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.
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