![]() This is by far the simplest solution and also quite easy to understand but it doesn't come to your mind without practicing. Now, if you check the #programming table again there won't be any duplicates. Over ( partition by name order by name) as rn Now, you can remove all the duplicates which are nothing but rows with rn > 1, as done by following SQL query: OVER ( partition by name order by name) as rn Now, it's easy to spot the duplicates in the derived table as shown in the following example: This way row number will restart as soon as a different name comes up but for the same name, all rows will get sequential numbers e.g. on a distinct clause in the first solution and on the partition by in the second solution.Īnyway, here is our temp table with test data, it is carefully constructed to have duplicates, you can see that C++ is repeated thrice while Java is repeated twice in the table.Ĭreate table #programming (name varchar(10)) - insert data with duplicate, C++ is repeated 3 times, while Java 2 times insert into #programming values ('Java') insert into #programming values ('C++') insert into #programming values ('JavaScript') insert into #programming values ('Python') insert into #programming values ('C++') insert into #programming values ('Java') insert into #programming values ('C++') - cleanup drop table #programming In such cases, you need to extend the solution by using those columns on key places e.g. name and city can be the same for two unique persons. In our table, I have just one column for simplicity, if you have multiple columns then the definition of duplicate depends on whether all columns should be equal or some key columns e.g. Since temp tables are cleaned up once you close the connection to the database, they are best suited for testing. I am using a temp table to avoid leaving test data into the database once we are done. It's a great course to start with T-SQL and SQL queries in SQL Server.īefore exploring a solution, let's first create the table and populate it with test data to understand both problems and solutions better. If you need a recommendation then I suggest you go through the Microsoft SQL for Beginners online course by Brewster Knowlton on Udemy. Now, let's see our solution to delete duplicate rows from a table in SQL Server.īy the way, if you are new to Microsoft SQL Server and T-SQL then I also suggest you join a comprehensive course to learn SQL Server fundamentals and how to work with T-SQL. This little bit of extra detail like row_number makes this problem challenging for many programmers who don't use SQL on a daily basis. Some candidate says that they will find duplicate by using group by and printing name which has counted more than 1, but when it comes to deleting this approach doesn't work, because if you delete using this logic both duplicate and unique row will get deleted. This is an interesting question because many candidates confuse themselves easily. I have shared a lot of good SQL-based problems on that article and users have also shared some excellent problems in the comments, which you should look at.ītw, this is the follow-up question of another popular SQL interview question, how do you find duplicate records in a table, which we have discussed earlier. The queries are also very interesting to check the candidate's logical reasoning ability.Įarlier, I have shared a list of frequently asked SQL queries from interviews and this article is an extension of that. No doubt that SQL queries are an integral part of any programming job interview which requires database and SQL knowledge. You can do that by using a common table expression (see T-SQL Fundamentals) or without it on Microsoft SQL Server. In this approach, all unique rows will have row number = 1 and duplicate rows will have row_number > 1, which gives you an easy option to remove those duplicate rows. ![]() It uses a ranking function like row_number() to assign a row number to each row.īy using partition by clause you can reset the row numbers on a particular column. The second approach doesn't require extra space as it removes duplicate rows directly from the table. This way, all duplicate rows will be removed, but with large tables, this solution will require additional space of the same magnitude as the original table. By using a temp table, you can first copy all unique records into a temp table and then delete all data from the original table and then copy unique records again to the original table. you can use temp tables or a window function like row_number() to generate artificial ranking and remove the duplicates. There are a couple of ways to remove duplicate rows from a table in SQL e.g. ![]()
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