The used SELECT statements have a different number of columns

This error usually occurs when you are trying to combine two or more SELECT statements with a UNION operator, but the columns in each SELECT statement do not match. To fix this error, you need to make sure that the number of columns and their data types are the same in all the SELECT statements.

For example, if you have two SELECT statements like this:

SELECT name, age FROM users;
SELECT email, phone FROM contacts;

You cannot combine them using a UNION operator because they have different columns. Instead, you need to modify them to have the same number of columns:

SELECT name, age, NULL AS email, NULL AS phone FROM users;
SELECT NULL AS name, NULL AS age, email, phone FROM contacts;

In this modified version, both SELECT statements have four columns, with two columns set to NULL in each statement. Now you can combine them using a UNION operator:

SELECT name, age, email, phone
FROM (
  SELECT name, age, NULL AS email, NULL AS phone FROM users
  UNION
  SELECT NULL AS name, NULL AS age, email, phone FROM contacts
) AS combined;

This will give you a result set with four columns: name, age, email, and phone, where some of the columns may have NULL values.