One of the useful operators provided by SQLite is the UNION ALL operator, which is used to combine the results of two or more SELECT statements into a single result set.
The UNION ALL operator is used to merge the rows returned by two or more SELECT statements into a single result set. Unlike the UNION operator, which removes duplicates, the UNION ALL operator does not remove duplicates from the result set. This means that if a row appears in more than one SELECT statement, it will appear in the result set multiple times.
Syntax
The basic syntax for using the UNION ALL operator in SQLite is as follows:
SELECT column1, column2, ... FROM table1 UNION ALL SELECT column1, column2, ... FROM table2;
In this syntax, we are using two SELECT statements to retrieve data from two different tables. The results of these SELECT statements are combined using the UNION ALL operator. Note that both SELECT statements must have the same number of columns, and the data types of corresponding columns must be compatible.
Example
Let’s take a look at a simple example to better understand how the UNION ALL operator works in SQLite. Suppose we have two tables, “employees” and “customers,” with the following data:
Table: employees
| id | name | age |
|---|---|---|
| 1 | John | 25 |
| 2 | Mary | 30 |
| 3 | Bob | 35 |
Table: customers
| id | name | age |
|---|---|---|
| 1 | Alice | 22 |
| 2 | David | 27 |
| 3 | Sarah | 31 |
To retrieve all the names and ages of both employees and customers, we can use the following query:
SELECT name, age FROM employees UNION ALL SELECT name, age FROM customers; The result of this query will be: name age John 25 Mary 30 Bob 35 Alice 22 David 27 Sarah 31
As we can see, the UNION ALL operator combines the results of the two SELECT statements and returns all the rows without removing any duplicates.
In summary, the SQLite UNION ALL operator is a useful tool for combining the results of multiple SELECT statements into a single result set. It allows us to retrieve data from multiple tables and combine them in a way that preserves all the rows returned by the individual SELECT statements.