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    How to Find Duplicate Records in SQL

    In this tutorial, we will learn about duplicates and the reasons we need to eliminate them. Also, we will focus on the methods with which we can remove duplicates from the data we have in our SQL database.

    What are Duplicates in SQL?

    Duplicates in SQL are mostly the data points that exist more than once in our data store.

    For example:

    If we consider a customer table and in it, we store the details of customers of any particular shop. Then here, duplicate records would be the entry of a single customer more than once.

    In such cases, the system has an increased load of handling large data stores and their processing.

    Why do we need to handle Duplicates in SQL?

    Some of the major reasons why we need to remove duplicates from our records are as follows:

    1. The size of data to be stored increases due to the duplicates.
    2. When we have duplicates in our data they can give rise to business errors also known as logical errors.
    3. Due to the increased use of resources, the overall cost of the handling resources rises.
    4. Errors arise when analytics are performed on such erroneous data.

    Let us now have a look at our demo database: IBM.

    Query:

    USE IBM;

    SELECT * FROM IBM;

    How to handle Duplicates in SQL?

    We have various solutions present to handle the duplicates which exist in our database. Some of them are as follows:

    1. Using the Distinct Keyword to eliminate duplicate values and count their occurences from the Query results.

      We can use the Distinct keyword to fetch the unique records from our database. This way we can view the unique results from our database.

      Syntax:

      SELECT col1, col2, COUNT(DISTINCT(col3)),…..

      FROM tableName;

    2. Using Distinct keyword to delete the Duplicate records from the database.

      Syntax:

      SELECT col1, col2, DISTINCT(col3),…..

      FROM tableName;

    3. Using the Count keyword and Group By to eliminate the duplicate records.

      Syntax:

      SELECT col1,col2,col3,......
      COUNT(*) AS aliasName
      FROM tableName
      GROUP BY col1,col2,col3,......
      HAVING COUNT(*) = 1;
      
    4. Using Joins to eliminate the Duplicate records.

      Syntax:

      SELECT col1,col2,col3,....
      FROM tableName alias
      WHERE EXISTS(SELECT 1 FROM tableName alias2 
      WHERE Alias.col1 = Alias1.col1 AND
      Alias.col2 = Alias2.col2 AND
      Alias.col2 = Alias2.col2 AND
      ...);
      
    5. We have various methods like the Distinct keyword and then we have also seen the use of count and the Group By clause. And at last, we have seen how we can use joins to eliminate duplicates from our data.


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