SQL Exercises: Find number of customers have listed their names

Last update on February 11 2025 08:02:04 (UTC/GMT +8 hours)

From the following table, write a SQL query to count the number of customers. Return number of customers.

Sample table: customer

 customer_id |   cust_name    |    city    | grade | salesman_id 
-------------+----------------+------------+-------+-------------
        3002 | Nick Rimando   | New York   |   100 |        5001
        3007 | Brad Davis     | New York   |   200 |        5001
        3005 | Graham Zusi    | California |   200 |        5002
        3008 | Julian Green   | London     |   300 |        5002
        3004 | Fabian Johnson | Paris      |   300 |        5006
        3009 | Geoff Cameron  | Berlin     |   100 |        5003
        3003 | Jozy Altidor   | Moscow     |   200 |        5007
        3001 | Brad Guzan     | London     |       |        5005

Sample Solution:

-- This query counts the total number of rows in the 'customer' table.
SELECT COUNT(*)
-- Specifies the table from which to retrieve the data (in this case, 'customer').
FROM customer;

Output of the Query:

count
8

Code Explanation:

The said SQL query that retrieve the total number of rows in the 'customer' table. The "SELECT COUNT(*)" statement specifies that the number of rows in the table should be counted. The "(*)" is a wildcard that counts all rows, regardless of the specific columns and their values.
The result of this query will be a single value that represents the total number of rows in the 'customer' table.

Relational Algebra Expression:

Relational Algebra Tree:

Explanation:

Visual presentation:

Practice Online

For more Practice: Solve these Related Problems:

• Write a SQL query to count the number of customers who are from the city 'London'.
• Write a SQL query to count the number of customers who have a grade of 200.
• Write a SQL query to count the number of customers who have made at least one purchase.
• Write a SQL query to count the number of customers who are assigned to salesman ID 5002.

Have another way to solve this solution? Contribute your code (and comments) through Disqus.

Previous SQL Exercise:Find the number of salesmen for each customer.
Next SQL Exercise: How many customers get a grade for their performance.