Google bigquery 如何在BigQuery中连接具有特定条件的表？

BigQuery中有两个表的结构如下

t1
源代码站点\u id当前\u阶段
1 TR 3453 1
2纽约3459534 3
2 PO 4456456 5
3 PO 2345345 5
3 TY 2342342 3
4 TY 9384775 2
5 TR 2345245 0
t2
源代码站点\u id应为\u阶段
1 TR 3453 2
2 PO 4456456
3 TY 2342空
6 YO 7879897 4

SELECT source, code, site_id, current_stage, 
  IF(t2.source IS NULL, current_stage, expected_stage) AS expected_stage 
FROM t1 
LEFT JOIN t2 
USING (source, code, site_id)