Add public factory method to SubselectExpression.

This allows using Select instances as Expression where this isn't yet anticipated.

Closes #1831

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

@@ -27,4 +27,14 @@ public class SubselectExpression extends Subselect implements Expression {
 
 		super(subselect);
 	}
+
+	/**
+	 * Wraps a Select in a {@link SubselectExpression}, for using it as an expression in function calls or similar.
+	 *
+	 * @author Jens Schauder
+	 * @since 3.4
+	 */
+	public static Expression of(Select subselect) {
+		return new SubselectExpression(subselect);
+	}
 }

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

@@ -24,6 +24,8 @@ import org.springframework.data.relational.core.dialect.RenderContextFactory;
 import org.springframework.data.relational.core.sql.*;
 import org.springframework.util.StringUtils;
 
+import java.util.List;
+
 /**
  * Unit tests for {@link SqlRenderer}.
  *
@@ -413,6 +415,29 @@ class SelectRendererUnitTests {
 				.isEqualTo("SELECT foo.bar FROM foo WHERE foo.bar IN (SELECT floo.bah FROM floo)");
 	}
 
+	@Test // GH-1831
+	void shouldRenderSimpleFunctionWithSubselect() {
+
+		Table foo = SQL.table("foo");
+
+		Table floo = SQL.table("floo");
+		Column bah = floo.column("bah");
+
+
+		Select subselect = Select.builder().select(bah).from(floo).build();
+
+		SimpleFunction func = SimpleFunction.create("func", List.of(SubselectExpression.of(subselect)));
+
+		Select select = Select.builder() //
+				.select(func.as("alias")) //
+				.from(foo) //
+				.where(Conditions.isEqual(func, SQL.literalOf(23))) //
+				.build();
+
+		assertThat(SqlRenderer.toString(select))
+				.isEqualTo("SELECT func(SELECT floo.bah FROM floo) AS alias FROM foo WHERE func(SELECT floo.bah FROM floo) = 23");
+	}
+
 	@Test // DATAJDBC-309
 	void shouldConsiderNamingStrategy() {