The UNION and UNION ALL operators in MySQL are used to combine the result sets of two or more SELECT statements. The main difference between the two operators is that UNION removes duplicate rows, while UNION ALL does not.
The syntax for UNION is as follows:
SELECT column_list
FROM table_name1
UNION
SELECT column_list
FROM table_name2;
The syntax for UNION ALL is as follows:
SELECT column_list
FROM table_name1
UNION ALL
SELECT column_list
FROM table_name2;
The column_list is a list of columns that you want to retrieve. You can specify all columns by using the asterisk (*), or you can specify specific columns.
The table_name1 and table_name2 are the names of the tables that you want to combine.
For example, the following query will combine the result sets of the users and orders tables:
SELECT *
FROM users
UNION
SELECT *
FROM orders;
This query will return all rows from the users table and the orders table. However, it will also return duplicate rows if there are any users who have placed orders.
The following query will combine the result sets of the users and orders tables, but it will not return any duplicate rows:
SELECT *
FROM users
UNION ALL
SELECT *
FROM orders;
This query will return all rows from the users table and the orders table, but it will only return each row once.
I hope this helps! Let me know if you have any other questions.