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ClickHouse server version 21.6.6
通过设置union_default_mode等于DISTINCT，相当于union的功能；
通过设置union_default_mode等于ALL，相当于union all的功能；
dblab-VirtualBox :) SET union_default_mode = 'DISTINCT';
SET union_default_mode = 'DISTINCT'
Query id: 8fcb74e5-8067-4ddf-a853-c722e0c62d91
Ok.
0 rows in set. Elapsed: 0.003 sec.
dblab-VirtualBox :) SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 2;
SELECT 1
UNION
SELECT 2
UNION
SELECT 3
UNION
SELECT 2
Query id: 83fa959e-3efc-42b7-8681-2ef4abaeb92a
┌─1─┐
│ 1 │
└───┘
┌─1─┐
│ 2 │
└───┘
┌─1─┐
│ 3 │
└───┘
3 rows in set. Elapsed: 0.004 sec.
dblab-VirtualBox :) SET union_default_mode = 'ALL';
SET union_default_mode = 'ALL'
Query id: 56747ae8-7d40-4875-95cc-0d822dd12b2a
Ok.
0 rows in set. Elapsed: 0.002 sec.
dblab-VirtualBox :) SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 2;
SELECT 1
UNION
SELECT 2
UNION
SELECT 3
UNION
SELECT 2
Query id: bfbfee71-ce88-4e9c-a248-78f8f4ca5338
┌─1─┐
│ 1 │
└───┘
┌─1─┐
│ 2 │
└───┘
┌─1─┐
│ 3 │
└───┘
┌─1─┐
│ 2 │
└───┘
4 rows in set. Elapsed: 0.003 sec.
