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简化的例子1:在子查询或主查询中,什么是优选的?
SELECT * FROM (
SELECT x, y, z FROM table1
WHERE x = 'test'
-- union, etc, etc, complicated stuff...
) AS t
-- union, etc, etc, complicated stuff...
简化示例2:
SELECT * FROM (
SELECT x, y, z FROM table1
-- union, etc, etc, complicated stuff...
) AS t
-- union, etc, etc, complicated stuff...
WHERE x = 'test'
哪以上是更受欢迎?更高性能?建议其他原因?在进行联合和类似操作之前,它是否有助于“尽早”筛选结果?谢谢。