Duplicate Registry Numbers

In addition to these unintended duplications, which should eventually be reconciled by CAS, there are many quasi-duplicates resulting from the liberal allocation of numbers. 

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The data base that is used in the CNMR search system consists currently of 4,100 CNMR spectra (3). As in the case of the MSSS, every compound has a CAS registry number, and all exact duplicate spectra have been removed from the file. A specific compound may still appear in this file more than once, however, because its CNMR spectrum may have been recorded in different solvents. The CNMR file is still small but is growing at a fairly steady rate and should benefit considerably from recent international agreements to the effect that all major compilations of CNMR data will, in the future, be pooled. 

CAS sometimes finds it necessary to register substances without full knowledge of their structures. Examples are trivially named natural products and trade name materials. This may lead to unintentional duplication in the registry system since the actual material may be indexed at another CA index name based on information from another literature source. Similar problems may arise when more than one structure is reported for the same chemical substance. When it is recognised that duplication has occurred and that a substance has been assigned two registry numbers, one of the numbers is retained as the preferred number, to which the other one is cross-referred. 

Like the natural join, the Cousin join combines the rows of the joined tables by equal registry number and produces a single registry number column in the result table. The duplicate data problem is eliminated by allowing each row of the joined tables to appear once, at most, in the result table. Additionally, the Cousin join provides the capability of the outer join to supply null values for missing data. For example, if the query above was performed and there was no data in the... 

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