feat: SelectDataset 新增 Exists/ExistsContext

生成 SELECT 1 FROM ... LIMIT 1 判断是否存在符合条件的行，不用调用方自己拼
COUNT(*) 或者查完整行再判断长度。

select_dataset_test.go

@@ -1618,6 +1618,58 @@ func (sds *selectDatasetSuite) TestSetError() {
 	sds.Equal(err1, err)
 }
 
+func (sds *selectDatasetSuite) TestExists() {
+	mDB, sqlMock, err := sqlmock.New()
+	sds.NoError(err)
+	sqlMock.ExpectQuery(`SELECT 1 FROM "items" LIMIT 1`).
+		WithArgs().
+		WillReturnRows(sqlmock.NewRows([]string{"1"}).FromCSVString("1"))
+
+	db := dbv2.New("mock", mDB)
+	exists, err := db.From("items").Exists()
+	sds.NoError(err)
+	sds.True(exists)
+}
+
+func (sds *selectDatasetSuite) TestExists_NotFound() {
+	mDB, sqlMock, err := sqlmock.New()
+	sds.NoError(err)
+	sqlMock.ExpectQuery(`SELECT 1 FROM "items" LIMIT 1`).
+		WithArgs().
+		WillReturnRows(sqlmock.NewRows([]string{"1"}))
+
+	db := dbv2.New("mock", mDB)
+	exists, err := db.From("items").Exists()
+	sds.NoError(err)
+	sds.False(exists)
+}
+
+func (sds *selectDatasetSuite) TestExists_WithWhere() {
+	mDB, sqlMock, err := sqlmock.New()
+	sds.NoError(err)
+	sqlMock.ExpectQuery(`SELECT 1 FROM "items" WHERE \("name" = 'Bob'\) LIMIT 1`).
+		WithArgs().
+		WillReturnRows(sqlmock.NewRows([]string{"1"}).FromCSVString("1"))
+
+	db := dbv2.New("mock", mDB)
+	exists, err := db.From("items").Where(dbv2.C("name").Eq("Bob")).Exists()
+	sds.NoError(err)
+	sds.True(exists)
+}
+
+func (sds *selectDatasetSuite) TestExists_WithPreparedStatement() {
+	mDB, sqlMock, err := sqlmock.New()
+	sds.NoError(err)
+	sqlMock.ExpectQuery(`SELECT 1 FROM "items" WHERE \("name" = \?\) LIMIT \?`).
+		WithArgs("Bob", 1).
+		WillReturnRows(sqlmock.NewRows([]string{"1"}).FromCSVString("1"))
+
+	db := dbv2.New("mock", mDB)
+	exists, err := db.From("items").Prepared(true).Where(dbv2.C("name").Eq("Bob")).Exists()
+	sds.NoError(err)
+	sds.True(exists)
+}
+
 func TestSelectDataset(t *testing.T) {
 	suite.Run(t, new(selectDatasetSuite))
 }