fix: allow LIMIT 0 for SELECT queries

This commit enables the correct queries of type `SELECT ... LIMIT 0`.
Before that, the limit-clause wasn't applied to the query.

internal/dbtest/db_test.go

@@ -234,6 +234,7 @@ func TestDB(t *testing.T) {
 		{testNilModel},
 		{testSelectScan},
 		{testSelectCount},
+		{testSelectLimit},
 		{testSelectMap},
 		{testSelectMapSlice},
 		{testSelectStruct},
@@ -348,6 +349,36 @@ func testSelectCount(t *testing.T, db *bun.DB) {
 	require.Equal(t, 3, count)
 }
 
+func testSelectLimit(t *testing.T, db *bun.DB) {
+	if !db.Dialect().Features().Has(feature.CTE) {
+		t.Skip()
+		return
+	}
+
+	values := db.NewValues(&[]map[string]interface{}{
+		{"num": 1},
+		{"num": 2},
+		{"num": 3},
+	})
+
+	q := db.NewSelect().
+		With("t", values).
+		Column("t.num").
+		TableExpr("t")
+
+	count, err := q.Limit(5).Count(ctx)
+	require.NoError(t, err)
+	require.Equal(t, 3, count)
+
+	count, err = q.Limit(2).Count(ctx)
+	require.NoError(t, err)
+	require.Equal(t, 2, count)
+
+	count, err = q.Limit(0).Count(ctx)
+	require.NoError(t, err)
+	require.Equal(t, 0, count)
+}
+
 func testSelectMap(t *testing.T, db *bun.DB) {
 	var m map[string]interface{}
 	err := db.NewSelect().
@@ -1344,6 +1375,9 @@ func testScanAndCount(t *testing.T, db *bun.DB) {
 	})
 
 	t.Run("no limit", func(t *testing.T) {
+		err := db.ResetModel(ctx, (*Model)(nil))
+		require.NoError(t, err)
+
 		src := []Model{
 			{Str: "str1"},
 			{Str: "str2"},
@@ -1357,6 +1391,24 @@ func testScanAndCount(t *testing.T, db *bun.DB) {
 		require.Equal(t, 2, count)
 		require.Equal(t, 2, len(dest))
 	})
+
+	t.Run("limit 0", func(t *testing.T) {
+		err := db.ResetModel(ctx, (*Model)(nil))
+		require.NoError(t, err)
+
+		src := []Model{
+			{Str: "str1"},
+			{Str: "str2"},
+		}
+		_, err = db.NewInsert().Model(&src).Exec(ctx)
+		require.NoError(t, err)
+
+		var dest []Model
+		count, err := db.NewSelect().Model(&dest).Limit(0).ScanAndCount(ctx)
+		require.NoError(t, err)
+		require.Equal(t, 0, count)
+		require.Equal(t, 0, len(dest))
+	})
 }
 
 func testEmbedModelValue(t *testing.T, db *bun.DB) {

query_select.go

@@ -48,6 +48,7 @@ func NewSelectQuery(db *DB) *SelectQuery {
 				conn: db.DB,
 			},
 		},
+		limit: -1,
 	}
 }
 
@@ -631,7 +632,7 @@ func (q *SelectQuery) appendQuery(
 				b = append(b, " ROWS"...)
 			}
 		} else {
-			if q.limit > 0 {
+			if q.limit >= 0 {
 				b = append(b, " LIMIT "...)
 				b = strconv.AppendInt(b, int64(q.limit), 10)
 			}
@@ -958,7 +959,7 @@ func (q *SelectQuery) scanAndCountConc(ctx context.Context, dest ...interface{})
 	var mu sync.Mutex
 	var firstErr error
 
-	if q.limit >= 0 {
+	if q.limit >= -1 {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
@@ -995,7 +996,7 @@ func (q *SelectQuery) scanAndCountConc(ctx context.Context, dest ...interface{})
 func (q *SelectQuery) scanAndCountSeq(ctx context.Context, dest ...interface{}) (int, error) {
 	var firstErr error
 
-	if q.limit >= 0 {
+	if q.limit >= -1 {
 		firstErr = q.Scan(ctx, dest...)
 	}