UNION 和 UNION ALL 有什么区别? [英] What is the difference between UNION and UNION ALL?
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问题描述
UNION 和 UNION ALL 有什么区别?
推荐答案
UNION 删除重复记录(结果中的所有列都相同)，UNION ALL 会不是.
UNION removes duplicate records (where all columns in the results are the same), UNION ALL does not.
使用 UNION 而不是 UNION ALL 会降低性能，因为数据库服务器必须做额外的工作来删除重复的行，但通常你不希望重复项(尤其是在开发报告时).
There is a performance hit when using UNION instead of UNION ALL, since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports).
SELECT 'foo' AS bar UNION SELECT 'foo' AS bar
结果:
+-----+
| bar |
+-----+
| foo |
+-----+
1 row in set (0.00 sec)
联合所有示例:
SELECT 'foo' AS bar UNION ALL SELECT 'foo' AS bar
结果:
+-----+
| bar |
+-----+
| foo |
| foo |
+-----+
2 rows in set (0.00 sec)
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