O'Reilly logo
  • B2B Insights View thinks this is interesting:

Relational databases follow certain rules to ensure data integrity and to make sure the data is always accessible. The first integrity rule states that the rows in an RDBMS table should be distinct. For most RDBMSs, a user can specify that duplicate rows are not allowed, in which case the RDBMS prevents duplicates. The second integrity rule states that column values must not have repeating groups or arrays. The third integrity rule is about the concept of a NULL value. In an RDBMS, there might be a situation where the value of a column is not known, which means the data is not available. NULL does not mean a missing value or zero.