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Selection SQL records with latest status

Started by leavins, Sep 22, 2022, 02:25 AM

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leavinsTopic starter

To ensure that only the most recent status change is visible on the webpage, the database maintains a log of all changes in the application's status history. I suspect that the root of the issue lies in the disparity between SQL and MySQL.

To display only the latest status update on the webpage, the database tracks the history of changes in the application's status. The issue at hand appears to stem from a potential discrepancy between SQL and MySQL.

Selecting the relevant data requires querying multiple tables, such as requests, request_statuses, and statuses, using inner joins to link the tables based on their respective IDs. This ensures that all relevant information is retrieved for displaying the latest status update.

select requests.fio, requests.stud_group, statuses.name, requests.type_name, request_statuses.comment
from requests
inner join request_statuses on requests.id=request_statuses.id_request
inner join statuses on statuses.id=request_statuses.id_status;
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cambridgesf

Performing a complex query without proper debugging can be very challenging and prone to error. In order to debug the query, it is necessary for the TS to publish scripts for creating and filling in tables in text format rather than a visual representation. Otherwise, it will be difficult to identify any errors, and it is also important to report the version of MySQL being used. However, it seems that the hint provided may not be useful in this case.

Debugging a complex query requires careful attention and precision, as even small mistakes can cause significant errors. To effectively debug queries, the TS must provide clear and detailed scripts for creating and filling tables in a readable text format.

A crucial aspect of debugging is also reporting the MySQL version. Without this information, it can be difficult to identify potential issues related to compatibility or functionality.

The provided code attempts to select only the most recent data for each request based on the "created at" timestamp. This requires joining multiple tables and grouping by the relevant fields. However, it's important to thoroughly test the code to ensure accuracy and reliability

SELECT
DISTINCT requests.fio, requests.stud_group, statuses.name, requests.type_name, request_statuses.comment
FROM
(SELECT requests.fio, MAX(requests.created_at) AS created_at
FROM requests GROUP BY requests.fio) AS r
 
INNER JOIN requests IN requests.created_at = r.created_at AND requests.fio = r.fio
 
INNER JOIN request_statuses ON requests.id=request_statuses.id_request
INNER JOIN statuses ON statuses.id=request_statuses.id_status;
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JimyChen

The provided SQL query is correctly joining the necessary tables to retrieve the relevant data for displaying the latest status update on the webpage. By joining the tables requests, request_statuses, and statuses using inner joins, the query can match the IDs of the tables to retrieve the desired information.

The query selects the fields fio and stud_group from the table requests, the field name from the table statuses, and the field comment from the table request_statuses. This combination of fields allows retrieving the necessary information for displaying the latest status update of an application.

To ensure that only the latest status update is shown, you might consider adding an order by clause to sort the results in descending order based on the timestamp or ID of the status change. For example:

CodeSelect
SELECT requests.fio, requests.stud_group, statuses.name, requests.type_name, request_statuses.comment
FROM requests
INNER JOIN request_statuses ON requests.id = request_statuses.id_request
INNER JOIN statuses ON statuses.id = request_statuses.id_status
ORDER BY request_statuses.timestamp DESC
LIMIT 1;
This modified query will retrieve the most recent status change and limit the result to only one row.


Here's an explanation of the modified SQL query:

```
SELECT requests.fio, requests.stud_group, statuses.name, requests.type_name, request_statuses.comment
FROM requests
INNER JOIN request_statuses ON requests.id = request_statuses.id_request
INNER JOIN statuses ON statuses.id = request_statuses.id_status
ORDER BY request_statuses.timestamp DESC
LIMIT 1;
```

- The `SELECT` statement specifies the fields we want to retrieve from the tables `requests`, `request_statuses`, and `statuses`. These fields include `fio` and `stud_group` from the `requests` table, `name` from the `statuses` table, and `comment` from the `request_statuses` table.

- The `FROM` clause specifies the main table we are querying from, which is `requests`.

- The `INNER JOIN` statements are used to connect the tables based on their respective IDs. In this case, we are joining the `request_statuses` table to the `requests` table using the `id` fields, and then joining the `statuses` table to the `request_statuses` table using the `id_status` fields.

- The `ORDER BY` clause is used to sort the results in descending order based on the `timestamp` field in the `request_statuses` table. This ensures that the most recent status update is listed first.

- The `LIMIT` clause is used to restrict the number of results returned to just one. This ensures that only the latest status update is displayed on the webpage.

By incorporating the `ORDER BY` and `LIMIT` clauses, the modified query guarantees that only the most recent status change is retrieved and displayed.


Here's an in-depth breakdown of the modified SQL query:

```sql
SELECT requests.fio, requests.stud_group, statuses.name, requests.type_name, request_statuses.comment
FROM requests
INNER JOIN request_statuses ON requests.id = request_statuses.id_request
INNER JOIN statuses ON statuses.id = request_statuses.id_status
ORDER BY request_statuses.timestamp DESC
LIMIT 1;
```

Let's go through each line of the query:

- `SELECT` statement: This specifies the fields we want to retrieve from the database. In this case, we are selecting the `fio` and `stud_group` fields from the `requests` table, the `name` field from the `statuses` table, and the `comment` field from the `request_statuses` table. These fields will provide the necessary information for displaying the latest status update.

- `FROM` clause: This indicates the main table we are querying from, which is the `requests` table in this case.

- The first `INNER JOIN` statement: This joins the `request_statuses` table to the `requests` table. The `ON` keyword specifies the condition for the join, which is that the `id` field in the `requests` table must match the `id_request` field in the `request_statuses` table. This join ensures that we can access the relevant status updates for each request.

- The second `INNER JOIN` statement: This joins the `statuses` table to the `request_statuses` table. The `ON` keyword again specifies the condition for the join, which is that the `id` field in the `statuses` table must match the `id_status` field in the `request_statuses` table. This join allows us to retrieve the name of the status associated with each status update.

- `ORDER BY` clause: This specifies the sorting order of the results. In this case, we are sorting the results based on the `timestamp` field in the `request_statuses` table, in descending order (`DESC`). This ensures that the most recent status update appears first in the result set.

- `LIMIT` clause: This limits the number of rows returned by the query. In this case, we are using `LIMIT 1`, which means we only want to retrieve the first row in the sorted result set. This will give us the latest status update.

By combining these elements, the modified query retrieves the necessary fields from the relevant tables and correctly sorts the results to display only the latest status update on the webpage.
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