DATAJDBC-534 - Derived Query support for count projection.

Original pull request: #215.

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/query/JdbcQueryCreator.java

@@ -34,6 +34,8 @@ import org.springframework.data.relational.core.mapping.RelationalPersistentProp
 import org.springframework.data.relational.core.query.Criteria;
 import org.springframework.data.relational.core.sql.Column;
 import org.springframework.data.relational.core.sql.Expression;
+import org.springframework.data.relational.core.sql.Expressions;
+import org.springframework.data.relational.core.sql.Functions;
 import org.springframework.data.relational.core.sql.Select;
 import org.springframework.data.relational.core.sql.SelectBuilder;
 import org.springframework.data.relational.core.sql.StatementBuilder;
@@ -54,6 +56,7 @@ import org.springframework.util.Assert;
  *
  * @author Mark Paluch
  * @author Jens Schauder
+ * @author Myeonghyeon Lee
  * @since 2.0
  */
 class JdbcQueryCreator extends RelationalQueryCreator<ParametrizedQuery> {
@@ -208,6 +211,8 @@ class JdbcQueryCreator extends RelationalQueryCreator<ParametrizedQuery> {
 
 			Column idColumn = table.column(entity.getIdColumn());
 			builder = Select.builder().select(idColumn).from(table);
+		} else if (tree.isCountProjection()) {
+			builder = Select.builder().select(Functions.count(Expressions.asterisk())).from(table);
 		} else {
 			builder = selectBuilder(table);
 		}

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/repository/query/PartTreeJdbcQueryUnitTests.java

@@ -55,6 +55,7 @@ import org.springframework.jdbc.core.namedparam.NamedParameterJdbcOperations;
  * @author Roman Chigvintsev
  * @author Mark Paluch
  * @author Jens Schauder
+ * @author Myeonghyeon Lee
  */
 @RunWith(MockitoJUnitRunner.class)
 public class PartTreeJdbcQueryUnitTests {
@@ -548,6 +549,17 @@ public class PartTreeJdbcQueryUnitTests {
 		assertThat(query.getParameterSource().getValue("user_street")).isEqualTo("Hello");
 	}
 
+	@Test // DATAJDBC-534
+	public void createsQueryForCountProjection() throws Exception {
+
+		JdbcQueryMethod queryMethod = getQueryMethod("countByFirstName", String.class);
+		PartTreeJdbcQuery jdbcQuery = createQuery(queryMethod);
+		ParametrizedQuery query = jdbcQuery.createQuery((getAccessor(queryMethod, new Object[] { "John" })));
+
+		assertThat(query.getQuery()).isEqualTo(
+			"SELECT COUNT(*) FROM " + TABLE + " WHERE " + TABLE + ".\"FIRST_NAME\" = :first_name");
+	}
+
 	private PartTreeJdbcQuery createQuery(JdbcQueryMethod queryMethod) {
 		return new PartTreeJdbcQuery(mappingContext, queryMethod, H2Dialect.INSTANCE, converter,
 				mock(NamedParameterJdbcOperations.class), mock(RowMapper.class));
@@ -639,6 +651,8 @@ public class PartTreeJdbcQueryUnitTests {
 		User findByAddressStreet(String street);
 
 		User findByAnotherEmbeddedList(Object list);
+
+		long countByFirstName(String name);
 	}
 
 	@Table("users")