I don't know how to use JPQL [AND] and [OR]

I made application what it will be able to search book of name and author.I use JPQL to realize it.When I fill two textbox(name of book and author),it search right.But I fill one textbox,it search not found.
How to fix？

Book.java

@Table(name = "books")
@NamedQueries({
		@NamedQuery(name = "searchNameAndAuthor", query = "SELECT b FROM Book AS b WHERE b.book_name =:book_name and b.author=:author"),
		@NamedQuery(name = "searchNameAndAuthorCount", query = "SELECT count(b) FROM Book AS b WHERE b.book_name =:book_name and b.author=:author")
})
@Entity
public class Book {
	@Id
	@Column(name = "id")
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	private Integer id;

	@Column(name = "isbn_code", length = 17)
	private Integer isbn_code;

	@Column(name = "book_name", length = 30, nullable = false)
	private String book_name;

	@Column(name = "author", length = 20)
	private String author;

	@Column(name = "publisher", length = 20)
	private String publisher;

	@Column(name = "publish_date", length = 8)
	private Date publish_date;

	@Column(name = "stock", length = 4)
	private Integer stock;

search.jsp

<c:import url="/WEB-INF/views/layout/app.jsp">
	<c:param name="content">
		<c:if test="${flush != null}">
			<div id="flush_success">
				<c:out value="${flush}"></c:out>
			</div>
		</c:if>
		<h2>図書館ようこそ</h2>
		<form method="POST" action="<c:url value='/books/search/result' />">
			<label for="book_name">調べたいタイトル</label><br /> <input type="text"
				name="book_name" value="${book_name}" /> <br />
			<br />
			<label for="author">著者</label><br /> <input type="text"
				name="author" value="${author}" /> <br />
			<br />
			<button type="submit">検索</button>
		</form>
	</c:param>
</c:import>

You need to handle the null case also. A way can be checking is the parameter is null as OR condition

SELECT b FROM Book AS b WHERE (:book_name IS NULL OR b.book_name =:book_name) 
                          and (:author IS NULL OR  b.author=:author)