Solution Explanation for LeetCode 1435: Create a Session Bar Chart

This problem requires creating a bar chart summarizing session durations from a Sessions table. The durations need to be binned into specific ranges: [0-5>, [5-10>, [10-15>, and 15 or more (minutes). The solution involves counting the number of sessions falling into each bin.

Approach

The most straightforward approach is to use SQL's UNION operator to combine the results of four separate COUNT(*) queries, each corresponding to a specific duration bin. Each query filters the Sessions table based on the duration range and counts the number of matching sessions. The AS clause assigns aliases to the counted column (total) and the bin label (bin).

Time Complexity Analysis

The time complexity is dominated by the database operations. Each COUNT(*) operation on the table scans a portion of the data (or uses indexes if available), giving a complexity of O(N) in the worst case, where N is the number of rows in the Sessions table. Since we perform four such counts, the overall time complexity remains O(N). The UNION operation has a time complexity proportional to the total number of rows returned by the four queries, which in the worst case is still O(N).

Space Complexity Analysis

The space complexity depends primarily on the size of the output result set (which is constant, at most four rows) and any temporary space used by the database during the queries (which will also be proportional to N in the worst case). Thus, the overall space complexity is O(N) in the worst case. However, if the database uses efficient indexing, the space used could be significantly less.

Code in MySQL

The provided MySQL solution effectively implements this approach:

SELECT '[0-5>' AS bin, COUNT(1) AS total FROM Sessions WHERE duration < 300
UNION
SELECT '[5-10>' AS bin, COUNT(1) AS total FROM Sessions WHERE 300 <= duration AND duration < 600
UNION
SELECT '[10-15>' AS bin, COUNT(1) AS total FROM Sessions WHERE 600 <= duration AND duration < 900
UNION
SELECT '15 or more' AS bin, COUNT(1) AS total FROM Sessions WHERE 900 <= duration;

Each SELECT statement performs a count for a specific duration range (converted from seconds to minutes). The UNION combines these results into a single table with two columns: bin and total. The COUNT(1) is used for efficiency; it counts rows instead of columns.

Note: Other SQL dialects might have slightly different syntax, but the core approach would remain similar using appropriate functions for filtering and counting. For example, you might use CASE statements in some databases to consolidate the logic into a single query, rather than using UNION four times. This single query approach could potentially be more efficient, depending on the database optimizer.