Ch1-What is DAX?

What is DAX?

數據分析表達式 (DAX) 是在 Analysis Services、Power BI 以及 Excel 中的 Power Pivot 使用的公式表達式語言。在初版PowerPivot for Microsoft Excel 2010時被建立。以後DAX日益流行，在Excel社區中，使用DAX構建Power Pivot數據模型，在BI社區中，使用DAX構建Power BI和Analysis Services模型。儘管DAX存在與不一樣的工具中，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。與須要花不少時間才能瞭解的編程語言相比，基礎的DAX學習很是簡單，只需幾個小時便可。但當開始接觸高級的內容如計算上下文，迭代，上下文轉換，一切都以爲很複雜。不要懼怕！保持耐心。當你開始理解這些概念後，會發現DAX實際上是很是簡單的語言。

evaluation contexts, iterations, and context transitions

第一章開始歸納說明什麼是數據模型，咱們建議全部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、模型和各類關係。

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。而後閱讀最後一節"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