How Can SQL's `IF` Function Be Used in `SELECT` Statements for Conditional Value Selection?

Conditional Value Selection in SQL SELECT Statements using IF

SQL's IF function is crucial for dynamically choosing output values based on conditions. A frequent use case involves selecting values from different columns based on criteria within the same row.

Imagine a 'report' table with 'id' and 'amount' columns, and a 'type' column ('P' for positive, 'N' for negative). To display the 'amount' adjusted for its 'type', use the IF function:

<code class="language-sql">SELECT id, 
       IF(type = 'P', amount, amount * -1) AS amount
FROM report;</code>

This query uses IF to check if 'type' is 'P'. If true, it returns the 'amount'; otherwise, it returns the negative of 'amount'.

Handling potential NULL values requires IFNULL:

<code class="language-sql">SELECT id, 
       IF(type = 'P', IFNULL(amount,0), IFNULL(amount,0) * -1) AS amount
FROM report;</code>

IFNULL ensures that if 'amount' is NULL, 0 is used instead. This prevents NULL results and maintains data integrity. The nested IFNULL handles both positive and negative cases.

The IF function within SELECT statements provides the power to conditionally select values from various columns based on row-specific conditions, significantly enhancing data retrieval accuracy and efficiency.

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