Answer / vivek

Anti-joins:
Anti-joins are written using the NOT EXISTS or NOT IN
constructs. An anti-join between two tables returns rows
from the first table for which there are no corresponding
rows in the second table. In other words, it returns rows
that fail to match the sub-query on the right side.

Suppose you want a list of departments with no employees.
You could write a query like this:
SELECT d.department_name
FROM departments d
MINUS
SELECT d.department_name
FROM departments d, employees e
WHERE d.department_id = e.department_id
ORDER BY department_name;

The above query will give the desired results, but it might
be clearer to write the query using an anti-join:
SELECT d.department_name
FROM departments d
WHERE NOT EXISTS (SELECT NULL
FROM employees e
WHERE e.department_id = d.department_id)
ORDER BY d.department_name;

have table with two columns with datatypes as number and varchar and the values in A column like 1,2,3 AND B column values like a,b,c. now need to display data in a single column as 1,a,2,b,3,c.

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