After reading a lot of materials related to the Primary key and Unique key, I am in doubt. Let us consider a primary key (AB) in table R (ABCDE). AB together can not be null as AB is a primary key. But, is it possible for a row that A is null? Or, B is null?

  • $\begingroup$ yes, It is possible for "either" of them to be null but not both as both being null would make the primary key null. This further depends on whether A and B allow null values or not $\endgroup$
    – Rinkesh P
    Mar 14, 2022 at 13:21
  • $\begingroup$ Thank you @RinkeshP, but then it is going to be the same with a unique key or not? Am I wrong? $\endgroup$
    – A Paul
    Mar 14, 2022 at 14:16
  • $\begingroup$ A unique key would allow at most 1 null value but a primary key won't allow null at all. $\endgroup$
    – Rinkesh P
    Mar 14, 2022 at 14:23


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