What is the difference between UNION and UNION ALL?

Question

What is the difference between UNION and UNION ALL?

Answer 1

UNION: This command is used to select the tuples which have related information from two or more tables.

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If you understand by watching videos, here’s the informative video about UNION

It’s similar to JOIN command. But when we are using UNION command, the selected columns must be of the same data type. It removes all the duplicate records from the final result.

Syntax

Select Column1, Column2, Column3 from Table A

UNION

Select Column1, Column2, Column3 from Table B

 

Example: There are two tables named Student and Teacher

TABLE 1: Student

StudentID	StudName	StudentAge
1	Bhumi	16
2	Manas	17
3	Mallik	18
4	Suraj	15
5	Raj	15.5

 

TABLE 2: Teacher

TeacherID	TeachName	TeacherAge
1	Betty	28
2	Mallik	29
3	James	45
4	Suraj	30

Query: 

SELECT StudName FROM Student

UNION

SELECT TeachName FROM Teacher

NO	StudName
1	Bhumi
2	Betty
3	James
4	Manas
5	Mallik
6	Raj
7	Suraj

Here, you can see the duplicate tuples are removed.

UNION ALL: This command is the same as UNION command. It just concatenates the records. Unlike UNION, UNION ALL pulls all the values from all the tables i.e. it doesn’t eliminate duplicate records.

Syntax

Select Column1, Column2, Column3 from Table A

UNION

Select Column1, Column2, Column3 from Table B

Example: Consider the above two tables Student and Teacher.

 Query:

Select Column1, Column2, Column3 from Table A

UNION ALL

Select Column1, Column2, Column3 from Table B

NO	StudName
1	Bhumi
2	Manas
3	Mallik
4	Suraj
5	Raj
6	Betty
7	Mallik
8	James
9	Suraj

You can observer here, the result is a combination of both the tables. 

UNION vs UNION ALL

The only difference between Union and Union All is that Union All will not removes duplicate rows or records, instead, it just selects all the rows from all the tables which meets the conditions of your specifics query and combines them into the result table.

 UNION doesn’t work with a column that has Text Data Type. Whereas, UNION ALL works with all data type columns.