union 和 union all 的區別

原創 雨沫流沙 2020-02-25 00:43

union與union all的區別：
union會把兩個記錄集中相同的記錄合併，而union all不會，性能上前者優。

使用場景：
當確認多個記錄集不會存在相同記錄，或者有可能有相同記錄但明確要合併的，用union
即使有相同記錄也不合並的，用union all

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