SSAS DAX Syntax Overview

This document provides a comprehensive overview of the DAX (Data Analysis Expressions) syntax used within SQL Server Analysis Services (SSAS) Tabular models.

Introduction to DAX

DAX is a collection of functions, operators, and constants that can be used in formulas to create custom calculations in Analysis Services, Power BI, and Power Pivot in Excel.

Core Concepts

• Measures: Calculations that aggregate data, often used in reports and visualizations.
• Calculated Columns: Columns added to tables that contain DAX expressions, evaluated row by row.
• Tables: DAX can create new tables based on expressions.
• Relationships: DAX leverages table relationships to perform calculations across related tables.

DAX Formula Structure

A typical DAX formula follows this structure:

<Result> = <Expression>
        

Where:

• <Result> is the name of the measure, calculated column, or calculated table.
• = is the assignment operator.
• <Expression> is the DAX formula that defines the calculation.

Common DAX Functions

DAX offers a rich library of functions categorized by their purpose:

Aggregation Functions

• SUM( ): Sums all the numbers in a column.
• AVERAGE( ): Computes the average of all numbers in a column.
• MAX( ): Finds the largest value in a column.
• MIN( ): Finds the smallest value in a column.
• COUNT( ): Counts the number of rows that contain non-blank values in a column.
• COUNTROWS( ): Counts the number of rows in a table.

  Logical Functions

  • IF( , , [value_if_false] ): Returns one value if a logical expression is true and another value if it's false.
  • AND( , ): Returns TRUE if all arguments are TRUE.
  • OR( , ): Returns TRUE if any argument is TRUE.

  Text Functions

  • CONCATENATE( , ): Joins two text strings.
  • LEFT( , ): Returns the specified number of characters from the start of a text string.
  • RIGHT( , ): Returns the specified number of characters from the end of a text string.
  • LEN( ): Returns the number of characters in a text string.

  Date and Time Functions

  • TODAY(): Returns the current date.
  • NOW(): Returns the current date and time.
  • CALENDAR( , ): Returns a table with a single column of dates.

  Filter Functions

  • FILTER(
  , ): Returns a table that has been filtered.
  ALL( [
  | [, ]...] ): Returns all the rows in a table, or all the values in a column, ignoring any filters that might have been applied.
  CALCULATE( [, [, ...] ] ): Evaluates an expression in a modified filter context.

  DAX Operators

  DAX supports various operators for performing operations:

  • Arithmetic Operators: +, -, *, /
  • Comparison Operators: =, >, <, >=, <=, <> (not equal to)
  • Text Concatenation Operator: &
  • Logical Operators: && (AND), || (OR), NOT()

  DAX Syntax Examples

  Here are some common DAX syntax examples:

  Example 1: Simple Measure (Total Sales)

  
  Total Sales = SUM( 'Sales'[SalesAmount] )
          

  Example 2: Measure with Filter Context (Sales Last Year)

  
  Sales Last Year =
  CALCULATE(
      [Total Sales],
      SAMEPERIODLASTYEAR( 'Date'[Date] )
  )
          

  Example 3: Calculated Column (Full Name)

  
  Full Name = 'Customers'[FirstName] & " " & 'Customers'[LastName]
          

  Example 4: Calculated Table (Product Categories)

  
  Product Categories =
  SUMMARIZE(
      'Products',
      'Products'[Category],
      "Product Count", COUNTROWS( 'Products' )
  )
          

  Best Practices

  • Use clear and descriptive names for measures and columns.
  • Leverage existing measures whenever possible.
  • Understand filter context and row context.
  • Optimize performance by writing efficient DAX expressions.
  • Use comments to explain complex logic.

  Further Learning

  For more in-depth information, please refer to the official DAX documentation.