How to delete duplicate rows in SQL Server?

Question

How can I delete duplicate rows where no unique row id exists?

My table is

col1  col2 col3 col4 col5 col6 col7
john  1          


        

Answer 1

There are two solutions in mysql:

A) Delete duplicate rows using DELETE JOIN statement

DELETE t1 FROM contacts t1
INNER JOIN contacts t2 
WHERE 
    t1.id < t2.id AND 
    t1.email = t2.email;

This query references the contacts table twice, therefore, it uses the table alias t1 and t2.

The output is:

1 Query OK, 4 rows affected (0.10 sec)

In case you want to delete duplicate rows and keep the lowest id, you can use the following statement:

DELETE c1 FROM contacts c1
INNER JOIN contacts c2 
WHERE
    c1.id > c2.id AND 
    c1.email = c2.email;

   

B) Delete duplicate rows using an intermediate table

The following shows the steps for removing duplicate rows using an intermediate table:

    1. Create a new table with the structure the same as the original table that you want to delete duplicate rows.

    2. Insert distinct rows from the original table to the immediate table.

    3. Insert distinct rows from the original table to the immediate table.

 

Step 1. Create a new table whose structure is the same as the original table:

CREATE TABLE source_copy LIKE source;

Step 2. Insert distinct rows from the original table to the new table:

INSERT INTO source_copy
SELECT * FROM source
GROUP BY col; -- column that has duplicate values

Step 3. drop the original table and rename the immediate table to the original one

DROP TABLE source;
ALTER TABLE source_copy RENAME TO source;

Source: http://www.mysqltutorial.org/mysql-delete-duplicate-rows/