AND, OR, and NOT

This lesson uses DDI_SQL_Training dataset version 1.0.0 on SQL Server 2019 or later. Real questions often have several conditions. Here the goal is North-region customers in Leeds or York, excluding North customers in other cities.

Run the example

SELECT CustomerID, City, Region
FROM dbo.Customers
WHERE Region = N'North'
  AND (City = N'Leeds' OR City = N'York')
ORDER BY CustomerID;

Read it step by step

1. Region = N'North' must be TRUE for every returned row because it is joined to the city group with AND.
2. The parentheses group City = N'Leeds' OR City = N'York'. A row passes that group when either city matches.
3. AND has higher precedence than OR. The parentheses make the intended grouping obvious to readers and prevent a different meaning if the expression grows.

Check the result

The four rows meet both the region rule and the grouped city rule. NOT can reverse a condition, for example NOT (Region = N'North'), but a comparison involving NULL may be UNKNOWN rather than TRUE or FALSE.

Practice

Find active products in category 1 or 3. Include parentheses so the active condition applies to both categories.

SELECT ProductID, CategoryID FROM dbo.Products WHERE IsActive = 1 AND (CategoryID = 1 OR CategoryID = 3) ORDER BY ProductID;