问题之间的差异 [英] Diffrence between question

问题描述

union和unionall有什么区别？



谢谢。

解决方案

你好< br $>


查看以下链接....



联盟与联盟之间的差异 - 最佳性能比较 [ ^ ]



使用SQL Server中的UNION和UNION ALL连接数据 [ ^ ]



问候，

GVPrabu。

http://bit.ly/WhPbdN [ ^ ]

UNION

UNION command用于从两个表中选择相关信息，非常类似于JOIN命令。但是，使用UNION命令时，所有选定的列都必须具有相同的数据类型。使用UNION时，只选择不同的值。



UNION ALL

UNION ALL命令等于UNION命令，除了UNION ALL选择所有值。



Union和Union all之间的区别在于Union all不会消除重复行，而只是拉取所有行从适合您的查询细节的所有表中，并将它们组合成一个表。



UNION语句有效地对结果集执行SELECT DISTINCT。如果您知道返回的所有记录都是您的联盟中唯一的，请改用UNION ALL，它会提供更快的结果。



示例：

表1：第一，第二，第三，第四，第五

表2：第一，第二，第五，第六



结果集：

UNION：第一，第二，第三，第四，第五，第六（这将删除重复值）

UNION ALL：第一，第一，第二，第二，第三，第四，第五，第五，第六，第六（这将重复数值）



祝你好运，

OI

What is the difference between union and unionall?

Thank You.

解决方案

Hi

Check the following links....

Difference Between Union vs. Union All – Optimal Performance Comparison[^]

Joining data using UNION and UNION ALL in SQL Server[^]

Regards,
GVPrabu.

http://bit.ly/WhPbdN[^]

UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.

UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.

The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table.

A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.

Example:
Table 1 : First,Second,Third,Fourth,Fifth
Table 2 : First,Second,Fifth,Sixth

Result Set:
UNION: First,Second,Third,Fourth,Fifth,Sixth (This will remove duplicate values)
UNION ALL: First,First,Second,Second,Third,Fourth,Fifth,Fifth,Sixth,Sixth (This will repeat values)

Good luck,
OI

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