does JOINING on TABLES ALWAYS create Cartesian products of those tables ?
if a problem can be solved by sub-query and Joining which should i prefer ?
which one is fast and memory saving ?
All the questions are tightly coupled, so please dont tell to open another thread :(
<MySQL 5
JOINING在TABLES ALWAYS上制造了这些表格的 Car产品?
是的,但正常的排位随后被过滤,而排位优化则足以采取更有效的办法。 不管怎样,所有产品都无条件地加入“援外”产品。
如果一个问题可以通过分局和Joining来解决,而后者应当优先解决吗?
取决于查询最佳程度和有关数据。 如同:引证。 仅靠.。
which one is fast and memory saving ?
还是试图这样做。
这确实不取决于所涉表层面积和军事术语。 基本上,在多数情况下,一种精炼的tes产品是不良的方案规划(最后,放弃了合适的条件)或非常坏的盘问计划。 在大多数情况下,汽车产品单价从未实现,而电梯优化使用另一种方法。
Yes, joining always creates a Cartesian product. That s why you usually add a condition to be met:
JOIN a, b ON key_a = key_b. If the key_b is a UNIQUE KEY in table b, you will have at most one row from b for each row of a.
在大多数情况下,合并的速度更快,但有一些例外(WHERE x IN ANY (subquery))。
见2。
Yes, JOINING on tables always create Cartesian products of those tables.
you can t prefer join or sub-query because it depends on your data size
so, you have to try the two ways then you can decide which one is more appropriate
to your query
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