SQL UNION Operator

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# SQL UNION Operator

The SQL UNION operator combines two or more SELECT statements.

## The SQL UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements.

Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.

### SQL UNION Syntax

    SELECT column_name(s) FROM table_name1
    UNION
    SELECT column_name(s) FROM table_name2

Note: The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL.

### SQL UNION ALL Syntax

    SELECT column_name(s) FROM table_name1
    UNION ALL
    SELECT column_name(s) FROM table_name2

PS: The column names in the result-set of a UNION are always equal to the column names in the first SELECT statement in the UNION.

## SQL UNION Example

Look at the following tables:

"Employees_Norway":

E_ID | E_Name
--- | ---
01 | Hansen, Ola
02 | Svendson, Tove
03 | Svendson, Stephen
04 | Pettersen, Kari

"Employees_USA":

E_ID | E_Name
--- | ---
01 | Turner, Sally
02 | Kent, Clark
03 | Svendson, Stephen
04 | Scott, Stephen

Now we want to list all the different employees in Norway and USA.

We use the following SELECT statement:

    SELECT E_Name FROM Employees_Norway
    UNION
    SELECT E_Name FROM Employees_USA

The result-set will look like this:

E_Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Scott, Stephen

Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them will be listed. The UNION command selects only distinct values.

## SQL UNION ALL Example

Now we want to list all employees in Norway and USA:

    SELECT E_Name FROM Employees_Norway
    UNION ALL
    SELECT E_Name FROM Employees_USA

Result

E_Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Svendson, Stephen
Scott, Stephen

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