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sqloracle-databaseselect-query

select count(1) from tablename where column=column

Question from interview:

  • We have table with name = tablename and some column.
  • We don't know column type.
  • SQL request: select count(1) from tablename where column=column

What result does this request return and why?

Solution

  • This returns the number of rows where column is not null, because NULL = NULL yields UNKNOWN which evaluates to false in the condition. For all other values column = column is true.