if a table contains a primary key there can be no duplicate records in a table
Table (database) In terms of the relational model of databases, a table can be considered a convenient representation of a relation, but the two are not strictly equivalent. For instance, an SQL table can potentially contain duplicate rows, whereas a true relation cannot contain duplicate tuples. Similarly, representation as a table implies a particular ordering to the rows and columns, whereas a relation is explicitly unordered. However, the database system does not guarantee any ordering of the rows unless an ORDER BY clause is specified in the SELECT statement that queries the table.
Blue moon Owing to the rarity of a blue moon, the term "blue moon" is used colloquially to mean a rare event, as in the phrase "once in a blue moon".[4][5]