A query string is the portion of a URL that begins with ? and contains parameters as key=value pairs separated by &. Servers and scripts read it to filter, search, sort or track.
Why it matters
Query strings let one page serve many views, like ?page=2 or ?q=shoes. They are also used for marketing tags such as utm_source. Many parameter combinations can create duplicate URLs for search engines, and sensitive data must never be placed in them.
When building URLs in code, use your language's URL or parameter-encoding functions rather than string concatenation, which is the usual source of broken encoding.
Example
https://example.com/search?q=blue+shoes&sort=price&page=2Spaces become + or %20. A literal ampersand in a value must be written as %26.
Common mistakes
- Putting passwords or tokens in the query string, where logs and referrers expose them.
- Forgetting to percent-encode special characters.
- Letting tracking parameters create thousands of duplicate indexable URLs.
Related
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