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.go

@@ -701,6 +701,17 @@ func (sd *SelectDataset) ScanValContext(ctx context.Context, i interface{}) (boo
 }
 
 // Generates the SELECT COUNT(*) sql for this dataset and uses Exec#ScanVal to scan the result into an int64.
+// Generates SELECT 1 FROM ... LIMIT 1 and returns true if any row exists.
+func (sd *SelectDataset) Exists() (bool, error) {
+	return sd.ExistsContext(context.Background())
+}
+
+// Generates SELECT 1 FROM ... LIMIT 1 and returns true if any row exists.
+func (sd *SelectDataset) ExistsContext(ctx context.Context) (bool, error) {
+	var v int64
+	return sd.Select(L("1")).Limit(1).ScanValContext(ctx, &v)
+}
+
 func (sd *SelectDataset) Count() (int64, error) {
 	return sd.CountContext(context.Background())
 }