本文阐述了ABAP CDS association的概念，并且展示了在CDS视图中和SQL语句中写路径表达式（Path Expression）代码的方法。我也会解释如何在CDS asociation中指定inner join——默认情况下是left outer join，以及如何为association添加过滤。
对于CDS的相关开发，SAP希望我们使用association而不是join，因为association更加接近“概念思维”。基本上，association本身不是join，它只是有关join连接可能性的元数据，它会按需成为join。真实的join会在路径表达式使用association的时候被创建。
一个简单的CDS association例子，它看起来和left outer join没区别：
@AbapCatalog.sqlViewName: 'ZCDS_ASSOC11'
define view zcds_assoc1 as select from scarr as scaassociation [0..1] to spfli as _spfli on sca.carrid = _spfli.carrid { * }暴露CDS association的例子：
@AbapCatalog.sqlViewName: 'ZCDS_ASSOC41'
@AccessControl.authorizationCheck: #NOT_REQUIRED
define view zcds_assoc4 as select from sairport as sairassociation [1..*] to spfli as _spfli on$projection.airportfrom = _spfli.airpfrom{ sair.id as airportfrom,sair.name,sair.time_zone,-- exposing association_spfli}在下面的例子里，你可以看到SPFLI表对SFLIGHT表和SAIRPORT表的association。通过别名alias _sfli和SFLIGHT和_sair，SAIRPORT的全部字段暴露在projection列表中。当路径表达式用于调用association时，会根据选择字段创建join条件：spfli.carrid = sflight.carrid and spfli.connid = sflight.connid and on spfli.airport = sairport.id。
@AbapCatalog.sqlViewName: 'ZCDS_ASSOC21'
@AccessControl.authorizationCheck: #NOT_REQUIRED
define view zcds_assoc2 as select from spfli association to sflight as _sfli on spfli.carrid = _sfli.carrid andspfli.connid = _sfli.connidassociation [1..1] to sairport as _sair on$projection.airportfrom = _sair.id{spfli.carrid,spfli.connid,spfli.airpfrom as airportfrom,-- exposing association_sfli,_sair }在下面的例子里，定义association时使用了上面的CDS entity，ZCDS_ASSOC2 ：
@AbapCatalog.sqlViewName: 'ZCDS_ASSOC31'
@AccessControl.authorizationCheck: #NOT_REQUIRED
define view zcds_assoc3 as select from scarrassociation [1..1] to zcds_assoc2 as _cdsassoc on$projection.carrierid = _cdsassoc.carrid{scarr.carrid as carrierid,-- Exposing Association _cdsassoc}
Open SQL语句中的路径表达式：在SQL语句中调用CDS association，需要使用如下的路径表达式。在上面的CDS association中，通过_cdsassoc暴露了完整的projection列表。
DATA(w_dbfeature) = cl_abap_dbfeatures=>use_features( requested_features = VALUE #( ( cl_abap_dbfeatures=>views_with_parameters ) ) ).
IF w_dbfeature IS NOT INITIAL.SELECT carrierid ,\_cdsassoc\_sfli-fldate AS flightdate,\_cdsassoc\_sair-name AS flightnameFROM zcds_assoc3WHERE carrierid = @carridINTO TABLE @DATA(t_data1).
ENDIF.
CDS视图中的路径表达式：
@AbapCatalog.sqlViewName: 'ZCDS_ON_ASSOC1'
define view zcds_on_assoc with parameters airport: S_FROMAIRP as select from zcds_assoc2 as cds2{cds2.carrid,cds2.connid,cds2.airportfrom, cds2._sair.name, -- use inner join...by default association uses left outer joincds2._sfli.planetype}where cds2.airportfrom = :airport
如我所提到的那样，在被路径表达式调用时，CDS association会默认创建left outer join。
在SPFLI表数据中，没有RTM机场的航班。
当我们输入参数airport = RTM的时候 ，下面的CDS视图的查询结果会是一条RTM机场的数据，但是这条记录里没有carrid。
@AbapCatalog.sqlViewName: 'ZCDS_ON_ASSOC41'
define view zcds_on_assoc4 with parameters airport: S_FROMAIRP as select from zcds_assoc4 as cds_assoc{cds_assoc.airportfrom,cds_assoc.name,cds_assoc.time_zone, cds_assoc._spfli.carrid -- use inner join...by default association uses left outer join}where cds_assoc.airportfrom = :airport
运行上面的CDS视图：
data preview中的结果：
为了把默认的left outer join变成inner join，我们需要使用[inner]，如下：
@AbapCatalog.sqlViewName: 'ZCDS_ON_ASSOC41'
define view zcds_on_assoc4 with parameters airport: S_FROMAIRP as select from zcds_assoc4 as cds_assoc{cds_assoc.airportfrom,cds_assoc.name,cds_assoc.time_zone, cds_assoc._spfli[inner].carrid -- use inner join...by default association uses left outer join}where cds_assoc.airportfrom = :airport
如果运行它，输入参数RTM，得到的结果为空：
目前，还不可以在CDS association中使用right outer join。
CDS association中的过滤例子如下：
@AbapCatalog.sqlViewName: 'ZCDS_ASSOC_FILT'
define view ZCDS_ASSOC_FILTER as select from zcds_assoc2 as cds2{cds2._sair[ id = 'TYO' ].name,cds2._sfli.planetype}
CDS视图的输出结果：
