sql: use of and difference between union and union

SQL UNION operator

UNION operation result set operator for combining two or more of the SELECT statement.

Please note that the interior of the UNION SELECT statement must have the same number of columns. The columns must also have similar data types. Meanwhile, the order of columns in each SELECT statement must be the same. PURPOSE check out the data type of consent

SQL UNION Syntax

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

Note: By default, UNION operator to select a different value. If you 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

In addition, UNION result set is always equal to the column names in the first column name UNION SELECT statement.

Below the original table used in the examples:

Employees_China:

E_ID	E_Name
01	Zhang, Hua
02	Wang, Wei
03	Carter Thomas
04	Yang, Ming

Employees_USA:

E_ID	E_Name
01	Adams, John
02	Bush, George
03	Carter Thomas
04	Gates, Bill

Use UNION command

Examples

List all the different names of employees in China and the United States:

SELECT E_Name FROM Employees_China
UNION
SELECT E_Name FROM Employees_USA

result

E_Name
Zhang, Hua
Wang, Wei
Carter Thomas
Yang, Ming
Adams, John
Bush, George
Gates, Bill

Note: This command does not list all employees in China and the United States. In the above example, we have two employees with the same name, only one of them was listed. UNION command selects only distinct values.

UNION ALL

UNION UNION ALL command and the command is almost equivalent, but UNION ALL command lists all values.

SQL Statement 1
UNION ALL
SQL Statement 2

Use UNION ALL command

Example:

List all employees in China and the United States:

SELECT E_Name FROM Employees_China
UNION ALL
SELECT E_Name FROM Employees_USA

result

E_Name
Zhang, Hua
Wang, Wei
Carter Thomas
Yang, Ming
Adams, John
Bush, George
Carter Thomas
Gates, Bill