UNION ALL in Oracle SQL – PLSQL

As the name suggests UNION ALL in Oracle SQL / PLSQL query is used to join or combine results from 2 or more SELECT statements, a UNION ALL does not excludes the duplicate rows returned by SQL SELECT statements.
Also, each SELECT statement must have the same number of column(s)/field(s) with similar data-types for the UNION ALL to work.

Syntax for UNION ALL in Oracle SQL / PLSQL is:

SELECT column_name1
,column_name2
.
.
,column_nameN
FROM table_name(s)
UNION ALL
SELECT column_name1
,column_name2
.
.
,column_nameN
FROM table_name(s);

Suppose have a table named ‘employee’ in the database as shown below:

Employee_Id Employee_Name Salary Department Commission
101 Emp A 10000 Sales 10
102 Emp B 20000 IT 20
103 Emp C 28000 IT 20
104 Emp D 30000 Support
105 Emp E 32000 Sales 10

Also we have one more table ‘comm’ as shown below:

Emp_Id Commission_Percent
102 20
103 20
104
105 10
106 15
107 25

Let’s take an example to understand UNION ALL, suppose we write our SQL query using UNION ALL as:

SELECT employee_id
FROM employee
UNION ALL
SELECT emp_id
FROM comm;

The result of the above query will be:

Emp_Id
101
102
102
103
103
104
104
105
105
106
107

Here we can see that the records for employee_id = ‘102’,’103,’104’ and ‘105’ appear twice in the result set because they are present in both (‘employee’ and ‘comm’) tables. Also note that all the ‘employee_id’ from both ‘employee’ and ‘comm’ tables are present in the result set.


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