Filtering, Sorting, Functions, Subqueries, and Useful Query Patterns
Move beyond basic SELECT statements and learn the query patterns used in dashboards, search screens, and back-office tools.
Inside this chapter
- Filtering for Business Questions
- Ordering and Limiting Results
- Useful Built-In Functions
- Subqueries and Reusable Thinking
Series navigation
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Filtering for Business Questions
Most useful queries do not read every row. They answer a question such as: which orders are still pending, which customers joined this month, which products are out of stock, or which invoices exceed a threshold. SQL filtering lets you ask those targeted questions directly.
SELECT order_id, customer_id, order_status
FROM orders
WHERE order_status IN ('PENDING', 'PACKING')
AND order_date >= '2026-01-01';Ordering and Limiting Results
SELECT product_name, unit_price
FROM products
WHERE is_active = 1
ORDER BY unit_price DESC
LIMIT 10;This pattern is common in leaderboards, report previews, admin dashboards, and API endpoints. Without a clear ordering rule, paginated results may become inconsistent or confusing.
Useful Built-In Functions
MariaDB includes string, numeric, date, and conditional functions that let you shape query results without shifting everything into application code.
SELECT
full_name,
UPPER(status) AS normalized_status,
DATE(created_at) AS signup_date
FROM customers;
SELECT
product_name,
ROUND(unit_price * 1.18, 2) AS price_with_tax
FROM products;Subqueries and Reusable Thinking
SELECT customer_id, full_name
FROM customers
WHERE customer_id IN (
SELECT customer_id
FROM orders
WHERE order_status = 'PENDING'
);Subqueries can help express business logic, but they should be used thoughtfully. Sometimes a join is more efficient or easier to read. Advanced SQL maturity means choosing the clearest and most performant form for the problem.