Java sql - delete half rows in DB table

Question

I want to delete a bunch of rows from a DB file that I have in a folder. Connecting and counting the amount of rows in the db file works but when I try to delete a specific amount of rows I get stuck.

Input:

sql = "SELECT COUNT(*) AS id FROM wifi_probe_requests";
...
sql = "DELETE FROM wifi_probe_requests LIMIT " + rowcount/2;
PreparedStatement pstmt = conn.prepareStatement(sql);
pstmt.executeUpdate();

Output:

54943
[SQLITE_ERROR] SQL error or missing database (near "LIMIT": syntax error)

Not using a limit works fine and I can delete the entire db table but what I want is to delete half the db rows as seen by the rowcount/2 I made.

UPDATE:

So far I have solved the problem by finding the id which is located at the n-rows/2 and then getting the value of it (ex. 264352). Then using that number to indicate what id rows are going to be deleted (ex. id.value < 264352).

sql = "SELECT COUNT(*) AS id FROM wifi_probe_requests";
int rowcount = COUNT(*);
sql = "DELETE FROM wifi_probe_requests WHERE id < (SELECT id FROM wifi_probe_requests ORDER BY id ASC LIMIT "+ rowcount/2 + ",1)";

rowcount = 50000
Delete valueof.id < valueof.id.50000/2

So all values of id below the value of an id at position 25000 will be deleted.

Answer 1

You can't. Some databases don't allow LIMIT in UPDATE or DELETE queries.

It seems that with SQLite it's possible to work around that, by compiling your own version , but if you're not willing to do that, you need to rewrite your query in a different way. For example if you have an autoincrement id in the table, you can calculate the "middle" id and use WHERE id < [middle id] as an alternative to LIMIT .

Answer 2

As stated by @Kayaman this is not possible using SQLITE. You can bypass this with a query such as;

DELETE FROM wifi_probe_requests WHERE id IN (SELECT id FROM wifi_probe_requests LIMIT 10)

One more thing; I don't think (rowcount/2) will work when you have an uneven amount of rows as it will not result in an integer. I think you will have to round it down/up.

Answer 3

How fancy do you want to make this? A simple solution would be something like:

SELECT COUNT(*) FROM mytable;
"SELECT id FROM mytable order by id LIMIT 1 OFFSET " + round(rowcount/2)
DELETE FROM mytable WHERE id < ?

If you go that route, you should be able to delete the first half of your rows by keyspace. If you just want just about half your rows deleted (and don't really care how many) you could probably find a way to use RANDOM() to do this. Probably like ( WARNING TOTALLY UNTESTED ):

DELETE FROM mytable WHERE random() < 0.5;