英文:
Using Union on the same subselect
问题
SELECT "a" as var1, foo FROM (SELECT foo FROM table1) as a
UNION
SELECT "b" as var1, foo FROM a
我能将这个UNION改成只执行一次子查询吗？
我问这个是因为目前这看起来很愚蠢，对于大表table1或更复杂的子查询（这只是一个最小的例子），有可能提升计算时间。
我尝试过这个，看起来像是我想要的，但不起作用：
SELECT "a" as var1, per FROM (SELECT per FROM selectpers) as a
UNION
SELECT "b" as var1, per FROM a
英文:
SELECT "a" as var1, foo FROM (SELECT foo FROM table1) as a
UNION`
SELECT "b" as var1, foo FROM (SELECT foo FROM table1) as b
Can I change this UNION to only do the sub select once?
I am asking because currently this looks stupid and for large table1 or more complex sub selects (this is just a minimal example) there is potential for make the computation time better.
I tried this, which would look like what i want but doesn't work:
SELECT "a" as var1, per FROM (SELECT per FROM selectpers) as a
UNION SELECT "b" as var1, per FROM a
答案1
得分: 0
SELECT x.var1, table1.foo
FROM table1
CROSS JOIN ( SELECT 'a' as var1
UNION ALL
SELECT 'b'
) x
英文:
SELECT x.var1, table1.foo
FROM table1
CROSS JOIN ( SELECT 'a' as var1
UNION ALL
SELECT 'b'
) x
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