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| toc_title |
|---|
| UNION |
UNION ALL Clause
You can use UNION ALL to combine any number of SELECT queries by extending their results. Example:
SELECT CounterID, 1 AS table, toInt64(count()) AS c
FROM test.hits
GROUP BY CounterID
UNION ALL
SELECT CounterID, 2 AS table, sum(Sign) AS c
FROM test.visits
GROUP BY CounterID
HAVING c > 0
Result columns are matched by their index (order inside SELECT). If column names do not match, names for the final result are taken from the first query.
Type casting is performed for unions. For example, if two queries being combined have the same field with non-Nullable and Nullable types from a compatible type, the resulting UNION ALL has a Nullable type field.
Queries that are parts of UNION ALL can’t be enclosed in round brackets. ORDER BY and LIMIT are applied to separate queries, not to the final result. If you need to apply a conversion to the final result, you can put all the queries with UNION ALL in a subquery in the FROM clause.
UNION DISTINCT Clause
The difference between UNION ALL and UNION DISTINCT is that UNION DISTINCT will do a distinct transform for union result, it is equivalent to SELECT DISTINCT from a subquery containing UNION ALL.
UNION Clause
By default, UNION has the same behavior as UNION DISTINCT, but you can specify union mode by setting union_default_mode, values can be 'ALL', 'DISTINCT' or empty string. However, if you use UNION with setting union_default_mode to empty string, it will throw an exception.
Implementation Details
Queries that are parts of UNION/UNION ALL/UNION DISTINCT can be run simultaneously, and their results can be mixed together.