MySQL：更新第一个可用列为空的行

Question

I have a table that I use to keep track of some associations between users and various other aspects of their website.

I need to be able to get the first available row and update one or two of it's columns ... the criteria is whether or not the user_id column has been used or not.

id | tag_id | user_id | product_id

If a row has a tag available where there is no user_id assigned, I want to be able to use and update that row for the latest purchased product.

1 | 100001 | 29 | 66  
2 | 100002 | 0  | 0 
3 | 100003 | 0  | 0

So as you can see, the second row would be the first eligible candidate.

I'm just not sure what the SQL needs to be in order to make that happen

Answer 1

UPDATE yourTablename SET user_id = 'your value for userid',
product_id='ur value for productid' WHERE id=(select min(id) where user_id='0');

alternative method already told are efficient but if your table has sorting with id

UPDATE yourTablename SET user_id = 'your value for userid',
product_id='ur value for productid' where user_id='0' LIMIT 1;