MySQL: Find Duplicate Entries in Database

With this MySQL query, we get all records that have the same value more than once in a given column:

SELECT * FROM tab GROUP BY col HAVING COUNT(*) > 1;

Thus we find all records from the table "tab", which have duplicates in the column "col".

We can also use the reverse of this query to find all records that exist only once in the table:

SELECT * FROM tab GROUP BY col HAVING COUNT(*) = 1;

Of course, you have to replace "col" in the query with the name of the column in which you would like to search for duplicates. Similarly, you have to substitute "tab" with the name of your table.