Cheat Sheet CDS ABAP

Data types in CDS ABAP and casting

In the CDS entities you can refer to the data elements of the Data Dictionary or directly to the ABAP data types. The data types must always fit exactly, otherwise there will be error messages when activating the CDS ABAP object. The SQL function CAST is used to convert the data types:

CAST( expression AS datatype [PRESERVING TYPE])

The ABAP data types are specified with abap.datatype, for example

• abap.int4 
• abap.char(length)
• abap.dec(length, decimal places)

Depending on the data type, the length and possibly also the number of decimal places must be specified in parentheses.   If the data type of the expression is technically identical to the specified data element, you can specify the addition PRESERVING TYPE. 

Session variables

There are a handful of session variables that can be accessed in CDS ABAP. 

$session.variable_name

So far there are the following variable names, which largely correspond to components of the SY or SYST structure in ABAP: 

• USER – The logged on ABAP user, equivalent to SY-UNAME. 
• CLIENT – The client to be used for the query. Corresponds to SY-MANDT, except for accesses with USING CLIENT or in AMDP with AMDP OPTIONS CDS SESSION CLIENT 
• SYSTEM_LANGUAGE – The logon language in internal ABAP format, equivalent to SY-LANGU
• SYSTEM_DATE – Current system date, equivalent to SY-DATUM
• SYSTEM_TIMEZONE – Time zone of the user, equivalent to SY-ZONLO
• USER_DATE – Current date of the user, equivalent to SY-DATLO

Arithmetic and operator expressions

In CDS ABAP the arithmetic operators for addition (+), subtraction (-) and multiplication (*) work as you would expect. However, the division differs from this. Basically, a whitespace is required before and after the division operator (/). 

• For operator expressions with ABAP.FLTP both operands must have this data type. This is especially disturbing since literals with decimal places are of the type ABAP.FLTP. The only thing that helps here is a CAST.
• Division with the division operator (/) is allowed in classic CDS views only for floating point data types. Therefore, use the SQL function DIVISION(numerator, denumerator, decimal places), which rounds automatically.
• For calculations that result in a decimal number, a check is made to ensure that the result is convenient for the ABAP.DEC data type. This means a maximum of 31 digits, 14 of which are after the decimal point. If this is not the case, the operands must be reduced beforehand per CAST. 

CASE Expressions

A CASE expression in CDS ABAP always returns exactly one value, depending on the conditions. The simple CASE expression compares an expression to several other expressions for equality:

CASE item_categ WHEN '10' THEN 'A'
                WHEN '20' THEN 'B'
                ELSE 'C'
    END 

The complex CASE expression (aka ‘searched case’) evaluates N independent conditions. The first case to be evaluated to TRUE, returns the result:

CASE WHEN weight_measure < 1 
       OR weight_unit = 'g' THEN 'A'
       WHEN weight_measure >= 1 
       AND weight_measure <= 5 THEN 'B'
       ELSE 'C'
   END 

If no condition was evaluated to TRUE, either the value from the ELSE clause or NULLis returned.

Associations

Associations can be used to describe the relationships between CDS entities. They define a JOIN that is executed only, if the fields are retrieved from the associated entity. These queries are created with so-called path expressions and can run across multiple CDS entities. 

DEFINE VIEW zcds_as_soi  //SalesOrderItem
    AS SELECT FROM snwd_so_i AS soi
    ASSOCIATION [1] TO snwd_pd AS _product 
    ON soi.product_guid = _product.node_key
    ASSOCIATION [1] TO zcds_as_so AS _sales_order 
    ON soi.parent_key = _sales_order.NodeKey
{
FeldListe
...
_product, 
_sales_order
}

The names of the associations always start with an underscore. They are included in the field list and thus they are available to the users of the CDS View. 

The cardinality can be specified in square brackets: [Min..Max]. If only Max is specified, the Min value is assumed to be 0. By default, it is [0..1]. Permitted values are e.g. [1], [*], [1..1], [1..*] or  [0..*].

Path expressions

Path expressions can be used to include fields from associated sources in the field list of the CDS view or query on the CDS entity. The path expressions can also go over several levels. It is also possible to add attributes to the evaluation of the associations, for example to filter or to define the join type:

_sales_order.BillingStatus,
_sales_order._buyer.company_name
_sales_order._buyer[LEFT OUTER 
         WHERE legal_form = 'GmbH'].company_name

In ABAP SQL

The path expressions can be used in ABAP SQL. However, the syntax is slightly different. Before associations, and as separator between them, there is always a backslash (\). The component is addressed (as usual in ABAP) with a hyphen (-):

SELECT \_sales_order\_buyer[LEFT OUTER
          WHERE legal_form = 'GmbH']-company_name
FROM zcds_my_view
INTO TABLE @DATA(lt_tmp).

NULL in CDS ABAP and ABAP

In the database there is the pseudo-value NULL, which signalizes the absence of a value. This is translated into an initial value in ABAP. This can lead to unexpected situations when aggregating, since two groups are formed – one for NULL and one for initial values in CDS ABAP – but both are initial in ABAP. 

Replace NULL

You can catch NULL values with the SQL function COALESCE(value1, value2), because then this function returns the 2nd value. 

Filter with NULL

A normal comparison with NULL always results in the logical value UNKNOWN. Thus, such a comparison is never valid in a WHERE condition. That is why the predicate IS (NOT) NULL is needed.