CODEWITHSUNDEEP
mysql
Nidheesh
Nidheesh

Reputation: 4562

Delete all rows except first N from a table having single column

I need a single query. Delete all rows from the table except the top N rows. The table has only one column. Like,

|friends_name|
==============
| Arunji     |
| Roshit     |
| Misbahu    |
| etc...     |

This column may contain repeated names as well.

Upvotes: 11

Views: 18278

Answers (3)

Unix One
Unix One

Reputation: 1181

I just wanted to follow up on this relatively old question because the existing answers don't capture the requirement and/or are incorrect. The question states the names can be repeated, but only the top N must be preserved. Other answers will delete incorrect rows and/or incorrect number of them.

For example, if we have this table:

|friends_name|
==============
| Arunji     |
| Roshit     |
| Misbahu    |
| Misbahu    |
| Roshit     |
| Misbahu    |
| Rohan      |

And we want to delete all but top 3 rows (N = 3), the expected result would be:

|friends_name|
==============
| Arunji     |
| Roshit     |
| Misbahu    |

The DELETE statement from the currently selected answer will result in:

|friends_name|
==============
| Arunji     |
| Misbahu    |
| Misbahu    |
| Misbahu    |

See this sqlfiddle. The reason for this is that it first sorts names alphabetically, then takes top 3, then deletes all that don't equal that. But since they are sorted by name they may not be the top 3 we want, and there's no guarantee that we'll end up with only 3.

In the absence of unique indexes and other fields to determine what "top N" means, we go by the order returned by the database. We could be tempted to do something like this (substitute 99999 with however high number):

DELETE FROM names LIMIT 99999 OFFSET 3

But according to MySQL docs, while the DELETE supports the LIMIT clause, it does not support OFFSET. So, doing this in a single query, as requested, does not seem to be possible; we must perform the steps manually.

Solution 1 - temporary table to hold top 3

CREATE TEMPORARY TABLE temp_names LIKE names;
INSERT INTO temp_names SELECT * FROM names LIMIT 3;
DELETE FROM names;
INSERT INTO names SELECT * FROM temp_names;

Here's the sqlfiddle for reference.

Solution 2 - new table with rename

CREATE TABLE new_names LIKE names;
INSERT INTO new_names SELECT * FROM names LIMIT 3;
RENAME TABLE names TO old_names, new_names TO names;
DROP TABLE old_names;

Here's the sqlfiddle for this one.

In either case, we end up with top 3 rows in our original table:

|friends_name|
==============
| Arunji     |
| Roshit     |
| Misbahu    |

Upvotes: 4

Edgar
Edgar

Reputation: 168

If you don't have an id field, i suppose you use an alphabetic order.

MYSQL

DELETE FROM friends 
WHERE friends_name 
NOT IN (
    SELECT * FROM (
        SELECT friends_name 
        FROM friends 
        ORDER BY friends_name ASC
        LIMIT 10) r
)

You delete all rows exept the 10 firsts (alphabetic order)

Upvotes: 4

fthiella
fthiella

Reputation: 49069

If you can order your records by friends_name, and if there are no duplicates, you could use this:

DELETE FROM names
WHERE
  friends_name NOT IN (
    SELECT * FROM (
      SELECT friends_name
      FROM names
      ORDER BY friends_name
      LIMIT 10) s
  )

Please see fiddle here.

Or you can use this:

DELETE FROM names ORDER BY friends_name DESC
LIMIT total_records-10

where total_records is (SELECT COUNT(*) FROM names), but you have to do this by code, you can't put a count in the LIMIT clause of your query.

Upvotes: 16

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