How to Efficiently Remove Duplicate Rows from a Table Without Unique Identifiers?

Efficiently remove duplicate rows without unique identifiers

Removing duplicates can be a challenge when a data table lacks unique row identifiers. This article provides an efficient solution for removing duplicate rows while retaining the first occurrence of the row.

Let’s look at a table with duplicate rows:

col1	col2	col3	col4	col5	col6	col7
john	1	1	1	1	1	1
john	1	1	1	1	1	1
sally	2	2	2	2	2	2
sally	2	2	2	2	2	2

The desired result after removing duplicate rows is:

col1	col2	col3	col4	col5	col6	col7
john	1	1	1	1	1	1
sally	2	2	2	2	2	2

Solution using CTE and ROW_NUMBER

This method utilizes the common table expression (CTE) and the ROW_NUMBER() function. CTE assigns each row a sequence number (RN) based on a specific order, allowing us to identify and eliminate duplicates.

Here is the SQL query with step-by-step instructions:

<code class="language-sql">WITH CTE AS (
   SELECT [col1], [col2], [col3], [col4], [col5], [col6], [col7],
       RN = ROW_NUMBER() OVER (PARTITION BY col1 ORDER BY col1) -- 为 col1 定义的每个组内分配序列号
   FROM dbo.Table1
)
DELETE FROM CTE WHERE RN > 1; -- 删除 RN 大于 1 的行（表示重复项）</code>

Instructions:

• CTE Creation: The WITH statement creates a CTE named CTE that contains the columns of the table and assigns RN values ​​to each row using the ROW_NUMBER() function. The PARTITION BY clause groups the rows based on the col1 column and sorts them within each group to determine the order.
• ROW_NUMBER() function: The ROW_NUMBER() function generates a sequence of integers starting from 1 for each row within each partition defined by the PARTITION BY clause.
• Delete operation: The DELETE statement deletes rows with RN greater than 1 in the CTE, thereby eliminating duplicate rows.

Output:

After executing the query, the updated table will contain:

col1	col2	col3	col4	col5	col6	col7
john	1	1	1	1	1	1
sally	2	2	2	2	2	2

The above is the detailed content of How to Efficiently Remove Duplicate Rows from a Table Without Unique Identifiers?. For more information, please follow other related articles on the PHP Chinese website!