I like Jira's JQL search functionality. (This is for rather structured data, which are issues in a project management system). It offers autocomplete for logic

I like Jira's JQL search functionality. (This is for rather structured data, which are issues in a project management system). It offers autocomplete for logical operators, search operators, field names and even for values (where there is a limited amount of data available to select). And it offers enough power for nearly all queries I could think of so far. (In contrast to SQL there are no subselects or explicit joins, but there are plugins that make subselects available).