INSERT OVERWRITE TABLE dw_rpt_report60010105049 SELECT COALESCE(n.char3, dw.tran_country) AS tran_country, dw.col2, dw.col3, ... FROM dw_rpt_report60010105049 dw LEFT JOIN ( SELECT char2, MAX(char3) AS char3 FROM orsuser_kara.para_nationality GROUP BY char2 ) n ON dw.tran_country = n.char2 WHERE n.char2 IS NOT NULL 此段代码中的主表的tran_country不是唯一的，可能会导致发散，怎么优化

可以考虑使用子查询或者临时表的方式，先将主表的tran_country进行去重，再进行JOIN操作。具体实现方式如下：

使用子查询：

INSERT OVERWRITE TABLE dw_rpt_report60010105049 
SELECT COALESCE(n.char3, t.tran_country) AS tran_country, t.col2, t.col3, ...
FROM (
   SELECT DISTINCT tran_country, col2, col3, ... 
   FROM dw_rpt_report60010105049
) t
LEFT JOIN (
   SELECT char2, MAX(char3) AS char3 
   FROM orsuser_kara.para_nationality 
   GROUP BY char2 
) n ON t.tran_country = n.char2 
WHERE n.char2 IS NOT NULL

使用临时表：

CREATE TEMPORARY TABLE temp_table AS 
SELECT DISTINCT tran_country, col2, col3, ... 
FROM dw_rpt_report60010105049;

INSERT OVERWRITE TABLE dw_rpt_report60010105049 
SELECT COALESCE(n.char3, t.tran_country) AS tran_country, t.col2, t.col3, ...
FROM temp_table t
LEFT JOIN (
   SELECT char2, MAX(char3) AS char3 
   FROM orsuser_kara.para_nationality 
   GROUP BY char2 
) n ON t.tran_country = n.char2 
WHERE n.char2 IS NOT NULL;

DROP TEMPORARY TABLE temp_table;

无论使用哪种方式，在进行JOIN操作之前，都先对主表进行去重，这样可以避免发散的情况发生，提高查询效率。