SELECT Statement

SELECT Statement

What it means

SELECT retrieves data from one or more tables — the most fundamental SQL statement. You specify which columns to return (or * for all), which table(s) to read from, and optionally filter, sort, and limit the results with other clauses (WHERE, ORDER BY, TOP, etc.).

Examples

SELECT * FROM Customers;                          -- every column, every row

SELECT Name, Email FROM Customers;                 -- specific columns only

SELECT Name AS CustomerName, Email FROM Customers;  -- column alias for output

SELECT DISTINCT Country FROM Customers;             -- unique values only, duplicates removed

SELECT TOP 10 * FROM Customers ORDER BY CreatedAt DESC;   -- first 10 rows, most recent first

SELECT Name, Price, Price * 1.08 AS PriceWithTax FROM Products;   -- computed column

Common mistake

Using SELECT * in production application code instead of naming the specific columns you need — it pulls unnecessary data over the network, breaks if the table’s columns change order or new ones are added, and can silently return more data than the application expects.

-- Fragile: pulls every column, including ones the app doesn't use,
-- and breaks assumptions if the schema changes later
SELECT * FROM Customers WHERE Id = 42;

-- Explicit, resilient to schema changes, and only transfers needed data
SELECT Id, Name, Email FROM Customers WHERE Id = 42;

Quick practice

1. What does SELECT DISTINCT Country FROM Customers; return?

   Answer

   Each unique value found in the Country column, with duplicates removed.

2. Why is SELECT * generally discouraged in application code, even though it’s convenient while exploring data?

   Answer

   It transfers unnecessary data, is fragile to schema changes (column additions/reordering), and makes it unclear exactly which columns the calling code actually depends on.

3. What does SELECT TOP 10 * FROM Customers ORDER BY CreatedAt DESC; return?

   Answer

   The 10 most recently created customers, since it sorts by CreatedAt descending before limiting to the first 10 rows.