in DAX as with Excel are SUM, AVERAGE, MIN & MAX. If your measure returns a table, you will also to use an iterator to define the row. Iterators would include SUMX & COUNTX which we will look at in the next section.

So when do you use a calculated column or a measure?

Well there are no real hard and fast rules. If you are coming from an Excel background, you might think it is best to expand the table and create a calculated column.  But Excel and DAX are different.  Measures have the ability to do things that calculated columns cannot do such as calculate the average sales per day or change v’s prior year.

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