Conditions are Expressions.

Selection of condition expressions yielding a boolean value is supported by some databases.

Closes #1007
Original pull request: #1079.

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

@@ -23,7 +23,7 @@ package org.springframework.data.relational.core.sql;
  * @since 1.1
  * @see Conditions
  */
-public interface Condition extends Segment {
+public interface Condition extends Segment, Expression {
 
 	/**
 	 * Combine another {@link Condition} using {@code AND}.

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

@@ -52,14 +52,14 @@ class ComparisonVisitor extends FilteredSubtreeVisitor {
 	@Override
 	Delegation enterNested(Visitable segment) {
 
-		if (segment instanceof Expression) {
-			ExpressionVisitor visitor = new ExpressionVisitor(context);
+		if (segment instanceof Condition) {
+			ConditionVisitor visitor = new ConditionVisitor(context);
 			current = visitor;
 			return Delegation.delegateTo(visitor);
 		}
 
-		if (segment instanceof Condition) {
-			ConditionVisitor visitor = new ConditionVisitor(context);
+		if (segment instanceof Expression) {
+			ExpressionVisitor visitor = new ExpressionVisitor(context);
 			current = visitor;
 			return Delegation.delegateTo(visitor);
 		}

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

@@ -28,6 +28,7 @@ import org.springframework.util.Assert;
  * and delegate nested {@link Expression} and {@link Condition} rendering.
  *
  * @author Mark Paluch
+ * @author Jens Schauder
  * @since 1.1
  */
 abstract class FilteredSingleConditionRenderSupport extends FilteredSubtreeVisitor {
@@ -55,18 +56,19 @@ abstract class FilteredSingleConditionRenderSupport extends FilteredSubtreeVisit
 	@Override
 	Delegation enterNested(Visitable segment) {
 
-		if (segment instanceof Expression) {
-			ExpressionVisitor visitor = new ExpressionVisitor(context);
+		if (segment instanceof Condition) {
+			ConditionVisitor visitor = new ConditionVisitor(context);
 			current = visitor;
 			return Delegation.delegateTo(visitor);
 		}
 
-		if (segment instanceof Condition) {
-			ConditionVisitor visitor = new ConditionVisitor(context);
+		if (segment instanceof Expression) {
+			ExpressionVisitor visitor = new ExpressionVisitor(context);
 			current = visitor;
 			return Delegation.delegateTo(visitor);
 		}
 
+
 		throw new IllegalStateException("Cannot provide visitor for " + segment);
 	}
 

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

@@ -480,6 +480,20 @@ class SelectRendererUnitTests {
 		assertThat(rendered).isEqualTo("SELECT CAST(User.name AS VARCHAR2) FROM User");
 	}
 
+	@Test // GH-1007
+	void shouldRenderConditionAsExpression() {
+
+		Table table = SQL.table("User");
+		Select select = StatementBuilder.select( //
+				Conditions.isGreater(table.column("age"), SQL.literalOf(18)) //
+		) //
+				.from(table) //
+				.build();
+
+		final String rendered = SqlRenderer.toString(select);
+		assertThat(rendered).isEqualTo("SELECT User.age > 18 FROM User");
+	}
+
 	@Test // GH-968
 	void rendersFullyQualifiedNamesInOrderBy() {