SQL Exercises: Find number of customers have listed their names

Last update on February 11 2025 08:01:07 (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.

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