It is very important that a developer understands that technically, CDS is an enhancement of SQL which provides a Data Definition Language (DDL) for defining semantically rich database tables/views (CDS entities) and user-defined types in the database. Unlike the SAP HANA CDS, ABAP CDS are independent of the database system. The entities of the models defined in ABAP CDS provide enhanced access functions compared with existing database tables and views defined in ABAP Dictionary, making it possible to optimize Open SQL-based applications. And it is because of these unparalleled advantages that ABAP CDS is the most preferred form of methodology when it comes to Code to Data paradigm.

1. In the first Blog, we covered how SAP introduced a new data
modeling infrastructure in the form of Core Data Services and how a
CDS view is different from SE11 view. The focus of this blog will be
entirely on the steps involved in creating CDS View using Eclipse, on
different types of CDS views, and on how to use CDS view in ABAP
program.
It is very important that a developer understands that technically,
CDS is an enhancement of SQL which provides a Data Definition
Language (DDL) for defining semantically rich database tables/views
(CDS entities) and user-defined types in the database. Unlike the SAP
HANA CDS, ABAP CDS are independent of the database system. The
entities of the models defined in ABAP CDS provide enhanced access
functions compared with existing database tables and views defined
in ABAP Dictionary, making it possible to optimize Open SQL-based
applications. And it is because of these unparalleled advantages that
ABAP CDS is the most preferred form of methodology when it comes
to Code to Data paradigm.
DDL and CDS view Entity
When creating a CDS view in ADT tool, 2 entities get created.
1. DDL SQL view: DDL SQL view is read-only classical database view
(SE 11 view) which is visible in ABAP data dictionary(SE11).
2. CDS view entity: CDS view entity is a DDL source file and the
actual CDS view. It is a database object which is visible in ADT
tool. This is not visible in SE11.
Following points should be considered for DDL SQL view and CDS view
entity:
1. SQL view and CDS view entity can have the same name but
should be kept differently.
2. SQL view can be viewed in SE11 but it is not editable there
whereas CDS view entity is not available in SE11.
3. CDS view entity is used as a data type in ABAP program and as
data source in open SQL. SQL view can be used in a similar way,
but it behaves like a classical SE11 view.
4. CDS view entity can be deleted. Both SQL view and CDS view are
deleted together.
Steps to create CDS view:
1. Download Eclipse and ADT plugin
First, prepare development environment for CDS view. Eclipse is a
tool where a user creates ABAP developments on HANA. Before
downloading eclipse, ensure that JRE and SAP GUI are available on
system.
Blog
Technical Overview of CDS View for ABAPer - Part II
By Rashmi Bansal, Accrete Solutions
Once Eclipse has been installed, open it by a double-click on its
icon. Now, get ADT plugin to connect with ABAP backend system.
Go to HELP-> Install new software.
Click on the ADD button, give a name to plugin and location as a
site (https://tools.hana.ondemand.com/oxygen)
Click on next, accept terms and agreement, finish this
process. Wait for 2-3 minutes, Eclipse will ask for a restart for
itself. After restarting eclipse, click on the workbench icon.
Choose ‘ABAP Development tools for SAP Netweaver’
2. Create ABAP project Go to file->new->project->ABAP
project.

2. Click on next. Enter system information.
Click on next. Give a name of CDS view.
Click on next button. Enter username and password.
Click on next. It will ask for the project name. Enter the project
name and finish this process. Choose ABAP perspective. The
project will be available with folder Local objects($tmp).
3. Create CDS view
Right click on local object folder. Go to new->other ABAP
repository objects->core data services->Data definition or search
for DDL source.
Click on next. Next screen would ask for a transport request, assign a
new name to it.
Click on next. Here it will ask to choose one template for CDS view.
There are 5 options available, I have chosen the 1st option.
Click on finish. It will open CDS view in source code editor where
provide a name for DDL SQL view i.e. visible in SE11 and data source
name(table name). Notice how the description and CDS view name is
already assigned here.
Below example is a display of material information from MARA table.
Save and activate view.
Run as ABAP objects. In the above example, CDS view
ZCDS_SQL_MAT1 is visible in SE11. But view ZCDS_MATERIAL is not
available in SE11.

3. CDS view with parameters
With this developer can pass parameters to CDS views which
can be used in where clause of a select query. For example, if it
is required to fetch material data for a plant then pass plant as
parameters. You need to use ‘$parameters’ with parameter’s
name.
CDS view with Joins
When it is required to fetch data from multiple tables, use
joins. CDS supports inner join, left/outer join and cross joins.
There is no limit on the number of join expressions in a SELECT
statement of a CDS view in the DDL, but there is an ATC check
that produces a message once a specific number of expressions
is reached. In nested join expressions, parentheses are
recommended for making the code easier to read. In the case
of inner and outer joins, the parentheses can be specified
wherever the ON conditions specify parentheses implicitly.
Below is an example of CDS view with joins and parameters
where order type and plant number is passed as a parameter to
fetch all order numbers, order status, material number and
serial number information where material type is ‘finished
goods' and movement type is ‘261’.
CDS view with Associations
Associations allow us to replace joins with simple path
expressions in queries. Association is basically not a join as
such, it is just metadata about possible join connections. It’s
Join on Demand. Actual join is created when the association is
used in a path expression.
To understand how the association works, have a look in below
CDS view where a CDS view with the association on table
MARA, MAKT is created.
The above CDS, when executed, will give the below output.
It means no joins have taken place in association on table MARA and
MAKT. This association will work when it is specifically called in path
expression in the select query. Find below example where CDS view is
called in select query and association is also called in select query
path expression.

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By default, association gets converted into the left outer join. To make association as an inner join, specify it as view itself. In association,
cardinality shows a minimum and a maximum number of records, it does not show one to one or one to many relations.
In a nutshell
To summarize, an ABAP CDS, because of its abstract data modeling techniques, is much more significant compared to the options that were
available earlier in the data dictionary. The different types of ABAP CDS provide the whole plethora of using this abstract data modeling
techniques thereby making it possible to write easier yet semantically enriched data models for different kinds of applications, for example,
search applications, analytical applications, and transactional applications. The application logic is moved from the application server to the
database server ("code push down") and the data is processed where it is saved, thereby improving the performance.
In next blog, we will discuss CDS with Join Vs CDS with Association