How to Find Duplicate Values in SQL

 Finding duplicates values in SQL comprises two key steps:

1. Using the GROUP BY clause to group all rows by the target column(s) – i.e. the column(s) you want to check for duplicate values on.
2. Using the COUNT function in the HAVING clause to check if any of the groups have more than 1 entry; those would be the duplicate values.

Examples : 

Duplicate Values in One Column

SELECT OrderID, COUNT(OrderID)

FROM Orders

GROUP BY OrderID

HAVING COUNT(OrderID) > 1




Duplicate Values in Multiple Columns

SELECT OrderID, ProductID, COUNT(*)

FROM OrderDetails

GROUP BY OrderID, ProductID

HAVING COUNT(*) > 1