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The number and names of columns returned by each select_statement have to be the same. Otherwise, a schema error is thrown.
UNION within a FROM Clause
If some additional processing has to be done on the result of the UNION, the entire statement expression can be embedded in a FROM clause like below:
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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) |
Unions in DDL and Insert Statements
Unions can be used in views, inserts, and CTAS (create table as select) statements. A query can contain multiple UNION clauses, as shown in the syntax above.
Applying Subclauses
To apply ORDER BY, SORT BY, CLUSTER BY, DISTRIBUTE BY or LIMIT to an individual SELECT, place the clause inside the parentheses that enclose the SELECT:
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SELECT key FROM src UNION SELECT key FROM src1 ORDER BY key LIMIT 10 |
Column Aliases for Schema Matching
UNION expects the same schema on both sides of the expression list. As a result, the following query When a specific column value is given in one or more SELECT statements (but not in all of them), the UNION may fail with an error message such as "FAILED: SemanticException 4:47 Schema of both sides of union should match." Column aliases should be used for such values, for example
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INSERT OVERWRITE TABLE target_table
SELECT name, id, category FROM source_table_1
UNION ALL
SELECT name, id, "Category159" FROM source_table_2 |
In such cases, column aliases can be used to force equal schemas:
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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 |
Column Type Conversion
Before HIVE-14251 in release 2.2.0, Hive tries to perform implicit conversion across Hive type groups. With the change of HIVE-14251, Hive will only perform implicit conversion within each type group including string group, number group or date group, not across groups. In order to union the types from different groups such as a string type and a date type, an explicit cast from string to date or from date to string is needed in the query.
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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 |
Version Information
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In Hive 0.12.0 and earlier releases, unions can only be used within a subquery such as "SELECT * FROM (select_statement UNION ALL select_statement UNION ALL ...) unionResult". As of Hive 0.13.0, unions can also be used in a top-level query: "select_statement UNION ALL select_statement UNION ALL ...". (See HIVE-6189.) Before Hive 1.2.0, only UNION ALL (bag union) is supported. UNION (or UNION DISTINCT) is supported since Hive 1.2.0. (See HIVE-9039.) |
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