SQL Exercises: How many customers get a grade for their performance

SQL Aggregate Functions: Exercise-5 with Solution

From the following table, write a SQL query to determine the number of customers who received at least one grade for their activity.

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 where the 'grade' column is not NULL in the 'customer' table.
SELECT COUNT(ALL grade)
-- Specifies the table from which to retrieve the data (in this case, 'customer').
FROM customer;

Output of the Query:

count
7

Code Explanation:

The said SQL statement is used to count the total number of grades present in the 'customer' table. The "COUNT" function is used to count the number of non-null values in the specified column, in this case, the "grade" column.
The "ALL" keyword is not necessary as it is the default behavior for the "COUNT" function to count all values, including duplicates.

Explanation:

visual presentation:

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