Allow for arbitrary conditions as join condition.

Closes #995.
Original pull request: #1014.

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

@@ -325,6 +325,18 @@ class DefaultSelectBuilder implements SelectBuilder, SelectAndFrom, SelectFromAn
 			return this;
 		}
 
+		@Override
+		public SelectFromAndJoinCondition on(Condition condition) {
+
+			if (this.condition == null) {
+				this.condition = condition;
+			} else {
+				this.condition = this.condition.and(condition);
+			}
+
+			return this;
+		}
+
 		/*
 		 * (non-Javadoc)
 		 * @see org.springframework.data.relational.core.sql.SelectBuilder.SelectOnConditionComparison#equals(org.springframework.data.relational.core.sql.Expression)
@@ -348,6 +360,12 @@ class DefaultSelectBuilder implements SelectBuilder, SelectAndFrom, SelectFromAn
 		}
 
 		private void finishCondition() {
+
+			// Nothing to do if a complete join condition was used.
+			if (from == null && to == null) {
+				return;
+			}
+
 			Comparison comparison = Comparison.create(from, "=", to);
 
 			if (condition == null) {

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

@@ -501,6 +501,17 @@ public interface SelectBuilder {
 		 * @see Table#column(String)
 		 */
 		SelectOnConditionComparison on(Expression column);
+
+		/**
+		 * Declare a join condition in one step.
+		 *
+		 * Using conditions allows more flexibility in comparison to  {@link #on(Expression)} which only allows for equality comparisons chained together with `AND`.
+		 *
+		 * @param condition must not be {@literal null}.
+		 * @return {@code this} builder.
+		 * @see Conditions
+		 */
+		SelectFromAndJoinCondition on(Condition condition);
 	}
 
 	/**

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

@@ -156,6 +156,28 @@ class SelectRendererUnitTests {
 				+ "AND employee.tenant = department.tenant");
 	}
 
+	@Test // GH-995
+	public void shouldRenderArbitraryJoinCondition() {
+
+		Table employee = SQL.table("employee");
+		Table department = SQL.table("department");
+
+		Select select = Select.builder() //
+				.select(employee.column("id"), department.column("name")) //
+				.from(employee) //
+				.join(department) //
+				.on(
+						Conditions.isEqual( employee.column("department_id"),department.column("id")) //
+								.or( //
+						Conditions.isNotEqual( employee.column("tenant"),department.column("tenant")) //
+				)) //
+				.build();
+
+		assertThat(SqlRenderer.toString(select)).isEqualTo("SELECT employee.id, department.name FROM employee " //
+				+ "JOIN department ON employee.department_id = department.id " //
+				+ "OR employee.tenant != department.tenant");
+	}
+
 	@Test // DATAJDBC-309
 	void shouldRenderMultipleJoinWithAnd() {