In SQL, the UNION ALL operator is used to combine the results of two or more SELECT statements into a single result set, including duplicates. This operator is similar to the UNION operator, but it does not remove the duplicates from the result set.

The syntax of the UNION ALL operator is as follows:

SELECT column1, column2, ...
FROM table1
UNION ALL
SELECT column1, column2, ...
FROM table2;

Here, the SELECT statements can include any valid SQL statement, including subqueries, functions, and expressions.

Let’s take an example to understand how the UNION ALL operator works in SQL:

Assume that we have two tables, Table1 and Table2, with the following data:

Table1:

Id	Name	Age	City
1	John	25	New York
2	Sarah	30	Chicago
3	Peter	28	Miami
4	Rachel	24	Boston

Table2:

Id	Name	Age	City
5	Michael	32	Dallas
6	Emily	27	Boston
7	David	29	Miami
8	Alice	26	Denver

Now, if we want to combine the data from both tables, including duplicates, we can use the UNION ALL operator, as shown below:

SELECT Id, Name, Age, City
FROM Table1
UNION ALL
SELECT Id, Name, Age, City
FROM Table2;

The result set of this query would be as follows:

Id	Name	Age	City
1	John	25	New York
2	Sarah	30	Chicago
3	Peter	28	Miami
4	Rachel	24	Boston
5	Michael	32	Dallas
6	Emily	27	Boston
7	David	29	Miami
8	Alice	26	Denver

As you can see, the result set includes all the rows from both tables, including duplicates.

In conclusion, the UNION ALL operator is used to combine the results of two or more SELECT statements into a single result set, including duplicates. It is useful when we want to retrieve all the data from multiple tables, without removing the duplicates.

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