The way to Discover Duplicate values in a Desk? SQL GROUP BY and Having Instance| Leetcode Answer

1. Discovering Duplicate parts By utilizing GROUP BY

The only answer to this drawback is by utilizing the GROUP BY and HAVING Clause. Use GROUP BY to group the consequence set on e-mail, this can convey all duplicate emails in a single group, now if the depend for a selected e-mail is bigger than 1 it means it’s a duplicate e-mail. Right here is the SQL question to search out duplicate emails :

# Write your MySQL question assertion under

SELECT Electronic mail FROM Particular person 
GROUP BY Electronic mail 
HAVING COUNT(Electronic mail) > 1

That is additionally my accepted reply on LeetCode. You’ll be able to see by utilizing the depend perform you’ll be able to depend a lot of parts within the group and in case your group comprises greater than 1 row then it is a duplicate worth that you simply wish to print. 

2. Discovering Duplicate values in a column By utilizing Self Be part of

By the best way, there are a few extra methods to unravel this drawback, one is by utilizing Self Be part of. For those who keep in mind, In Self Be part of we be a part of two situations of the identical desk to match one report to a different. Now if an e-mail from one report within the first occasion of the desk is the same as the e-mail of one other report within the second desk it means the e-mail is duplicate. Right here is the SQL question utilizing Self Be part of

# Write your MySQL question assertion under

SELECT DISTINCT a.Electronic mail FROM Particular person a 
JOIN  Particular person b ON a.Electronic mail = b. Electronic mail 
WHERE a.Id != b.Id 

Keep in mind to make use of the key phrase distinct right here as a result of it’s going to print the duplicate e-mail as many instances it seems within the desk. That is additionally an accepted answer in Leetcode. If you wish to be taught extra about how be a part of works and find out how to use left and proper out be a part of, I counsel you be a part of an SQL course that works with actual information like SQL for Newbs: Knowledge Evaluation for Learners on Udemy.

3. Discovering duplicate emails By utilizing Sub-query with EXISTS:

You’ll be able to even resolve this drawback utilizing a correlated subquery. In a correlated subquery, the internal question is executed for every report within the outer question. So one e-mail is in comparison with the remainder of the e-mail in the identical desk utilizing a correlated subquery and EXISTS clause in SQL as proven under. 

SELECT DISTINCT p1.Electronic mail
FROM Particular person p1
WHERE EXISTS(
    SELECT *
    FROM Particular person p2
    WHERE p2.Electronic mail = p1.Electronic mail
    AND p2.Id != p1.Id
)

For those who nonetheless want extra steering then becoming a member of a complete SQL course like The Full SQL Bootcamp course by Jose Portilla on Udemy can also be a good way to know find out how to discover duplicate parts in a desk utilizing GROUP BY  with HAVING clause.

That is all about find out how to discover duplicates in SQL utilizing the GROUP BY and HAVING clause. I’ve additionally proven you how one can resolve this drawback utilizing Self-join and a subquery with the EXISTS clause as effectively. When you get accustomed to the sample you’ll be able to resolve many such issues. If you wish to be taught extra take a look at the next assets.

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P.S. – If you’re inquisitive about studying Database and SQL and on the lookout for some free assets to begin your journey, then you may also check out this listing of Free SQL Programs for Learners to kick-start your studying.