SQL Basics — SELECT, WHERE, ORDER BY & Operators

0. The Sample Table Used Throughout

Every example below uses this simple customers table. Read it once before moving on — the queries will make far more sense when you can picture the data.

1. The SELECT Statement

SELECT is how you retrieve data. Its job is to decide which columns you want to see. Think of it as choosing the vertical slices of a table.

Selecting specific columns

-- Choose just the name and balance of every customer
SELECT customer_name, balance
FROM customers;

Selecting all columns

-- The asterisk (*) means "every column"
SELECT *
FROM customers;

Giving a column a friendly name (alias)

SELECT customer_name AS name,
       balance
FROM customers;

Use AS to rename a column in the output. It doesn't change the table — only the display.

2. Filtering Rows with WHERE

WHERE decides which rows to keep. While SELECT picks columns, WHERE picks rows based on a condition.

-- Customers from Mumbai only
SELECT customer_name, city
FROM customers
WHERE city = 'Mumbai';

Comparison operators

-- Customers with a balance above 50,000
SELECT customer_name, balance
FROM customers
WHERE balance > 50000;

-- Customers who joined in 2023 or later
SELECT customer_name, join_date
FROM customers
WHERE join_date >= '2023-01-01';

3. Logical Operators — AND, OR, NOT

Combine multiple conditions in a single WHERE clause.

AND — both conditions must be true

-- Mumbai customers with a balance above 20,000
SELECT customer_name, city, balance
FROM customers
WHERE city = 'Mumbai'
  AND balance > 20000;

OR — at least one condition must be true

-- Customers in Mumbai OR Delhi
SELECT customer_name, city
FROM customers
WHERE city = 'Mumbai'
   OR city = 'Delhi';

NOT — reverses a condition

-- Customers NOT in Mumbai
SELECT customer_name, city
FROM customers
WHERE NOT city = 'Mumbai';

4. IN, BETWEEN, and LIKE

IN — match any value in a list

Much cleaner than writing a long chain of ORs.

-- Customers in Mumbai, Delhi, or Chennai
SELECT customer_name, city
FROM customers
WHERE city IN ('Mumbai', 'Delhi', 'Chennai');

BETWEEN — match a range (inclusive)

-- Customers with a balance between 20,000 and 60,000
SELECT customer_name, balance
FROM customers
WHERE balance BETWEEN 20000 AND 60000;

BETWEEN is inclusive — both 20,000 and 60,000 are included.

LIKE — match a pattern with wildcards

-- Names starting with 'A'
SELECT customer_name
FROM customers
WHERE customer_name LIKE 'A%';

-- Names ending with 'Sharma'
SELECT customer_name
FROM customers
WHERE customer_name LIKE '%Sharma';

5. Handling NULL Values

NULL means "missing" or "unknown". It is not the same as zero or an empty string. This is one of the most common interview traps.

-- Find rows where a column is missing
SELECT customer_name
FROM customers
WHERE city IS NULL;

-- Find rows where a column has a value
SELECT customer_name
FROM customers
WHERE city IS NOT NULL;

6. ORDER BY, DISTINCT, and LIMIT

ORDER BY — sort the results

Default is ascending (ASC); use DESC for descending.

-- Highest balance first
SELECT customer_name, balance
FROM customers
ORDER BY balance DESC;

-- Sort by city (A→Z), then by balance (high→low) within each city
SELECT city, customer_name, balance
FROM customers
ORDER BY city ASC, balance DESC;

DISTINCT — remove duplicates

-- List each unique city exactly once
SELECT DISTINCT city
FROM customers;

Without DISTINCT, "Mumbai" and "Delhi" would appear twice. With it, each city appears once.

LIMIT — return only the top N rows

-- The 3 highest-balance customers
SELECT customer_name, balance
FROM customers
ORDER BY balance DESC
LIMIT 3;

LIMIT is standard in MySQL, PostgreSQL, and SQLite. SQL Server uses SELECT TOP 3 ... instead — know both, because the question changes by database.