SQL SELECT Statement: Retrieve Data from a Table (Examples)

SQL Tutorial · Chapter 3 of 32

The SQL SELECT statement retrieves data from one or more tables and returns it as a result set of rows and columns. You name the columns you want after SELECT and the table after FROM. SELECT is the most used command in SQL: nearly every report, dashboard and analysis starts with it.

SQL SELECT Syntax

SELECT column1, column2, ...
FROM table_name;

-- all columns
SELECT * FROM table_name;

The column list controls which columns come back and in what order. The asterisk * is shorthand for every column in the table.

Sample Tables

Example 1: Select Specific Columns

SELECT Name, Country
FROM Customers;

Only the two named columns are returned, in the order you listed them, for all five rows.

Example 2: Select All Columns

SELECT * FROM Orders;

This returns the complete Orders table shown above: six rows and five columns. SELECT * is handy for exploring a table, but in production queries list the columns explicitly so the result does not change when someone adds a column.

Example 3: Calculated Columns

SELECT can return expressions, not just stored columns. Here we add an 18% tax to each order amount and give the new column a name with an alias.

SELECT OrderID, Amount, Amount * 1.18 AS AmountWithTax
FROM Orders;

Example 4: Text Expressions and Constants

SELECT Name, City || ', ' || Country AS Location
FROM Customers;

|| is the standard concatenation operator; see the dialect notes below for MySQL and SQL Server.

Example 5: SELECT Without a Table

SELECT 7 * 6 AS Answer;

Useful for testing functions and expressions. Oracle requires FROM DUAL for this form.

Works in MySQL, SQL Server, PostgreSQL, SQLite

• Concatenation: || works in PostgreSQL, SQLite and Oracle. MySQL uses CONCAT(City, ', ', Country); SQL Server uses + or CONCAT().
• Integer division: 7 / 2 returns 3 in SQL Server and PostgreSQL but 3.5 in MySQL and SQLite. Use 7.0 / 2 for a decimal result.
• Row limiting: covered in the TOP / LIMIT chapter.

How the Database Processes a SELECT

Although you write SELECT first, the database evaluates it almost last. The logical order is FROM (find the table), then WHERE (filter rows), GROUP BY and HAVING, then SELECT (compute the output columns), then ORDER BY and finally LIMIT. This explains two rules you will meet in later chapters: a column alias defined in SELECT can be used in ORDER BY but not in WHERE, and aggregate functions belong to the grouping stage rather than the row filter.

Knowing the order also helps with performance thinking. Every column you leave out of the SELECT list is data the database does not have to read from disk or send over the network, which is the practical reason to avoid SELECT * in reports and applications.

Common Mistakes

• Forgetting the comma between column names, which makes the second name an alias of the first.
• Using SELECT * in application code, so the query breaks or slows when the table grows.
• Referencing a column that does not exist in the FROM table; check spelling and the table name.

Practice Exercise

1. Return the Product and Amount of every order, with a third column showing the Amount doubled, named DoubleAmount.
2. Return only the City column from Customers.

-- 1
SELECT Product, Amount, Amount * 2 AS DoubleAmount FROM Orders;

-- 2
SELECT City FROM Customers;
-- Mumbai, London, Madrid, Singapore, Sydney

Related Chapters

• SQL SELECT DISTINCT – return unique values only
• SQL WHERE – filter which rows SELECT returns
• SQL Aliases – naming calculated columns
• SQL from Scratch: full course overview

FAQ

What does SELECT * mean in SQL?

The asterisk selects every column of the table in the order they were defined. It is convenient for exploring data but explicit column lists are better in saved queries and application code.

Can SELECT return calculated values?

Yes. Any expression such as Amount * 1.18, text concatenation or a function call can appear in the SELECT list. Give it a readable name with AS.

Does SELECT change the data in the table?

No. SELECT only reads data. Changing rows requires INSERT, UPDATE or DELETE, covered later in this course.

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