Ch1-What is DAX?

What is DAX?

数据分析表达式 (DAX) 是在 Analysis Services、Power BI 以及 Excel 中的 Power Pivot 使用的公式表达式语言。在第一版PowerPivot for Microsoft Excel 2010时被创建。之后DAX日益流行，在Excel社区中，使用DAX构建Power Pivot数据模型，在BI社区中，使用DAX构建Power BI和Analysis Services模型。尽管DAX存在与不同的工具中，都共享相同的内部引擎"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是一种简单的语言。与需要花很多时间才能了解的编程语言相比，基础的DAX学习非常简单，只需几个小时即可。但当开始接触高级的内容如计算上下文，迭代，上下文转换，一切都觉得很复杂。不要害怕！保持耐心。当你开始理解这些概念后，会发现DAX其实是非常简单的语言。

evaluation contexts, iterations, and context transitions

第一章开始概括说明什么是数据模型，我们建议所有经验水平的读者仔细阅读本节，以熟悉整本书中都探讨的表格、模型和各种关系。

tables, models, and different kinds of relationships

在下面的部分中，对有Microsoft Excel、SQL和MDX经验的读者而言，会分别介绍DAX与这三种语言的比较。然后阅读最后一节"DAX for Power BI"，进入下一站开始DAX语言的真正旅程。

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