In-depth description of SQL SERVER merge operations such as Union, Except T, and Intersect

There are several processing methods for the result set, which are worth explaining.
1. union (Union, union all)
This is simple. It combines two result sets horizontally. For example
SELECT * FROM
UNION
SELECT * FROM B
[Note] union deletes duplicate values. That is to say, duplicate rows in A and B will only appear once, while union all will keep duplicate rows.

2. Variance t)
It is the unique part of the two sets. For example
SELECT * FROM
EXCEPT
SELECT * FROM B
This means that no row exists in Table A of table B.

3. Intersection (intersect)
Is the common part of the two sets. For example
SELECT * FROM
INTERSECT
SELECT * FROM B
This means that records appear in both A and B