2.4 — Multi-Condition Filtering

What You'll Learn

• How to use AND, OR, and parentheses to combine filters
• How this compares to advanced Excel filters

Combining Conditions

Excel:
Use "Custom Filter" to show rows where Product is "Apples" and Amount is greater than 100.

SQL:

SELECT * FROM sales
WHERE Product = 'Apples' AND Amount > 100;

• AND means both conditions must be true.

Using OR

Show sales for "Apples" or "Oranges":

SELECT * FROM sales
WHERE Product = 'Apples' OR Product = 'Oranges';

• OR means either condition can be true.

Grouping with Parentheses

Show sales for "Apples" with Amount > 100, or any sales for "Oranges":

SELECT * FROM sales
WHERE (Product = 'Apples' AND Amount > 100)
   OR Product = 'Oranges';

• Parentheses control the order of logic, just like in Excel formulas.

Special Note on NULL Values

• NULL means missing or unknown data.

• Comparisons like = NULL or <> NULL do not work.

• Use IS NULL or IS NOT NULL:

  WHERE Amount IS NULL

Common Mistakes to Avoid

• Using = or <> with NULL:

  WHERE Amount = NULL   -- Incorrect!
  WHERE Amount <> NULL  -- Incorrect!

  Always use IS NULL or IS NOT NULL for NULL checks.

• Forgetting Parentheses in Complex Conditions:

  WHERE Product = 'Apples' AND Amount > 100 OR Product = 'Oranges'

  This may not filter as you expect. Use parentheses to clarify:

  WHERE (Product = 'Apples' AND Amount > 100) OR Product = 'Oranges'

• Forgetting quotes around text values:

  WHERE Product = Apples   -- Incorrect!
  WHERE Product = 'Apples' -- Correct

• Case sensitivity:
  Some databases treat 'apples' and 'Apples' as different. Use LOWER() for case-insensitive comparisons if needed:

  WHERE LOWER(Product) = 'apples'

Key Points

• Combine conditions with AND, OR, and parentheses for advanced filtering.
• Always use IS NULL to check for missing data.

Next Steps

In the next module, you'll learn how to transform and sort your data—just like using formulas and sorting in Excel.