Support SimpleFunction as order by expression

To allow ordering results on for example the highest updated date, when
joining with multiple tables;

    ORDER BY GREATEST(table1.created_at, table2.created_at) ASC

Author: koenpunt

spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/OrderByClauseVisitor.java

@@ -17,21 +17,27 @@
 
 import org.springframework.data.relational.core.sql.Column;
 import org.springframework.data.relational.core.sql.OrderByField;
+import org.springframework.data.relational.core.sql.SimpleFunction;
 import org.springframework.data.relational.core.sql.Visitable;
+import org.springframework.lang.Nullable;
 
 /**
  * {@link PartRenderer} for {@link OrderByField}s.
  *
  * @author Mark Paluch
  * @author Jens Schauder
  * @author Chirag Tailor
+ * @author Koen Punt
  * @since 1.1
  */
 class OrderByClauseVisitor extends TypedSubtreeVisitor<OrderByField> implements PartRenderer {
 
 	private final RenderContext context;
 
 	private final StringBuilder builder = new StringBuilder();
+
+	@Nullable private PartRenderer delegate;
+
 	private boolean first = true;
 
 	OrderByClauseVisitor(RenderContext context) {
@@ -68,8 +74,22 @@ Delegation leaveMatched(OrderByField segment) {
 		return Delegation.leave();
 	}
 
+	@Override
+	Delegation enterNested(Visitable segment) {
+		if (segment instanceof SimpleFunction) {
+			delegate = new SimpleFunctionVisitor(context);
+			return Delegation.delegateTo((SimpleFunctionVisitor)delegate);
+		}
+
+		return super.enterNested(segment);
+	}
+
 	@Override
 	Delegation leaveNested(Visitable segment) {
+		if (delegate instanceof SimpleFunctionVisitor) {
+			builder.append(delegate.getRenderedPart());
+			delegate = null;
+		}
 
 		if (segment instanceof Column) {
 			builder.append(NameRenderer.fullyQualifiedReference(context, (Column) segment));

spring-data-relational/src/test/java/org/springframework/data/relational/core/sql/render/OrderByClauseVisitorUnitTests.java

@@ -22,13 +22,18 @@
 import org.springframework.data.relational.core.sql.OrderByField;
 import org.springframework.data.relational.core.sql.SQL;
 import org.springframework.data.relational.core.sql.Select;
+import org.springframework.data.relational.core.sql.SimpleFunction;
 import org.springframework.data.relational.core.sql.Table;
 
+import java.util.Arrays;
+import java.util.List;
+
 /**
  * Unit tests for {@link OrderByClauseVisitor}.
  *
  * @author Mark Paluch
  * @author Jens Schauder
+ * @author Koen Punt
  */
 class OrderByClauseVisitorUnitTests {
 
@@ -88,4 +93,22 @@ void shouldRenderOrderByFullyQualifiedNameWithTableAlias() {
 		assertThat(visitor.getRenderedPart().toString()).isEqualTo("emp.name ASC");
 	}
 
+	@Test // GH-1348
+	void shouldRenderOrderBySimpleFunction() {
+
+		Table employee = SQL.table("employee").as("emp");
+		Column column = employee.column("name");
+		List<Expression> columns = Arrays.asList(employee.column("id"), column);
+
+		SimpleFunction simpleFunction = SimpleFunction.create("GREATEST", columns);
+
+		Select select = Select.builder().select(column).from(employee)
+				.orderBy(OrderByField.from(simpleFunction).asc(), OrderByField.from(column).asc()).build();
+
+		OrderByClauseVisitor visitor = new OrderByClauseVisitor(new SimpleRenderContext(NamingStrategies.asIs()));
+		select.visit(visitor);
+
+		assertThat(visitor.getRenderedPart().toString()).isEqualTo("GREATEST(emp.id, emp.name) ASC, emp.name ASC");
+	}
+
 }