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Entity Framework Extensions - Boost EF Core 9
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SQL - Union Operator

The UNION operator is used to combine result set of two or more SELECT queries. The UNION operator automatically removes duplicate rows from SELECT statement result set.

Syntax:

SELECT column_name1, column_name2,... FROM tables [WHERE Condition] UNION SELECT column_name1, column_name2, ... FROM tables [WHERE Condition];

For the demo purpose, we will use the following tables in all examples.

Employee
EmpIdFirstNameLastNameEmailSalaryHireDate
1'John''King''[email protected]'330002018-07-25
2'James''Bond'2018-07-29
3'Neena''Kochhar''[email protected]'170002018-08-22
4'Lex''De Haan''[email protected]'150002018-09-8
5'Amit''Patel'180002019-01-25
6'Abdul''Kalam''[email protected]'250002020-07-14
Employee_backup
EmpIdFirstNameLastNameEmailSalaryHireDate
1'John''King''[email protected]'330002018-07-25
2'James''Bond'2018-07-29
3'Neena''Kochhar''[email protected]'170002018-08-22
6'Abdul''K''[email protected]'250002020-07-14
7'Swati''Karia''[email protected]'220002020-09-18

Consider the following query with the UNION operator.

SQL Script: UNION Operator
SELECT * FROM Employee
UNION
    SELECT * from Employee_backup

Above query returns the distinct records in both the tables, as shown below.

EmpIdFirstNameLastNameEmailSalaryHireDate
1'John''King''[email protected]'330002018-07-25
2'James''Bond'2018-07-29
3'Neena''Kochhar''[email protected]'170002018-08-22
4'Lex''De Haan''[email protected]'150002018-09-8
5'Amit''Patel'180002019-01-25
6'Abdul''Kalam''[email protected]'250002020-07-14
6'Abdul''K''[email protected]'250002020-07-14
7'Swati''Karia''[email protected]'220002020-09-18

Note that both the queries must have equal number of expressions in their SELECT clause. The following query will raise an error.

SQL Script: Error with UNION
SELECT * FROM Employee
UNION
    SELECT EmpId, FirstName from Employee_backup

You can use the WHERE clause with any or all queries, as shown below.

SQL Script: UNION Query
SELECT * FROM Employee
WHERE Salary > 18000
UNION
    SELECT * from Employee_backup
WHERE Salary > 18000

Above query returns the distinct records in both the tables, as shown below.

EmpIdFirstNameLastNameEmailSalaryHireDate
1'John''King''[email protected]'330002018-07-25
6'Abdul''Kalam''[email protected]'250002020-07-14
6'Abdul''K''[email protected]'250002020-07-14
7'Swati''Karia''[email protected]'220002020-09-18
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