Calculating Library Status and Next Open Time with SQL

Understanding the Problem and Database Schema

In this article, we’ll delve into a complex database query problem involving two tables: library_details and library_timing. We need to calculate the status of a library based on its open and close times.

Table Creation and Insertion

First, let’s look at the table creation and insertion scripts provided in the question:

CREATE TABLE `library_details` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `library_name` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`);
);

INSERT INTO library_details VALUES(1,"library1");

CREATE TABLE `library_timing` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `library_id` int(11) DEFAULT NULL,
  `start_time` time DEFAULT NULL,
  `end_time` time DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_library_timing_1` (`library_id`),
  CONSTRAINT `fk_library_timing_1` FOREIGN KEY (`library_id`) REFERENCES `library_details` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
);

INSERT INTO library_timing VALUES(1,1,08:30,18:00);

Query Explanation

The provided query in the question uses a combination of SQL functions and logic to calculate the status and next open time:

SELECT 
  ld.id
  , ld.library_name
  , CASE WHEN curtime() BETWEEN lt.start_time AND lt.end_time THEN '1' ELSE '0' END "status"
  , CASE 
      WHEN (curtime() - lt.end_time) > (curtime() - lt.start_time) 
       AND curtime() NOT BETWEEN lt.start_time AND lt.end_time THEN
        SUBTIME(curtime(), lt.end_time)
      WHEN (curtime() - lt.end_time) < (curtime() - lt.start_time) 
       AND curtime() NOT BETWEEN lt.start_time AND lt.end_time THEN
        SUBTIME(lt.start_time, curtime())
      ELSE '00:00:00'
   END "next open time"
FROM library_details ld
JOIN library_timing lt ON lt.id = ld.id;

This query does the following:

It selects data from both library_details (ld) and library_timing (lt) tables.
For each row, it checks if the current time is within the open hours of the library. If so, it assigns a status of ‘1’, otherwise it assigns ‘0’.
For libraries that are closed during the current time, it calculates the next open time based on whether the current time is earlier or later than the end time.
- If the current time is earlier than the end time and later than the start time, it calculates the duration between the start and end times.
- If the current time is later than the end time but earlier than the start time, it subtracts the duration between the start and end times from the current time to calculate the next open time.
The result includes the library ID, name, status, and next open time.

Demo and Further Explanation

The question provides a demo query that illustrates how to use the SUBTIME function to calculate the next open time:

addtime(subtime(lt.start_time, curtime()), "24:00:00")

However, this is not necessary in the main query. The calculation of the next open time can be simplified as follows:

CASE 
  WHEN (curtime() - lt.end_time) > (curtime() - lt.start_time) 
   AND curtime() NOT BETWEEN lt.start_time AND lt.end_time THEN
    SUBTIME(lt.start_time, curtime())
  WHEN (curtime() - lt.end_time) < (curtime() - lt.start_time) 
   AND curtime() NOT BETWEEN lt.start_time AND lt.end_time THEN
    curtime()
  ELSE '00:00:00'
END "next open time"

In this simplified query, if the current time is earlier than the end time and later than the start time, it uses the SUBTIME function to calculate the duration between the start and end times. If the current time is later than the end time but earlier than the start time, it simply returns the current time.

Understanding SQL Functions and Logic

In this query, several SQL functions are used:

curtime(): Returns the current date and time.
BETWEEN operator: Used to check if a value falls within a specified range.
CASE statement: Used to perform conditional logic based on certain conditions.
SUBTIME function: Used to calculate the difference between two times.

The logic behind this query is as follows:

If the current time is within the open hours of the library, it assigns a status of ‘1’ and sets the next open time to ‘00:00:00’.
If the current time is outside the open hours but not exactly at the start or end time, it calculates the duration between the start and end times using the SUBTIME function.
If the current time is exactly at the start time or end time, it simply returns the next open time as either the current time or the previous day’s closing time.

Conclusion

The provided query demonstrates how to calculate a library’s status (open or closed) and next open time based on the current date and time. It uses SQL functions and logic to determine whether the library is open during the current hours, taking into account start and end times as well as any overlap between them.

Last modified on 2025-01-16