How Can I Efficiently Remove Duplicates from a Large MySQL Database While Prioritizing Specific Data?

Efficient Duplicate Removal from Large MySQL Databases

Keeping large MySQL databases free of duplicates is crucial for data integrity and performance. However, identifying and removing duplicates can be a daunting task for extensive tables. One common challenge faced by users is the need to swiftly remove duplicates from a substantial database containing millions of rows, where duplicate removal can often become a time-consuming process.

A typical scenario encountered in duplicate removal involves a table with columns id, text1, text2, and text3, where the combination of text1 and text2 should be unique. If any duplicates exist, only one combination with a non-NULL value for text3 should remain. For example, given the data:

| id | text1 | text2 | text3 |
| --- | ----- | ----- | ----- |
| 1   | abc   | def   | NULL   |
| 2   | abc   | def   | ghi    |
| 3   | abc   | def   | jkl    |
| 4   | aaa   | bbb   | NULL   |
| 5   | aaa   | bbb   | NULL   |

...the desired outcome would be:

| id | text1 | text2 | text3 |
| --- | ----- | ----- | ----- |
| 1   | abc   | def   | ghi   |
| 2   | aaa   | bbb   | NULL  |

While solutions such as CREATE TABLE tmp SELECT text1, text2, text3 FROM my_tbl; GROUP BY text1, text2; or SELECT DISTINCT may work for smaller databases, they often encounter prolonged execution times when dealing with large tables.

To address this challenge, an efficient approach is to use a combination of on duplicate key and ifnull():

create table tmp like yourtable;

alter table tmp add unique (text1, text2);

insert into tmp select * from yourtable
    on duplicate key update text3 = ifnull(text3, values(text3));

rename table yourtable to deleteme, tmp to yourtable;

drop table deleteme;

This approach employs an optimized strategy. It creates a new table tmp similar to the original table. Then, it adds a unique constraint on text1 and text2 to enforce uniqueness. Subsequently, data from the yourtable is inserted into tmp, taking advantage of the on duplicate key clause. This clause ensures that if any duplicate rows are encountered, the text3 column from the new data takes precedence over the existing value. To finalize the process, the original yourtable is renamed to deleteme, while tmp is renamed to yourtable, effectively replacing the old table with the duplicate-free data. Finally, the deleteme table is dropped.

This method eliminates the need for computationally expensive operations like GROUP BY or DISTINCT and leverages MySQL's sophisticated query optimization capabilities. As a result, it offers significant improvements in execution time, enabling swift duplicate removal from even large-scale databases.

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