Adds support for windowing functions.

This allows us to use analytic functions aka windowing functions in generated select statements.

Closes #1019

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

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.relational.core.sql;
+
+import java.util.Arrays;
+
+/**
+ * Represents an analytic function, also known as windowing function
+ * 
+ * @author Jens Schauder
+ * @since 2.7
+ */
+public class AnalyticFunction extends AbstractSegment implements Expression {
+
+	private final SimpleFunction function;
+	private final Partition partition;
+	private final OrderBy orderBy;
+
+	public static AnalyticFunction create(String function, Expression... arguments) {
+
+		return new AnalyticFunction(SimpleFunction.create(function, Arrays.asList(arguments)), new Partition(),
+				new OrderBy());
+	}
+
+	private AnalyticFunction(SimpleFunction function, Partition partition, OrderBy orderBy) {
+
+		super(function, partition, orderBy);
+
+		this.function = function;
+		this.partition = partition;
+		this.orderBy = orderBy;
+	}
+
+	public AnalyticFunction partitionBy(Expression... partitionBy) {
+
+		return new AnalyticFunction(function, new Partition(partitionBy), orderBy);
+	}
+
+	public AnalyticFunction orderBy(OrderByField... orderBy) {
+		return new AnalyticFunction(function, partition, new OrderBy(orderBy));
+	}
+
+	public AnalyticFunction orderBy(Expression... orderByExpression) {
+
+		final OrderByField[] orderByFields = Arrays.stream(orderByExpression) //
+				.map(OrderByField::from) //
+				.toArray(OrderByField[]::new);
+
+		return new AnalyticFunction(function, partition, new OrderBy(orderByFields));
+	}
+
+	public AliasedAnalyticFunction as(String alias) {
+		return new AliasedAnalyticFunction(this, SqlIdentifier.unquoted(alias));
+	}
+
+	public AliasedAnalyticFunction as(SqlIdentifier alias) {
+		return new AliasedAnalyticFunction(this, alias);
+	}
+
+	public static class Partition extends SegmentList<Expression> {
+		Partition(Expression... expressions) {
+			super(expressions);
+		}
+	}
+
+	private static class AliasedAnalyticFunction extends AnalyticFunction implements Aliased {
+
+		private final SqlIdentifier alias;
+
+		AliasedAnalyticFunction(AnalyticFunction analyticFunction, SqlIdentifier alias) {
+
+			super(analyticFunction.function, analyticFunction.partition, analyticFunction.orderBy);
+			this.alias = alias;
+		}
+
+		@Override
+		public SqlIdentifier getAlias() {
+			return alias;
+		}
+	}
+}

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

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.relational.core.sql;
+
+/**
+ * Represents an `ORDER BY` clause. Currently, only used in {@link AnalyticFunction}.
+ *
+ * @author Jens Schauder
+ * @since 2.7
+ */
+public class OrderBy extends SegmentList<OrderByField> {
+
+	OrderBy(OrderByField... fields) {
+		super(fields);
+	}
+}

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

@@ -46,24 +46,24 @@ public class OrderByField extends AbstractSegment {
 	}
 
 	/**
-	 * Creates a new {@link OrderByField} from a {@link Column} applying default ordering.
+	 * Creates a new {@link OrderByField} from an {@link Expression} applying default ordering.
 	 *
-	 * @param column must not be {@literal null}.
+	 * @param expression must not be {@literal null}.
 	 * @return the {@link OrderByField}.
 	 */
-	public static OrderByField from(Column column) {
-		return new OrderByField(column, null, NullHandling.NATIVE);
+	public static OrderByField from(Expression expression) {
+		return new OrderByField(expression, null, NullHandling.NATIVE);
 	}
 
 	/**
-	 * Creates a new {@link OrderByField} from a {@link Column} applying a given ordering.
+	 * Creates a new {@link OrderByField} from an {@link Expression} applying a given ordering.
 	 *
-	 * @param column must not be {@literal null}.
+	 * @param expression must not be {@literal null}.
 	 * @param direction order direction
 	 * @return the {@link OrderByField}.
 	 */
-	public static OrderByField from(Column column, Direction direction) {
-		return new OrderByField(column, direction, NullHandling.NATIVE);
+	public static OrderByField from(Expression expression, Direction direction) {
+		return new OrderByField(expression, direction, NullHandling.NATIVE);
 	}
 
 	/**

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

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.relational.core.sql;
+
+/**
+ * A list of {@link Segment} instances. Normally used by inheritance to derive a more specific list.
+ *
+ * @see org.springframework.data.relational.core.sql.AnalyticFunction.Partition
+ * @see OrderBy
+ * @param <T> the type of the elements.
+ */
+public class SegmentList<T extends Segment> extends AbstractSegment {
+	SegmentList(T... segments) {
+		super(segments);
+	}
+}

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

@@ -29,8 +29,8 @@ import org.springframework.util.StringUtils;
  */
 public class SimpleFunction extends AbstractSegment implements Expression {
 
-	private String functionName;
-	private List<Expression> expressions;
+	private final String functionName;
+	private final List<Expression> expressions;
 
 	private SimpleFunction(String functionName, List<Expression> expressions) {
 

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

@@ -0,0 +1,102 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.relational.core.sql.render;
+
+import org.springframework.data.relational.core.sql.AnalyticFunction;
+import org.springframework.data.relational.core.sql.OrderBy;
+import org.springframework.data.relational.core.sql.SimpleFunction;
+import org.springframework.data.relational.core.sql.Visitable;
+import org.springframework.lang.Nullable;
+
+/**
+ * Renderer for {@link AnalyticFunction}. Uses a {@link RenderTarget} to call back for render results.
+ *
+ * @author Jens Schauder
+ * @since 2.7
+ */
+class AnalyticFunctionVisitor extends TypedSingleConditionRenderSupport<AnalyticFunction> implements PartRenderer {
+
+	private final StringBuilder part = new StringBuilder();
+	private final RenderContext context;
+	@Nullable private PartRenderer delegate;
+	private boolean addSpace = false;
+
+	AnalyticFunctionVisitor(RenderContext context) {
+		super(context);
+		this.context = context;
+	}
+
+	@Override
+	Delegation enterNested(Visitable segment) {
+
+		if (segment instanceof SimpleFunction) {
+
+			delegate = new SimpleFunctionVisitor(context);
+			return Delegation.delegateTo((DelegatingVisitor) delegate);
+		}
+
+		if (segment instanceof AnalyticFunction.Partition) {
+
+			delegate = new SegmentListVisitor("PARTITION BY ", ", ", new ExpressionVisitor(context));
+			return Delegation.delegateTo((DelegatingVisitor) delegate);
+		}
+
+		if (segment instanceof OrderBy) {
+
+			delegate = new SegmentListVisitor("ORDER BY ", ", ", new OrderByClauseVisitor(context));
+			return Delegation.delegateTo((DelegatingVisitor) delegate);
+		}
+		return super.enterNested(segment);
+	}
+
+	@Override
+	Delegation leaveNested(Visitable segment) {
+
+		if (delegate instanceof SimpleFunctionVisitor) {
+
+			part.append(delegate.getRenderedPart());
+			part.append(" OVER(");
+		}
+
+		if (delegate instanceof SegmentListVisitor) {
+
+			final CharSequence renderedPart = delegate.getRenderedPart();
+			if (renderedPart.length() != 0) {
+
+				if (addSpace) {
+					part.append(' ');
+				}
+				part.append(renderedPart);
+				addSpace = true;
+			}
+		}
+
+		return super.leaveNested(segment);
+	}
+
+	@Override
+	Delegation leaveMatched(AnalyticFunction segment) {
+
+		part.append(")");
+
+		return super.leaveMatched(segment);
+	}
+
+	@Override
+	public CharSequence getRenderedPart() {
+		return part;
+	}
+}

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

@@ -82,6 +82,13 @@ class ExpressionVisitor extends TypedSubtreeVisitor<Expression> implements PartR
 			return Delegation.delegateTo(visitor);
 		}
 
+		if (segment instanceof AnalyticFunction) {
+
+			AnalyticFunctionVisitor visitor = new AnalyticFunctionVisitor(context);
+			partRenderer = visitor;
+			return Delegation.delegateTo(visitor);
+		}
+
 		if (segment instanceof Column) {
 
 			Column column = (Column) segment;

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

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.relational.core.sql.render;
+
+import org.springframework.data.relational.core.sql.SegmentList;
+import org.springframework.data.relational.core.sql.Visitable;
+import org.springframework.util.Assert;
+
+/**
+ * A part rendering visitor for lists of segments. It can be set up depending on the elements in the list it should
+ * handle and the way elemnts should get separated when rendered.
+ * 
+ * @author Jens Schauder
+ * @since 2.7
+ */
+class SegmentListVisitor extends TypedSubtreeVisitor<SegmentList<?>> implements PartRenderer {
+
+	private final StringBuilder part = new StringBuilder();
+	private final String start;
+	private final String separator;
+	private final DelegatingVisitor nestedVisitor;
+
+	private boolean first = true;
+
+	/**
+	 * @param start a {@literal String} to be rendered before the first element if there is at least one element. Must not
+	 *          be {@literal null}.
+	 * @param separator a {@literal String} to be rendered between elements. Must not be {@literal null}.
+	 * @param nestedVisitor the {@link org.springframework.data.relational.core.sql.Visitor} responsible for rendering the
+	 *          elements of the list. Must not be {@literal null}.
+	 */
+	SegmentListVisitor(String start, String separator, DelegatingVisitor nestedVisitor) {
+
+		Assert.notNull(start, "Start must not be null.");
+		Assert.notNull(separator, "Separator must not be null.");
+		Assert.notNull(nestedVisitor, "Nested Visitor must not be null.");
+		Assert.isInstanceOf(PartRenderer.class, nestedVisitor, "Nested visitor must implement PartRenderer");
+
+		this.start = start;
+		this.separator = separator;
+		this.nestedVisitor = nestedVisitor;
+	}
+
+	@Override
+	Delegation enterNested(Visitable segment) {
+
+		if (first) {
+			part.append(start);
+			first = false;
+		} else {
+			part.append(separator);
+		}
+
+		return Delegation.delegateTo(nestedVisitor);
+	}
+
+	@Override
+	Delegation leaveNested(Visitable segment) {
+
+		part.append(((PartRenderer) nestedVisitor).getRenderedPart());
+
+		return super.leaveNested(segment);
+	}
+
+	@Override
+	public CharSequence getRenderedPart() {
+
+		return part;
+	}
+}

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

@@ -33,7 +33,6 @@ class SelectListVisitor extends TypedSubtreeVisitor<SelectList> implements PartR
 	private final StringBuilder builder = new StringBuilder();
 	private final RenderTarget target;
 	private boolean requiresComma = false;
-	private boolean insideFunction = false; // this is hackery and should be fix with a proper visitor for
 	private ExpressionVisitor expressionVisitor;
 	// subelements.
 
@@ -86,7 +85,7 @@ class SelectListVisitor extends TypedSubtreeVisitor<SelectList> implements PartR
 			requiresComma = true;
 		}
 
-		if (segment instanceof Aliased && !insideFunction) {
+		if (segment instanceof Aliased) {
 			builder.append(" AS ").append(NameRenderer.render(context, (Aliased) segment));
 		}
 

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

@@ -17,6 +17,7 @@ package org.springframework.data.relational.core.sql.render;
 
 import static org.assertj.core.api.Assertions.*;
 
+import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 import org.springframework.data.relational.core.dialect.PostgresDialect;
 import org.springframework.data.relational.core.dialect.RenderContextFactory;
@@ -265,8 +266,7 @@ class SelectRendererUnitTests {
 
 		Select select = Select.builder().select(column).from(employee).orderBy(OrderByField.from(column).asc()).build();
 
-		assertThat(SqlRenderer.toString(select))
-				.isEqualTo("SELECT emp.name FROM employee emp ORDER BY emp.name ASC");
+		assertThat(SqlRenderer.toString(select)).isEqualTo("SELECT emp.name FROM employee emp ORDER BY emp.name ASC");
 	}
 
 	@Test // GH-968
@@ -544,9 +544,7 @@ class SelectRendererUnitTests {
 
 		Table table = SQL.table("User");
 		Select select = StatementBuilder.select( //
-				Conditions.isGreater(table.column("age"), SQL.literalOf(
-						18))
-		) //
+				Conditions.isGreater(table.column("age"), SQL.literalOf(18))) //
 				.from(table) //
 				.build();
 
@@ -575,4 +573,95 @@ class SelectRendererUnitTests {
 		assertThat(rendered)
 				.isEqualTo("SELECT * FROM tableA JOIN tableB ON tableA.id = tableB.id ORDER BY tableA.name, tableB.name");
 	}
+
+	/**
+	 * Tests the rendering of analytic functions.
+	 */
+	@Nested
+	class AnalyticFunctionsTests {
+
+		Table employee = SQL.table("employee");
+		Column department = employee.column("department");
+		Column age = employee.column("age");
+		Column salary = employee.column("salary");
+
+		@Test // GH-1019
+		void renderEmptyOver() {
+
+			Select select = StatementBuilder.select( //
+					AnalyticFunction.create("MAX", salary) //
+			) //
+					.from(employee) //
+					.build();
+
+			String rendered = SqlRenderer.toString(select);
+
+			assertThat(rendered).isEqualTo("SELECT MAX(employee.salary) OVER() FROM employee");
+		}
+
+		@Test // GH-1019
+		void renderPartition() {
+
+			Select select = StatementBuilder.select( //
+					AnalyticFunction.create("MAX", salary) //
+							.partitionBy(department) //
+			) //
+					.from(employee) //
+					.build();
+
+			String rendered = SqlRenderer.toString(select);
+
+			assertThat(rendered)
+					.isEqualTo("SELECT MAX(employee.salary) OVER(PARTITION BY employee.department) FROM employee");
+		}
+
+		@Test // GH-1019
+		void renderOrderBy() {
+
+			Select select = StatementBuilder.select( //
+					AnalyticFunction.create("MAX", salary) //
+							.orderBy(age) //
+			) //
+					.from(employee) //
+					.build();
+
+			String rendered = SqlRenderer.toString(select);
+
+			assertThat(rendered).isEqualTo("SELECT MAX(employee.salary) OVER(ORDER BY employee.age) FROM employee");
+		}
+
+		@Test // GH-1019
+		void renderFullAnalyticFunction() {
+
+			final Select select = StatementBuilder.select( //
+					AnalyticFunction.create("MAX", salary) //
+							.partitionBy(department) //
+							.orderBy(age) //
+			) //
+					.from(employee) //
+					.build();
+
+			String rendered = SqlRenderer.toString(select);
+
+			assertThat(rendered).isEqualTo(
+					"SELECT MAX(employee.salary) OVER(PARTITION BY employee.department ORDER BY employee.age) FROM employee");
+		}
+
+		@Test // GH-1019
+		void renderAnalyticFunctionWithAlias() {
+
+			final Select select = StatementBuilder.select( //
+					AnalyticFunction.create("MAX", salary) //
+							.partitionBy(department) //
+							.orderBy(age) //
+							.as("MAX_SELECT")) //
+					.from(employee) //
+					.build();
+
+			String rendered = SqlRenderer.toString(select);
+
+			assertThat(rendered).isEqualTo(
+					"SELECT MAX(employee.salary) OVER(PARTITION BY employee.department ORDER BY employee.age) AS MAX_SELECT FROM employee");
+		}
+	}
 }