Basic OR Usage

When you use the OR operator, at least one condition must be TRUE for a row to be included in the results. If all conditions are FALSE or NULL, the row is excluded.

Combining Multiple OR Conditions

You can chain multiple conditions with OR to create more inclusive filters. If any condition is TRUE, the row will be included.

Truth Table for OR

The OR operator follows this truth table for two conditions A and B:

Order of Operations: OR and AND

When combining OR and AND operators, AND has higher precedence than OR. This means that AND conditions are evaluated first, then OR conditions.

Warning: This is a common source of logical errors in SQL. Always use parentheses to make your intention clear.

Combining OR with AND Using Parentheses

Use parentheses to explicitly control the order of evaluation when combining OR and AND.

Note: The first query finds both tutorials and guides, but only if they have over 1000 views. The second finds all tutorials regardless of views, plus guides with over 1000 views.

Combining OR with NOT

Use NOT with OR to create complex filtering conditions.

Tip: When using NOT with OR, remember that NOT (A OR B) is equivalent to (NOT A) AND (NOT B) - this is De Morgan's Law.

Example 1: Multi-Keyword Content Search

This example shows how to search for videos with multiple keyword patterns:

Optimization Tip: For searching across many values in the same column, consider using the IN operator instead of multiple OR conditions. Example: WHERE title IN ("tutorial", "guide", "how to")

Example 2: Flexible Content Discovery

This example shows how to find popular content using different criteria:

Example 3: Complex Filtering with OR and AND

This example demonstrates proper use of parentheses with OR and AND:

Translation: This query finds videos that are: (1) either about tutorials OR education, AND (2) either have few views (< 1000) OR are very popular (> 50000 views), AND (3) uploaded this year.

When checking a column against multiple possible values, you can use either OR or the IN operator. The IN operator is generally more concise and often performs better.

1. Use Parentheses for Clarity

Always use parentheses when combining OR with AND operators to make your query's logic clear and avoid unexpected results due to operator precedence.

2. Consider Using IN Instead of Multiple ORs

When checking the same column against multiple values, the IN operator is more concise and often performs better than multiple OR conditions.

3. Be Aware of Performance Implications

OR conditions can sometimes prevent the database from using indexes effectively. In performance-critical scenarios, consider restructuring the query or using UNION to combine separate queries.

4. Understand NULL Behavior with OR

Remember that OR with NULL follows three-valued logic. If one condition is TRUE, the result is TRUE regardless of the other. If one condition is NULL and the other is FALSE, the result is NULL.

This can lead to unexpected filtering behavior. When a WHERE clause filters with an OR condition:

• Rows are included when the entire condition is TRUE
• Both FALSE and NULL results are excluded from the result set
• TRUE OR NULL always evaluates to TRUE, so rows will be included