Delete duplicate rows with same id in mysql

Delete duplicate rows with same id in mysql

Joinsubscribers and get a daily digest of news, geek trivia, and our feature articles. When you are working with spreadsheets in Microsoft Excel and accidentally copy rows, or if you are making a composite spreadsheet of several others, you will encounter duplicate rows which you need to delete. This can be a very mindless, repetitive, time consuming task, but there are several tricks that make it simpler.

Today we will talk about a few handy methods for identifying and deleting duplicate rows in Excel. Once you have downloaded and opened the resource, or opened your own document, you are ready to proceed. If you are using Microsoft Office Suite you will have a bit of an advantage because there is a built in feature for finding and deleting duplicates. Begin by selecting the cells you want to target for your search.

Once you have clicked on it, a small dialog box will appear. You will notice that the first row has automatically been deselected. In this case, all the rows with duplicate information except for one have been deleted and the details of the deletion are displayed in the popup dialog box.

Let us start again by opening up the Excel spreadsheet. In this case, two were left because the first duplicates were found in row 1. This method automatically assumes that there are headers in your table. If you want the first row to be deleted, you will have to delete it manually in this case. If you actually had headers rather than duplicates in the first row, only one copy of the existing duplicates would have been left. This method is great for smaller spreadsheets if you want to identify entire rows that are duplicated.

delete duplicate rows with same id in mysql

You will need to begin by opening the spreadsheet you want to work on. Once it is open, you need to select a cell with the content you want to find and replace and copy it. The reason for this is that sometimes your word may be present in other cells with other words. If you do not select this option, you could inadvertently end up deleting cells that you need to keep. Ensure that all the other settings match those shown in the image below.

The reason we used the number 1 is that it is small and stands out. Now you can easily identify which rows had duplicate content. The Best Tech Newsletter Anywhere. Joinsubscribers and get a daily digest of news, comics, trivia, reviews, and more. Windows Mac iPhone Android. Smarthome Office Security Linux. The Best Tech Newsletter Anywhere Joinsubscribers and get a daily digest of news, geek trivia, and our feature articles. Skip to content. How-To Geek is where you turn when you want experts to explain technology.

Since we launched inour articles have been read more than 1 billion times.

Subscribe to RSS

Want to know more?A few days ago I faced an issue in my Ruby on Rails application. The issue was caused by some duplicate rows in a very large the MySQL table more than 1,5 million rows. The obvious solution of course is to remove duplicate rows from the table and add a unique constraint on some columns to make sure no duplicate rows will ever appear again. I want to make sure that combined value will always be unique.

What is the issue with this method? It is a self-join and it is extremely slow. Remember that we have more than 1 million rows so don't expect to see the above query complete soon. Luckily, I found another solution which seems to fit my requirement.

The query is:. But I soon realized it did not work. Finally I found the "best" solution. The steps:. That's it. The script took about 20 seconds to copy data from the original table into the temporary table, but everything else is super fast. The question is how to remove duplicate rows? Searching on Google and I quickly found the solution. The steps: Create a temporary table similar with similar schema to the existing table add UNIQUE constraint to the columns we want run INSERT IGNORE to copy data from the original table to the temporary table, any duplicate rows will not be inserted into the temporary table because they violates the UNIQUE constraint and we simply ignore the error rename the original table to something else and rename temporary table to the original table.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Like so:. This will drop all the duplicate rows. As always, you may want to take a backup before running something like this Since you have a column which has unique IDs e. MySQL has restrictions about referring to the table you are deleting from. You can work around that with a temporary table, like:. This query could be faster:.

Deleting duplicates on MySQL tables is a common issue, that's genarally the result of a missing constraint to avoid those duplicates before hand. But this common issue usually comes with specific needs The approach should be different depending on, for example, the size of the data, the duplicated entry that should be kept generally the first or the last onewhether there are indexes to be kept, or whether we want to perform any additional action on the duplicated data.

This limitation can be overcome by using an inner query with a temporary table as suggested on some approaches above. But this inner query won't perform specially well when dealing with big data sources. However, a better approach does exist to remove duplicates, that's both efficient and reliable, and that can be easily adapted to different needs.

The general idea is to create a new temporary table, usually adding a unique constraint to avoid further duplicates, and to INSERT the data from your former table into the new one, while taking care of the duplicates. This approach relies on simple MySQL INSERT queries, creates a new constraint to avoid further duplicates, and skips the need of using an inner query to search for duplicates and a temporary table that should be kept in memory thus fitting big data sources too.

This is how it can be achieved. Given we have a table employeewith the following columns:.

delete duplicate rows with same id in mysql

In order to delete the rows with a duplicate ssn column, and keeping only the first entry found, the following process can be followed:. Chetanfollowing this process, you could fast and easily remove all your duplicates and create a UNIQUE constraint by running:. Of course, this process can be further modified to adapt it for different needs when deleting duplicates.

Some examples follow. Sometimes we need to perform some further processing on the duplicated entries that are found such as keeping a count of the duplicates. Sometimes we use an auto-incremental field and, in order the keep the index as compact as possible, we can take advantage of the deletion of the duplicates to regenerate the auto-incremental field in the new temporary table.

Many further modifications are also doable depending on the desired behavior. As an example, the following queries will use a second temporary table to, besides 1 keep the last entry instead of the first one; and 2 increase a counter on the duplicates found; also 3 regenerate the auto-incremental field id while keeping the entry order as it was on the former data.

Then we have a different solution. I forgot to tell you that this query doesn't remove the row with the lowest id of the duplicated rows. If this works for you try this query:.

How to Remove All Duplicate Rows Except One in SQL?

The faster way is to insert distinct rows into a temporary table. Using delete, it took me a few hours to remove duplicates from a table of 8 million rows.

Using insert and distinct, it took just 13 minutes. This will succeed only on one of the duplicated rows because of the new constraint. I suggest that you keep the constraint you added, so that new duplicates are prevented in the future.We should follow certain best practices while designing objects in SQL Server. For example, a table should have primary keys, identity columns, clustered and non-clustered indexes, constraints to ensure data integrity and performance.

Even we follow the best practices, and we might face issues such as duplicate rows. We might also get these data in intermediate tables in data import, and we want to remove duplicate rows before actually inserting in the production tables. Suppose your SQL table contains duplicate rows and you want to remove those duplicate rows. Many times, we face these issues. It is a best practice as well to use the relevant keys, constrains to eliminate the possibility of duplicate rows however if we have duplicate rows already in the table.

We need to follow specific methods to clean up duplicate data. This article explores the different methods to remove duplicate data from the SQL table. For example, execute the following query, and we get those records having occurrence greater than 1 in the Employee table. We require to keep a single row and remove the duplicate rows.

We need to remove only duplicate rows from the table. For example, the EmpID 1 appears two times in the table. We want to remove only one occurrence of it. In the following screenshot, we can see that the above Select statement excludes the Max id of each duplicate row and we get only the minimum ID value. To remove this data, replace the first Select with the SQL delete statement as per the following query.

Once you execute the delete statement, perform a select on an Employee table, and we get the following records that do not contain duplicate rows. It is available starting from SQL Server In the output, if any row has the value of [DuplicateCount] column greater than 1, it shows that it is a duplicate row.

In the screenshot, you can note that we need to remove the row having Rank greater than one.

sql - Remove duplicate rows in MySQL

SQL Server integration service provides various transformation, operators that help both administrators and developers in reducing manual effort and optimize the tasks. We can use a Sort operator to sort the values in a SQL table. You might ask how data sorting can remove duplicate rows? For the configuration of the Sort operator, double click on it and select the columns that contain duplicate values.

We can also use the ascending or descending sorting types for the columns. The default sort method is ascending. In the sort order, we can choose the column sort order. Sort order 1 shows the column which will be sorted first. On the bottom left side, notice a checkbox Remove rows with duplicate sort values. It will do the task of removing duplicate rows for us from the source data. We can add SQL Server destinations to store the data after removing duplicate rows.

We only want to check that sort operator is doing the task for us or not. To view the distinct data, right-click on the connector between Sort and Multicast.Like so:. This will drop all the duplicate rows. As always, you may want to take a backup before running something like this Since you have a column which has unique IDs e. MySQL has restrictions about referring to the table you are deleting from.

You can work around that with a temporary table, like:. This query could be faster:. Deleting duplicates on MySQL tables is a common issue, that's genarally the result of a missing constraint to avoid those duplicates before hand. But this common issue usually comes with specific needs The approach should be different depending on, for example, the size of the data, the duplicated entry that should be kept generally the first or the last onewhether there are indexes to be kept, or whether we want to perform any additional action on the duplicated data.

This limitation can be overcome by using an inner query with a temporary table as suggested on some approaches above.

Clustered and nonclustered indexes in sql server Part 36

But this inner query won't perform specially well when dealing with big data sources. However, a better approach does exist to remove duplicates, that's both efficient and reliable, and that can be easily adapted to different needs. The general idea is to create a new temporary table, usually adding a unique constraint to avoid further duplicates, and to INSERT the data from your former table into the new one, while taking care of the duplicates. This approach relies on simple MySQL INSERT queries, creates a new constraint to avoid further duplicates, and skips the need of using an inner query to search for duplicates and a temporary table that should be kept in memory thus fitting big data sources too.

delete duplicate rows with same id in mysql

This is how it can be achieved. Given we have a table employeewith the following columns:. In order to delete the rows with a duplicate ssn column, and keeping only the first entry found, the following process can be followed:. Chetanfollowing this process, you could fast and easily remove all your duplicates and create a UNIQUE constraint by running:. Of course, this process can be further modified to adapt it for different needs when deleting duplicates. Some examples follow. Sometimes we need to perform some further processing on the duplicated entries that are found such as keeping a count of the duplicates.

Sometimes we use an auto-incremental field and, in order the keep the index as compact as possible, we can take advantage of the deletion of the duplicates to regenerate the auto-incremental field in the new temporary table. Many further modifications are also doable depending on the desired behavior. As an example, the following queries will use a second temporary table to, besides 1 keep the last entry instead of the first one; and 2 increase a counter on the duplicates found; also 3 regenerate the auto-incremental field id while keeping the entry order as it was on the former data.

I forgot to tell you that this query doesn't remove the row with the lowest id of the duplicated rows.

delete duplicate rows with same id in mysql

If this works for you try this query:.Something is not right. Your beautifully designed application is behaving oddly. After some investigation, you discover a problem in the code logic, and realise the database table contains duplicate rows. And now you have to delete them. Why does your table contain duplicates? The answer in the vast majority of cases is that your table has been badly designed.

There is no one else to blame but the database designer. So before you fix the symptom and I know it's urgent and I'm getting thereyou need to think about fixing the cause of the problem.

All tables should have a unique primary key. Not having one is an exception that is usually not wise. I suggest you read some more about the topic of database normalizationand follow the steps to correct the structure of your database.

A normalized database almost never has this kind of data integrity problems. All too many books and tutorials dive straight into providing novices with the tools needed to create database tables without teaching the fundamentals of good database design.

However, I'll get off my soapbox now and get to the topic at hand. What to do about it when it happens to you. First, let's create our sample badly-designed table, and populate it with some data.

Notice the table has no primary key. Alarm bells should be ringing. Here is a suggestion for how the table should have looked:. I hope that you would not have done such a thing using raw SQL, and can at least blame the application, but nevertheless, you're stuck with duplicate records and need to remove them. In some cases there may only be one duplicate, as in the sample data we have created. In this case, the simplest way is just to delete that one record.

Many people do not know about a handy way to do this. Of course the statement:.In this article we look at ways to remove all duplicate rows except one in an SQL database. For all examples in this article, we'll be using MySQL and the following table called user with names of users:. You can simply create a new table with unique values copied over from the original table like so:. You will have to add original table's constraints and indexes on the new table as they aren't copied over.

You can use a temporary table to copy over unique values from the original table, and add them back to the original table. This can be useful when a primary key does not exist. Be careful when using this method because if there are foreign key constraints referencing the original table, rows deleted in step 2 may delete referenced rows in child tables as well to maintain referential integrity.

To make sure there are no duplicates added in future, we can extend the previous set of queries to add a UNIQUE constraint to the column.

Using the methods listed below, row with the lowest id is kept, all other duplicates are removed. Using the methods listed below, row with the highest id is kept, all other duplicates are removed. Hope you found this post useful. It was published 12 Jun, and was last revised 31 May, Please show your love and support by sharing this post. Find out different queries and techniques you can use to remove all duplicate values except one in SQL.

Daniyal Hamid 31 May, Use Temporary Table to Fill Original Table With Unique Rows You can use a temporary table to copy over unique values from the original table, and add them back to the original table.


replies on “Delete duplicate rows with same id in mysql”

Leave a Reply

Your email address will not be published. Required fields are marked *