LeetCode 1907. Count Salary Categories SQL Solution | Explanation + Code

Description

Table: Accounts

+-------------+------+
| Column Name | Type |
+-------------+------+
| account_id  | int  |
| income      | int  |
+-------------+------+
account_id is the primary key (column with unique values) for this table.
Each row contains information about the monthly income for one bank account.

 

Write a solution to calculate the number of bank accounts for each salary category. The salary categories are:

• "Low Salary": All the salaries strictly less than $20000.
• "Average Salary": All the salaries in the inclusive range [20000,50000].
• "High Salary": All the salaries strictly greater than $50000.

The result table must contain all three categories. If there are no accounts in a category, return 0.

Return the result table in any order.

The result format is in the following example.

 

Example 1:

Input: 
Accounts table:
+------------+--------+
| account_id | income |
+------------+--------+
| 3          | 108939 |
| 2          | 12747  |
| 8          | 87709  |
| 6          | 91796  |
+------------+--------+
Output: 
+----------------+----------------+
| category       | accounts_count |
+----------------+----------------+
| Low Salary     | 1              |
| Average Salary | 0              |
| High Salary    | 3              |
+----------------+----------------+
Explanation: 
Low Salary: Account 2.
Average Salary: No accounts.
High Salary: Accounts 3, 6, and 8.

Solutions

Solution 1: Temporary Table + Grouping + Left Join

We can first create a temporary table containing all salary categories, and then count the number of bank accounts for each salary category. Finally, we use a left join to connect the temporary table with the result table to ensure that the result table contains all salary categories.

sql

# Write your MySQL query statement below WITH S AS ( SELECT 'Low Salary' AS category UNION SELECT 'Average Salary' UNION SELECT 'High Salary' ), T AS ( SELECT CASE WHEN income < 20000 THEN "Low Salary" WHEN income > 50000 THEN 'High Salary' ELSE 'Average Salary' END AS category, COUNT(1) AS accounts_count FROM Accounts GROUP BY 1 ) SELECT category, IFNULL(accounts_count, 0) AS accounts_count FROM S LEFT JOIN T USING (category);(code-box)

Solution 2: Filtering + Merging

We can filter out the number of bank accounts for each salary category separately, and then merge the results. Here, we use UNION to merge the results.

sql

# Write your MySQL query statement below SELECT 'Low Salary' AS category, IFNULL(SUM(income < 20000), 0) AS accounts_count FROM Accounts UNION SELECT 'Average Salary' AS category, IFNULL(SUM(income BETWEEN 20000 AND 50000), 0) AS accounts_count FROM Accounts UNION SELECT 'High Salary' AS category, IFNULL(SUM(income > 50000), 0) AS accounts_count FROM Accounts;(code-box)