ETL is closely related to data modeling,[3] and for transformation, Power Query can be used to develop a logical data model in those cases where the data does not already have one, or where there is a need to further develop the data model.
History
Power Query was first announced in 2011 under the codename "Data Explorer" as part of Azure SQL Labs. In 2013, in order to expand on the self-service business intelligence capabilities of Microsoft Excel, the project was redesigned to be packaged as an add-in Excel and was renamed "Data Explorer Preview for Excel",[4] and was made available for Excel 2010 and Excel 2013.[5] In July 2013 the add-in was removed from preview and renamed to "Power Query".[6][7] Monthly updates for the add-in were released until 2016, when Power Query was included in Excel natively. In Excel 2016, the function was renamed "Get & Transform"[8] for a short time, but has since been changed back to Power Query.
In April 2017, Power Query was made available in Microsoft Analysis Services. With the launch of the Common Data Service in March 2018, Power Query was included as its main data import tool.
M Formula language
Power Query is built on what was then[when?] a new query language called M. It is a mashup language (hence the letter M) designed to create queries that mix together data. It is similar to the F# programming language, and according to Microsoft it is a "mostly pure, higher-order, dynamically typed, partially lazy, functional language." The M language is case-sensitive.
Much of the user interaction with Power Query can be done via graphical user interfaces with wizards, and this can be used for many common or basic tasks. It is also possible to use the advanced editing mode where the developer can write in the M formula language; this gives greater expressive power, more possibilities, and can also be used to change the code generated by the graphical wizards.
Let expression
User queries are typically written with a top level let expression. The let expression contains a list of comma-separated named reference (variables) bindings and an in expression which is what the let expression evaluates to. The in expression can reference the variables and the variables can reference each other. Backwards and forward referencing is allowed, and self-referencing is allowed by prefixing the @ on the variable. Variables are recursively evaluated as needed to evaluate the in expression. No variable is evaluated more than once.
Variables are not typed in Power Query. Instead, an expression can have a type assertion which will evaluate to an error when the expression does not evaluate to a value compatible with the assertion. Assertions can be preceded by nullable to include null in the allowed values.
Datatypes/Assertions
Name
Description
Datatype
Assertion
number
Assertion for integer and floating-point numbers
No
Yes
int
Signed 32-bit integer
Yes
No
long
Signed 64-bit integer
Yes
No
double
IEEE 754 float
Yes
No
decimal
128-bit float. Same as C#'s decimal
Yes
No
time
Time of day
Yes
Yes
date
A calendar date ranging from 1 CE to 9999 CE in the Georgian Calendar
Yes
Yes
datetime
A composite of the date and time datatypes
Yes
Yes
duration
A measurement of elapsed time (can be negative)
Yes
Yes
logical
Represents a Boolean true or false value
Yes
Yes
text
A Unicode string
Yes
Yes
guid
A Globally Unique Identifier (Converts to a text as needed automatically)
Yes
No
list
An ordered list of values
Yes
Yes
record
An ordered map from text to any value
Yes
Yes
table
A 2D matrix where each column has a unique name and type (type not checked on table contents)
Yes
Yes
function
A power query function
Yes
Yes
type
Represents a datatype and may contain assertion information
Yes
Yes
action
An internally used datatype
Yes
Yes
null
The null singleton
Yes
Yes
any
Represents all values
No
Yes
anynonnull
Represents all values except null
No
Yes
none
Represents no values and always fails as an assertion
No
Yes
error
A pseudo value representing an error
No
No
Comments
Power Query supports the C block (/* ... */) and C line (// ...) comments.
DirectQuery
In Power BI, use of M-code is somewhat limited in DirectQuery, as opposed to Import which has all capabilities. This is due to the requirement that M-code in DirectQuery has to be translated from DAX into SQL and then into the data source's native query language (if it isn't SQL) upon user interaction with the data.
Query Folding
Query Folding is the ability for the Power Query steps to be transpiled into a single query at the data source (for example in Transact SQL). As such, Query Folding works like a traditional ETL process, and enables working on the data before loading. Query Folding is not always supported. Steps like filtering, selecting columns and simple SQL arithmetic are supported. Steps like creating index and appending or merging non foldable sources with foldable sources are not. Folding indicators (such as folding, not folding, might fold, opaque, unknown) might indicate up to which step a query might fold. Non-folding queries will have to be performed on the client-side. The order of queries can determine how many of the steps which get folded.
Connectors
A connector in power query is a library which enables queries to connect to a datasource. End users of Power BI can write custom connectors in the Power Query language. If a user is the owner/maintainer of the datasource technology they can request Microsoft certify their connector and include it in future releases of Power Bi after the review of the code. Connectors that are not certified can still be shared with other people who enable developer mode in Power Bi, or by Power Bi online domain administrators.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
