How To Find Duplicate Values in MySQL

Find duplicate values in one column

SELECT 
    col, 
    COUNT(col)
FROM
    table_name
GROUP BY col
HAVING COUNT(col) > 1;

Find duplicate values in multiple columns

SELECT 
    col1, COUNT(col1),
    col2, COUNT(col2),
    ...

FROM
    table_name
GROUP BY 
    col1, 
    col2, ...
HAVING 
       (COUNT(col1) > 1) AND 
       (COUNT(col2) > 1) AND 
       ...