case usage in oracle

CASE expressions in Oracle are used to evaluate conditions and return results based on different conditions. The syntax is: CASE WHEN condition THEN result ELSE default result END. Usage includes: 1. Determine conditions; 2. Return results; 3. Specify default results (optional). CASE expressions are readable, scalable, and improve performance, but the conditions must be mutually exclusive and the default result prevents NULL values ​​from being returned.

CASE usage in Oracle

CASE expression is used in Oracle to return different results based on given conditions powerful tool. The syntax is as follows:

<code class="sql">CASE
    WHEN 条件1 THEN 结果1
    WHEN 条件2 THEN 结果2
    ...
    ELSE 默认结果
END</code>

Usage:

• Determine the conditions: Specify the different conditions that need to be evaluated. A condition can be a Boolean expression, a logical expression, or a comparison.
• Return results: Specify the results to be returned for each condition. The result can be of any data type but should be compatible with the condition type.
• Default result (optional): Specify the default result returned when no conditions are met.

Example:

Query the customer’s membership level and classify them into different levels based on points:

<code class="sql">SELECT CASE
    WHEN points < 100 THEN 'Basic'
    WHEN points >= 100 AND points < 500 THEN 'Silver'
    WHEN points >= 500 THEN 'Gold'
    ELSE 'Unknown'
END AS membership_level
FROM customers;</code>

Advantages:

• High readability: CASE expressions are easy to read and understand because they organize conditions and results in a way similar to natural language.
• Extensibility: Conditions can be easily added or removed, adapting expressions to changing business needs.
• Improving performance: Compared to using multiple IF-THEN-ELSE statements, the CASE expression can improve query performance because it only evaluates the expression once.

Note:

• Each condition must be mutually exclusive, that is, any given record can only satisfy one condition.
• The default result is optional, but it prevents NULL values ​​from being returned if the conditions are not mutually exclusive.
• CASE expressions can be nested to handle more complex situations.

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