What is DAX?
資料分析運算式 (DAX) 是在 Analysis Services、Power BI 以及 Excel 中的 Power Pivot 使用的公式運算式語言，在第一版PowerPivot for Microsoft Excel 2010時被創建，之后DAX日益流行，在Excel社區中，使用DAX構建Power Pivot資料模型，在BI社區中，使用DAX構建Power BI和Analysis Services模型，盡管DAX存在與不同的工具中，都共享相同的內部引擎"Tabular"-表格模型，
DAX, which stands for Data Analysis eXpressions,is the programming language of Microsoft Power BI,Microsoft Analysis Services
DAX is present in many different tools, all sharing the same internal engine named Tabular
Tabular models
DAX是一種簡單的語言，與需要花很多時間才能了解的編程語言相比，基礎的DAX學習非常簡單，只需幾個小時即可，但當開始接觸高級的內容如計算背景關系，迭代，背景關系轉換，一切都覺得很復雜，不要害怕！保持耐心，當你開始理解這些概念后，會發現DAX其實是非常簡單的語言，
evaluation contexts, iterations, and context transitions
第一章開始概括說明什么是資料模型，我們建議所有經驗水平的讀者仔細閱讀本節，以熟悉整本書中都探討的表格、模型和各種關系，
tables, models, and different kinds of relationships
在下面的部分中，對有Microsoft Excel、SQL和MDX經驗的讀者而言，會分別介紹DAX與這三種語言的比較，然后閱讀最后一節"DAX for Power BI"，進入下一站開始DAX語言的真正旅程，
Understanding the data model
A data model is a set of tables, linked by relationships
table is a set of rows containing data, with each row divided into columns.
Each column has a data type and contains a single piece of information
row in a table as a record
A relationship is a link between two tables
Two tables in a relationship one-side and many-side
Special kinds of relationships are 1:1 and weak relationships
On the one-side, the column needs to have a unique value for each row, and it cannot contain blanks.
When a column has a unique value for each row, it is called a key for the table
Relationships can form a chain
We usually discourage the use of bidirectional filters
Understanding the direction of a relationship
Filtering always happens from the one-side of the relationship to the many-side
DAX for Excel users
Cells versus tables
Two functional languages
Iterators in DAX
DAX requires theory
DAX for SQL developers
Relationship handling
DAX is a functional language
DAX as a programming and querying language
Subqueries and conditions in DAX and SQL
DAX for MDX developers
Multidimensional versus Tabular
DAX as a programming and querying language
Hierarchies
Leaf-level calculations
DAX for Power BI users
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