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SQL大坑：union会合并后去重，union all不会有distinct的结果
col1, col2的结果会去重： select col1, col2 from the_table1 union select col1, col2 from the_table2 col1, col2的结果不会去重： select col1, col2 from the_table1 union all select col1, col2 from the_table2 Spark的union…- 1
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