overwrite mysql table data - mysql

Question

I have a web crawler. The web crawler gathers the links from web pages I give it but when it is retrieving the links, some links are duplicated due to the website. is there a way in MYSQL to overwrite data if a new row is the exact same as an old row.
Say if I have http://www.facebook.com in a link field
I also manage to pick up http://www.facebook.com again, I would like the latter to overwrite the old row. therefore I don't have clashes on my search engine.

Answer 1

I'm assuming that you want to update a last_updated date if the url already exists. Else there is no good reason to do an update.
INSERT INTO `scrapping_table`
(`url`)
VALUES
("www.facebook.com")
ON DUPLICATE KEY UPDATE
`date_updated` = `datetime.now()`

Answer 2

look into ON DUPLICATE KEY actions
http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html
Basically make the columns you're concerned with a unique key write your insert statement and then add
ON DUPLICATE KEY UPDATE col = overwriting value

Answer 3

If your link field is unique than you can use
INSERT INTO "mytable" (link_field, x_column, y_column) VALUES ("www.facebook.com",'something new for x','something new for y')
ON DUPLICATE KEY UPDATE x_column='something new for x', y_column='something new for y'
Just make sure your link field is unique and if you have more unique fields in your column, I suggest use this second method because they suggest avoid using an ON DUPLICATE KEY clause on tables with multiple unique indexes.
set your link field as unique.
before inserting a row try
Select "primary_id" from mytable where link_field="www.facebook.com"
Count the number of returned row from this SQL.
=>If count>0 then UPDATE the row using the "primary_id" we just grabbed through the SELECT SQL
=> if count==0 , just insert your row

Answer 4

beware!!
while operating a web crawler that probably will find millions of links
you want to minimize the query's each "crawl" process fires...
do you want to create a unique link table that will feed the bots? or do you want to prevent duplicate search results?
unique url pool table:
while crawling the page - you should save url's to an array (or list) and making sure (!in_array()) that the its a unique value array, you will find that each page you crawl includes alot of repeated links - so clean them before using sql.
covert the urls to hashes ("simhash" of 32 digits [1,0]).
now open a connection to db and check if exists if it does dump them! don't update (its making a second process). you should match the links using the hashes over an indexed table it will be far more faster.
prevent duplicate results search:
if you indexed the url in the above methodology you should not find duplicate url's, if you have, it means there is a problem in your crawling operation.
even if you have duplicate values in another table and you want to search it but not returning duplicate results you can use DISTINCT in your query.
good luck!