Orphan composite foreign keys

Orphan composite foreign keys

I would like to determine the values that are different between a composite foreign key and composite primary key. I cannot currently create this relationship because there are values in the child table that are not in the parent table. The foreign key is:

ALTER TABLE EDI.HM_MCSMD
 ADD CONSTRAINT HM_MCSMD_02_FK 
     FOREIGN KEY
       ( TPR_PCSG_CO_ID
       , MXFG_ID
       , MCSMS_TRNS_HDR_ID
       , MCSMS_MVEI_ID )
    REFERENCES EDI.HM_MCSMS
       ( TPR_PCSG_CO_ID
       , MXFG_ID
       , MCSMS_TRNS_HDR_ID
       , MCSMS_MVEI_ID )
    ENABLE

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You need to investigate and fix these anomalies, perhaps by deleting the child rows, or adding the parent rows that are missing. Here's the query to find them:

select TPR_PCSG_CO_ID
     , MXFG_ID
     , MCSMS_TRNS_HDR_ID
     , MCSMS_MVEI_ID
  from EDI.HM_MCSMD   as D
 where not exists
       ( select 1 
           from EDI.HM_MCSMS
          where TPR_PCSG_CO_ID 
            = D.TPR_PCSG_CO_ID
            and MXFG_ID 
            = D.MXFG_ID
            and MCSMS_TRNS_HDR_ID
            = D.MCSMS_TRNS_HDR_ID
            and MCSMS_MVEI_ID
            = D.MCSMS_MVEI_ID )

There are a number of ways to write a query to find missing related rows, but I like NOT EXISTS with a correlated subquery because it says what you want. Well, except for the SELECT 1 bit, but that's another story (see How does WHERE EXISTS ( SELECT NULL... ) work? 22 February 2002).

If your decision to fix the anomalies is to delete the orphans, just change the query above to say DELETE FROM instead of SELECT ... FROM.

On the other hand, if you will be creating parent rows for the orphans to relate to, feed the query above into an INSERT statement:

insert 
  into EDI.HM_MCSMS
     ( TPR_PCSG_CO_ID
     , MXFG_ID
     , MCSMS_TRNS_HDR_ID
     , MCSMS_MVEI_ID )
select ...

The insert will only work if all the other columns in the parent table, i.e. the non-key columns not mentioned in the INSERT column list, have default values or are allowed to go null.

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This was first published in February 2003

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