Shulou(Shulou.com)06/02 Report--

In this issue, the editor will bring you about the difference between UNION and UNION ALL in SQL. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

Most UNION uses union all to connect directly, and all values are fetched. Records may have duplicates. Union is the only value, and records are not duplicated.

1. The syntax of UNION is as follows:

[SQL statement 1]

UNION

[SQL statement 2]

2. The syntax of UNION ALL is as follows:

[SQL statement 1]

UNION ALL

[SQL statement 2]

Efficiency:

Both the UNION and UNION ALL keywords combine two result sets into one, but they are different in terms of usage and efficiency.

1. Processing of duplicate results: UNION will filter out duplicate records after table linking, and Union All will not remove duplicate records.

2. Processing of sorting: Union will sort according to the order of fields; UNION ALL will simply merge the two results and return them.

UNION ALL is much faster than UNION in terms of efficiency, so use UNION ALL if you can confirm that the two merged result sets do not contain duplicate data and do not need to be sorted.

The above is the difference between UNION and UNION ALL in SQL shared by Xiaobian. If you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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