The LeetCode problem “Find Customer Referee” requires us to retrieve the names of customers who either have no referee or whose referee is not customer ID 2.
High-Level Strategy
To solve this problem:
1. Filter by Conditions: Use SQL’s WHERE clause to apply conditions that exclude customers with referee_id = 2 but include customers with NULL referee values.
2. Select Specific Column: Use SELECT to retrieve only the name column of the filtered customers.
This problem uses conditional filtering and NULL handling in SQL, common techniques for managing data where certain fields may have missing values or need to meet specific criteria. This approach is useful in various SQL tasks, such as selecting records with optional fields or excluding data based on certain conditions.
SQL Query Solution
SELECT name
FROM Customer
WHERE referee_id != 2 OR referee_id IS NULL;
Explanation of Each Line
1. SELECT name:
• Specifies that we want to retrieve only the name column, as per the problem’s requirements.
2. FROM Customer:
• Indicates that the data source is the Customer table, where customer information is stored.
3. WHERE referee_id != 2 OR referee_id IS NULL:
• Filters the data to include only those rows where the referee_id is not 2 or where referee_id is NULL.
• The OR operator allows us to include customers who have no referee (indicated by NULL) while also excluding those with referee_id = 2.
How This Code Solves the Problem
• Filters customers with referee_id values other than 2.
• Includes customers with no referee (referee_id IS NULL).
• Outputs only the names of the customers who meet these criteria.
Broader Context
This query highlights SQL’s ability to filter records based on multiple conditions, including handling NULL values. These techniques are essential in SQL for managing data that includes optional fields or requires filtering based on exclusion criteria.
