add simple tests

Author: elianddb

enginetest/queries/join_queries.go

@@ -808,6 +808,41 @@ var JoinScriptTests = []ScriptTest{
 			},
 		},
 	},
+	{
+		// https://github.com/dolthub/dolt/issues/9807
+		Name: "FULL OUTER JOIN with empty tables",
+		SetUpScript: []string{
+			"CREATE TABLE t1 (i INT);",
+			"CREATE TABLE t2 (j INT);", 
+			"INSERT INTO t2 VALUES (1);",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "SELECT i, j FROM t1 FULL OUTER JOIN t2 ON 1=1;",
+				Expected: []sql.Row{
+					{nil, 1},
+				},
+			},
+			{
+				Query: "SELECT j, i FROM t2 FULL OUTER JOIN t1 ON 1=1;",
+				Expected: []sql.Row{
+					{1, nil},
+				},
+			},
+			{
+				Query: "SELECT i, j FROM t1 FULL OUTER JOIN t2 ON 1=0;",
+				Expected: []sql.Row{
+					{nil, 1},
+				},
+			},
+			{
+				Query: "SELECT j, i FROM t2 FULL OUTER JOIN t1 ON 1=0;",
+				Expected: []sql.Row{
+					{1, nil},
+				},
+			},
+		},
+	},
 	{
 		Name:        "Simple join query",
 		SetUpScript: []string{},

sql/rowexec/join_iters.go

@@ -516,6 +516,8 @@ func (i *fullJoinIter) Next(ctx *sql.Context) (sql.Row, error) {
 
 	for {
 		if i.r == nil {
+			// Phase 2 of FULL OUTER JOIN: return unmatched right rows as (null, rightRow).
+			// Use parentRow instead of leftRow since leftRow is nil when left side is empty.
 			iter, err := i.b.Build(ctx, i.rp, i.parentRow)
 			if err != nil {
 				return nil, err