Explanation:
This statement returns the rows in the CUSTOMERS table that do not have a postal code. The correct syntax to check NULL values is usage of “IS NULL” clause.
Incorrect Answers:
1: “CONTAINS NULL” is incorrect clause in Oracle.
2: This statement will just check if postal code equals to string ‘________';
4: Usage of “IS NVL” is incorrect in Oracle. But there is a function NVL() you can use to process NULL values.
5: You can not use equal comparison to check whether value is NULL or not. Use construction “IS NULL” or “IS NOT NULL” to do that.
OCP Introduction to Oracle 9i: SQL Exam Guide, Jason Couchman, p. 31-32
Chapter 1: Overview of Oracle Databases

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