SQL Exercises: Filter rows against NULL

Last update on February 10 2025 07:55:32 (UTC/GMT +8 hours)

From the following table, write a SQL query to find all those customers who does not have any grade. Return customer_id, cust_name, city, grade, salesman_id.

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 selects all columns from the 'customer' table.
SELECT *
-- Specifies the table from which to retrieve the data (in this case, 'customer').
FROM customer
-- Filters the rows to only include those where the 'grade' column is NULL.
WHERE grade IS NULL;

Output of the Query:

customer_id	cust_name	city	grade	salesman_id
3001		Brad Guzan	London		5005

Code Explanation:

The said SQL query to retrieve all columns (denoted by "SELECT *") from the 'customer' table where the "grade" column is NULL. The "WHERE grade IS NULL" condition filters the results to only retrieve rows where the "grade" column is missing a value or has a NULL value.

Relational Algebra Expression:

Relational Algebra Tree:

Explanation :

Visual presentation :

Practice Online

For more Practice: Solve these Related Problems:

• Write a SQL query to find customers who do not have a grade and live in 'New York'. Return customer_id, cust_name, city, grade, salesman_id.
• Write a SQL query to retrieve customers who do not have a grade and their salesman_id is greater than 5003. Return customer_id, cust_name, city, grade, salesman_id.
• Write a SQL query to find customers who do not have a grade and their city starts with the letter 'L'. Return customer_id, cust_name, city, grade, salesman_id.
• Write a SQL query to list customers who do not have a grade and their cust_name contains the letter 'a'. Return customer_id, cust_name, city, grade, salesman_id.

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