When relational databases (RDBs) appeared there was a deal of confusion about what they were. Everybody now knows that they are only a collection of very simple records in ‘card‐indexed’ files, where anything is automatically cross‐referenced to everything else, but as late as 1986 I had to explain to the MD of a software house that his product was not relational even though it allowed cross‐references to be built between files. Relational databases provide automatic cross‐referencing and keep it up to date. They also check it is not self‐contradictory, or in RDB jargon, ensure it has ‘referential integrity’.
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