Remove Duplicate Rows in Mysql

If you don't want to alter the column properties, then you can use the query below.

Since you have a column which has unique IDs (e.g., auto_increment columns), you can use it to remove the duplicates:

DELETE `a`
FROM
    `jobs` AS `a`,
    `jobs` AS `b`
WHERE
    -- IMPORTANT: Ensures one version remains
    -- Change "ID" to your unique column's name
    `a`.`ID` < `b`.`ID`

    -- Any duplicates you want to check for
    AND (`a`.`title` = `b`.`title` OR `a`.`title` IS NULL AND `b`.`title` IS NULL)
    AND (`a`.`company` = `b`.`company` OR `a`.`company` IS NULL AND `b`.`company` IS NULL)
    AND (`a`.`site_id` = `b`.`site_id` OR `a`.`site_id` IS NULL AND `b`.`site_id` IS NULL);

In MySQL, you can simplify it even more with the NULL-safe equal operator (aka "spaceship operator"):

DELETE `a`
FROM
    `jobs` AS `a`,
    `jobs` AS `b`
WHERE
    -- IMPORTANT: Ensures one version remains
    -- Change "ID" to your unique column's name
    `a`.`ID` < `b`.`ID`

    -- Any duplicates you want to check for
    AND `a`.`title` <=> `b`.`title`
    AND `a`.`company` <=> `b`.`company`
    AND `a`.`site_id` <=> `b`.`site_id`;

MySQL has restrictions about referring to the table you are deleting from. You can work around that with a temporary table, like:

create temporary table tmpTable (id int);

insert  into tmpTable
        (id)
select  id
from    YourTable yt
where   exists
        (
        select  *
        from    YourTabe yt2
        where   yt2.title = yt.title
                and yt2.company = yt.company
                and yt2.site_id = yt.site_id
                and yt2.id > yt.id
        );

delete  
from    YourTable
where   ID in (select id from tmpTable);

From Kostanos' suggestion in the comments:
The only slow query above is DELETE, for cases where you have a very large database. This query could be faster:

DELETE FROM YourTable USING YourTable, tmpTable WHERE YourTable.id=tmpTable.id

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... that do require specific approaches. 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 one), whether there are indexes to be kept, or whether we want to perform any additional action on the duplicated data.

There are also some specificities on MySQL itself, such as not being able to reference the same table on a FROM cause when performing a table UPDATE (it'll raise MySQL error #1093). 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 employee, with the following columns:

employee (id, first_name, last_name, start_date, ssn)

In order to delete the rows with a duplicate ssn column, and keeping only the first entry found, the following process can be followed:

-- create a new tmp_eployee table
CREATE TABLE tmp_employee LIKE employee;

-- add a unique constraint
ALTER TABLE tmp_employee ADD UNIQUE(ssn);

-- scan over the employee table to insert employee entries
INSERT IGNORE INTO tmp_employee SELECT * FROM employee ORDER BY id;

-- rename tables
RENAME TABLE employee TO backup_employee, tmp_employee TO employee;

Technical explanation

• Line #1 creates a new tmp_eployee table with exactly the same structure as the employee table
• Line #2 adds a UNIQUE constraint to the new tmp_eployee table to avoid any further duplicates
• Line #3 scans over the original employee table by id, inserting new employee entries into the new tmp_eployee table, while ignoring duplicated entries
• Line #4 renames tables, so that the new employee table holds all the entries without the duplicates, and a backup copy of the former data is kept on the backup_employee table

⇒ Using this approach, 1.6M registers were converted into 6k in less than 200s.

Chetan, following this process, you could fast and easily remove all your duplicates and create a UNIQUE constraint by running:

CREATE TABLE tmp_jobs LIKE jobs;

ALTER TABLE tmp_jobs ADD UNIQUE(site_id, title, company);

INSERT IGNORE INTO tmp_jobs SELECT * FROM jobs ORDER BY id;

RENAME TABLE jobs TO backup_jobs, tmp_jobs TO jobs;

Of course, this process can be further modified to adapt it for different needs when deleting duplicates. Some examples follow.