SQL SELECT, Filtering, Sorting, and CASE

1. Introduction

Most analytical SQL starts by reading a table and narrowing it to records relevant to a question. Building a query in small steps makes it easier to understand why a row appears or is excluded.

2. Understand the concept

SELECT chooses output expressions; WHERE filters individual rows; ORDER BY controls display order. CASE creates a conditional value. SQL uses three-valued logic with NULL, so comparisons to NULL use IS NULL rather than equals.

3. Syntax and interpretation

SELECT order_id, amount,
       CASE WHEN amount >= 100 THEN 'Large' ELSE 'Small' END AS order_band
FROM orders
WHERE status = 'Complete' AND order_date >= '2026-01-01'
ORDER BY amount DESC;

Filtering occurs before the result is sorted. Specify the date boundary and required columns so the query’s scope is visible.

4. Apply it to an analysis

Build a readable SQL query by selecting columns, filtering rows, sorting, and deriving categories. Start with a small, known example and state what each row or value represents. After the transformation, check row counts, types, keys, and units against the original data. If an assumption fails, stop and investigate instead of presenting the output as a valid result.

5. Common mistakes

• Writing = NULL, which does not test for missing SQL values.
• Relying on implicit ordering without an ORDER BY clause.

6. Practice task

Select customer_id and amount for paid orders above 100, sorting largest amount first.

7. One possible solution

SELECT customer_id, amount
FROM orders
WHERE status = 'Paid' AND amount > 100
ORDER BY amount DESC;

8. Summary and next step

Build a readable SQL query by selecting columns, filtering rows, sorting, and deriving categories. Use the next lesson in this module to extend the idea with another technique and a more realistic analysis case.