﻿ SQL: Find number of customers have listed their names

# SQL Exercises: Find number of customers have listed their names

## SQL Aggregate Functions: Exercise-4 with Solution

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:

## Query Visualization:

Duration:

Rows:

Cost:

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

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