How to check if a value already exists to avoid duplicates?

Question

I've got a table of URLs and I don't want any duplicate URLs. How do I check to see if a given URL is already in the table using PHP/MySQL? JavaScript questions and answers, JavaScript questions pdf, JavaScript question bank, JavaScript questions and answers pdf, mcq on JavaScript pdf, JavaScript questions and solutions, JavaScript mcq Test , Interview JavaScript questions, JavaScript Questions for Interview, JavaScript MCQ (Multiple Choice Questions)

Answer 1

If you don't want to have duplicates you can do following: add uniqueness constraint use "REPLACE" or "INSERT ... ON DUPLICATE KEY UPDATE" syntax If multiple users can insert data to DB, method suggested by @Jeremy Ruten, can lead to an error: after you performed a check someone can insert similar data to the table.