SQL UNION Operator

The SQL UNION operator combines two or more SELECT statements.

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
  1. SELECT column_name(s) FROM table_name1  
  2. UNION  
  3. 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
  1. SELECT column_name(s) FROM table_name1  
  2. UNION ALL  
  3. SELECT column_name(s) FROM table_name2  
Note

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_England"

E_ID E_NAME
1
  Barnes, Stephen
2
 Dean, Alex
 3Taylor, John 
 4 Young, Colin

E_ID E_NAME
1
Taylor, John
2
Johnson, James
3 Brown, Mike
4 Miller, Andrew

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

We use the following SELECT statement,
  1. SELECT E_Name FROM Employees_England  
  2. UNION  
  3. SELECT E_Name FROM Employees_USA  
The result-set will look like this,

E_NAME
Barnes, Stephen
Dean, Alex
Taylor, John
Young, Colin
Johnson, James
Brown, Mike
Miller, Andrew

Note

This command cannot be used to list all employees in England 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 England and USA,
  1. SELECT E_Name FROM Employees_England  
  2. UNION ALL  
  3. SELECT E_Name FROM Employees_USA  
Result

E_NAME
Barnes, Stephen
Dean, Alex
Taylor, John
Young, Colin
Taylor, John
Johnson, James
Brown, Mike
Miller, Andrew