SQL is a language used to communicate with databases. The WordPress blog system uses MySQL as its default database.
When doing database queries, SQL wildcards are the ‘%’ (per cent) character. However, Wordpress escapes ‘%’ and ‘_’ characters for security purposes, to prevent SQL injection attacks.
If you are modifying a WordPress database query string, either manually or through one of the query classes, you’ll want to wrap your search term with a regular regexp wildcard instead, like this:
WordPress returns the results with the wildcard search parameters.