What is the difference between UNION and UNION ALL?

Question

What is the difference between UNION and UNION ALL?

Answer 1

Suppose that you have two table Teacher & Student

Both have 4 Column with different Name like this

Teacher - ID(int), Name(varchar(50)), Address(varchar(50)), PositionID(varchar(50))

Student- ID(int), Name(varchar(50)), Email(varchar(50)), PositionID(int)

You can apply UNION or UNION ALL for those two table which have same number of columns. But they have different name or data type.

When you apply UNION operation on 2 tables, it neglects all duplicate entries(all columns value of row in a table is same of another table). Like this

SELECT * FROM Student
UNION
SELECT * FROM Teacher

the result will be

When you apply UNION ALL operation on 2 tables, it returns all entries with duplicate(if there is any difference between any column value of a row in 2 tables). Like this

SELECT * FROM Student
UNION ALL
SELECT * FROM Teacher

Output

Performance:

Obviously UNION ALL performance is better that UNION as they do additional task to remove the duplicate values. You can check that from Execution Estimated Time by press ctrl+L at MSSQL