Oracle SQL Certification 1Z0-071 Course | Section 9: Conditional Expressions

In this section, we cover the most important conditional expressions supported by Oracle SQL, focusing on syntax, behavior, and exam relevant details.

Conditional expressions allow you to introduce decision making logic directly inside SQL statements. Instead of returning raw column values, you can evaluate conditions and return different results depending on the data. This is a critical skill for real world SQL usage and a frequently tested topic in the Oracle SQL 1Z0-071 certification exam.

CASE Expression

The CASE expression is the SQL equivalent of an if else statement. It evaluates conditions sequentially and returns a value when the first matching condition is found.

Key characteristics of CASE
  • Evaluates conditions from top to bottom
  • Returns a single value
  • Can be used in SELECT, WHERE, ORDER BY, and other clauses
  • Supports both numeric and character results
Basic CASE syntax

A CASE expression consists of:

  • WHEN clauses that define conditions
  • THEN clauses that define returned values
  • An optional ELSE clause
  • A mandatory END keyword

If no WHEN condition matches and the ELSE clause is omitted, the CASE expression returns NULL.

Exam notes
  • CASE expressions are evaluated row by row
  • All returned values must be compatible data types
  • CASE is ANSI SQL and preferred over DECODE in modern SQL

CASE expressions are essential for readable and flexible SQL queries and appear often in certification questions.

DECODE Function

DECODE is an Oracle specific function that provides conditional logic similar to CASE. It compares an expression to a list of values and returns a corresponding result.

How DECODE works

DECODE evaluates equality only. It does not support complex conditions such as greater than or less than comparisons.

The structure follows a simple pattern:

  • Expression to evaluate
  • Search value
  • Result value
  • Optional default value

If no match is found and no default is provided, DECODE returns NULL.

CASE vs DECODE
  • CASE is more flexible and readable
  • DECODE is more compact for simple equality checks
  • DECODE is commonly found in legacy Oracle SQL
Exam notes
  • DECODE performs implicit equality comparisons only
  • Default value behavior is frequently tested
  • CASE is generally recommended unless DECODE is explicitly required
NVL and NVL2

NULL values can cause calculations and comparisons to behave unexpectedly. Oracle provides NVL and NVL2 to handle NULL values safely.

NVL function

NVL replaces a NULL value with a specified replacement value.

Important rules:

  • Both arguments must be compatible data types
  • NVL returns the data type of the first argument

Common use cases include replacing NULL salaries, commissions, or numeric values used in calculations.

NVL2 function

NVL2 evaluates whether a value is NULL and returns one result if it is not NULL and another if it is NULL.

This allows you to apply conditional logic without writing a full CASE expression.

Exam notes
  • NVL is often used in arithmetic expressions
  • NVL2 checks NULL status, not value equality
  • Understanding NULL behavior is critical for exam success
NULLIF Function

NULLIF compares two expressions and returns NULL if they are equal. If they are not equal, it returns the first expression.

When to use NULLIF
  • Detect unchanged values
  • Prevent divide by zero errors
  • Simplify conditional comparisons

A common example is comparing two scores or values and returning NULL when they match.

Exam notes
  • NULLIF returns NULL only when values are equal
  • Data types must be compatible
  • Often tested together with NVL and CASE
Key Exam Takeaways
  • CASE is the most flexible conditional expression and heavily tested
  • DECODE is Oracle specific and limited to equality checks
  • NVL and NVL2 are essential for handling NULL values correctly
  • NULLIF simplifies equality based NULL logic
  • Conditional expressions always return a single value per row

Mastering conditional expressions will significantly improve your ability to write clean, reliable SQL and will help you answer many Oracle SQL certification questions with confidence.

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