Filtering with WHERE

The WHERE clause filters which rows a query returns, based on one or more conditions evaluated against each row.

Comparison and Logical Operators

WHERE supports standard comparison operators, and conditions can be combined with AND/OR to build precise filters.

SELECT * FROM Products
WHERE Price > 100 AND Category = 'Electronics';

SELECT * FROM Orders
WHERE Status = 'Pending' OR Status = 'Processing';

SELECT * FROM Employees
WHERE Department IN ('Sales', 'Marketing');

Pattern Matching and NULL

LIKE matches text patterns using wildcards (% for any sequence, _ for a single character). NULL requires special handling — it represents "unknown", so it must be checked with IS NULL, never = NULL.

SELECT * FROM Customers
WHERE Email LIKE '%@gmail.com';

SELECT * FROM Employees
WHERE ManagerId IS NULL; -- top-level employees with no manager

Common Mistakes

• Writing WHERE column = NULL instead of WHERE column IS NULL — the equals operator never matches NULL.
• Forgetting parentheses when mixing AND and OR, which can change a condition's meaning due to operator precedence.
• Using LIKE '%text%' on very large tables without an appropriate index, which can be slow.
• Case-sensitivity assumptions — T-SQL's default collation is often case-insensitive, which can surprise developers coming from other languages.