<p>I've evaluated some plugins from official IntelliJ repository, but none of them seems to provide more then basic syntax highlighting. Even highlighting is limited. For example, Database Navigator doesn't highlight IF. It would be great to have proper highlighting. And if it would have auto-complete feature and provide file structure view I would be completely happy. 
I use SQL Query Plugin when I need to post some little stuff to database or to query something from it. I use PL/SQL Developer when I need to make more complex things, or when I need to modify PL/SQL. But most of the time I need to navigate through mix of Java, JavaScript and PL/SQL code. Unfortunately, SQL Query Plugin doesn't help with that as well as PL/SQL Developer.</p>

Have you tried
SQL Query Plugin
? It comes with a great host of features, including multiple query windows, direct editing of tables, and of course, proper highlighting for PL/SQL syntax and a browser of the internal database structure.
It also integrates well with the main editor: you can right-click on any selected Java string containing SQL and "Send to SQLPlugin" to strip out all of the quotes and non-SQL syntax.