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.

Limitations 

Only UNION ALL is supported. The regular UNION (UNION DISTINCT) is not supported. If you need UNION DISTINCT, you can write SELECT DISTINCT from a subquery containing UNION ALL.

Implementation Details 

Queries that are parts of UNION ALL can be run simultaneously, and their results can be mixed together.

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