说明 HIVE 1.2.0版本之前仅支持： UNION ALL (bag union) 1.2.0版本之后：UNION的默认结果是去重后的结果
UNION within a FROM Clause：
/* 例子1 */ SELECT * FROM ( select_statement UNION ALL select_statement ) unionResult; /* 例子2 */ SELECT u.id, actions.date FROM ( SELECT av.uid AS uid FROM action_video av WHERE av.date = '2008-06-03' UNION ALL SELECT ac.uid AS uid FROM action_comment ac WHERE ac.date = '2008-06-03' ) actions JOIN users u ON (u.id = actions.uid); 使用 ORDER BY, SORT BY, CLUSTER BY, DISTRIBUTE BY or LIMIT： /* 例子1、修饰单个SELECT */ SELECT key FROM (SELECT key FROM src ORDER BY key LIMIT 10)subq1 UNION SELECT key FROM (SELECT key FROM src1 ORDER BY key LIMIT 10)subq2; /* 例子2、修饰整个UNION结果 */ SELECT key FROM src UNION SELECT key FROM src1 ORDER BY key LIMIT 10; /* 例子3、错误用法（×）：以下用法是错的 */ INSERT OVERWRITE TABLE target_table SELECT name, id, category FROM source_table_1 UNION ALL SELECT name, id, "Category159" FROM source_table_2; /* 例子4、正确用法 */ INSERT OVERWRITE TABLE target_table SELECT name, id, category FROM source_table_1 UNION ALL SELECT name, id, "Category159" as category FROM source_table_2; 列的值类型转换： （在HIVE-2.2.0版本之前，HIVE尽量试图执行类型转换，但是之后只在类型组内转换，即string group\ number group \ date group三个组内默认转换，跨组转换需要显示进行） SELECT name, id, cast('2001-01-01' as date) d FROM source_table_1 UNION ALL SELECT name, id, hiredate as d FROM source_table_2; 参考文献 https://cwiki.apache.org/confluence/display/Hive/LanguageManual+Union