Question

describe the commands union and minus, what are common pitfalls of the methods

Answer 1

You can easily combine different queries by using the set operators named union and minus.

Every set operator has equal precedence.

If the SQL statement includes different set operators, the oracle database actually evaluates everyone from left to right direction unless the parentheses specify the other orders.

If the component queries choose character data, then datatype of the return values are determined properly.