MYSQL INTERVIEW QUESTIONS
Printed From: One Stop GATE
Category: GATE Technical Discussions
Forum Name: GATE CS
Forum Discription: General Technical Discussions, Queries, doubts etc. for GATE in CS.
URL: http://forum.onestopgate.com/forum_posts.asp?TID=343
Printed Date: 07Feb2025 at 7:51am
Topic: MYSQL INTERVIEW QUESTIONS
Posted By: Priya
Subject: MYSQL INTERVIEW QUESTIONS
Date Posted: 15Feb2007 at 9:59am
How we can count duplicate entery in particular table against Primary Key ? What are constraints?
Answer
:
The syntax in the previous answer (where count(*) > 1) is very
questionable. suppose you think that you have duplicate employee
numbers. there's no need to count them to find out which values were
duplicate but the followin SQL will show only the empnos that are
duplicate and how many exist in the table:
Select empno, count(*)
from employee
group by empno
having count(*) > 1
Generally speaking aggregate functions (count, sum, avg etc.) go in
the HAVING clause. I know some systems allow them in the WHERE clause
but you must be very careful in interpreting the result. WHERE COUNT(*)
> 1 will absolutely NOT work in DB2 or ORACLE. Sybase and SQLServer
is a different animal.
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