teleren/devto huggingface dataset:
How to find the third and fifth-highest values in a PostgreSQL table?
Tags: postgres, database, howto, sql
Published: 2024-04-26 17:53:24

To find the third and fifth highest values in a PostgreSQL table, you can use a subquery or window functions.

Let’s consider an example using a table named example with a column value where we want to find these values.

Using a Subquery with DISTINCT and ORDER BY

This method is straightforward and works well for tables with a small number of rows:

```
SELECT DISTINCT value
FROM example
ORDER BY value DESC
LIMIT 1 OFFSET 2; - For the third highest value

SELECT DISTINCT value
FROM example
ORDER BY value DESC
LIMIT 1 OFFSET 4; - For the fifth highest value
```

In these queries, unique values from the value column are first selected, then they are sorted in descending order.

The LIMIT 1 OFFSET 2 command returns one value, skipping the two highest values to reach the third.

Similarly, LIMIT 1 OFFSET 4 skips the four highest values to access the fifth.

Using the Window Function ROW_NUMBER()

This method is more flexible and efficient for larger datasets:

```
WITH RankedValues AS (
 SELECT value, ROW_NUMBER() OVER (ORDER BY value DESC) as rank
 FROM (
 SELECT DISTINCT value
 FROM example
 ) as DistinctValues
)
SELECT value
FROM RankedValues
WHERE rank IN (3, 5);
```

In this query:

A subquery is first created that selects unique values from value.
The window function ROW_NUMBER() is then applied, which assigns a rank to each value in descending order.

In the outer query from the CTE (WITH query), rows where the rank is 3 or 5 are selected.

Both methods are effective, but using window functions is usually preferred for processing large data volumes due to their high performance and flexibility.

You can choose the method depending on the size of your data and specific query requirements.

`ask_dima@yahoo.com`
Understanding of NULL in PostgreSQL database.
Tags: postgressql, database, sql, howto
Published: 2024-04-26 17:51:46

In PostgreSQL, as in many other database management systems, NULL represents a special value used to denote the absence of a value. NULL is neither a zero value nor an empty string; it is a marker indicating that a cell’s value in the database is unknown or not applicable.

Here are some key aspects of working with NULL in PostgreSQL:

Comparison with NULL:

Comparison operations involving NULL, such as =, <>, <, >, and others, always return NULL. This means that any direct comparison with NULL cannot be true or false and returns NULL. To check for NULL, you should use IS NULL or IS NOT NULL.

Arithmetic Operations:

Arithmetic operations with NULL (e.g., addition or subtraction) also return NULL. For example, 5 + NULL returns NULL.

Logical Operations:

In logical operations, such as AND, OR, and NOT, NULL is handled by special rules. For example, TRUE AND NULL returns NULL, while FALSE AND NULL returns FALSE.

Functions and Aggregates:

Most functions and aggregate operations, such as SUM, AVG, MAX, and MIN, ignore NULL values. However, the function count(*) counts all rows, including those with NULL, whereas count(column_name) counts only rows where the specified column is not NULL.

Usage Considerations:

When working with NULL, it is important to remember its special behavior in SQL queries and functions. For example, the COALESCE function returns the first non-NULL argument from its list of arguments, which is often used to replace NULL values with alternatives.

Examples of Using NULL.

Checking for NULL:

```
SELECT * FROM users WHERE last_login IS NULL;
```

Using COALESCE to Replace NULL:

```
SELECT COALESCE(address, 'Not specified') FROM users;
```

Working with Aggregate Functions:

```
SELECT AVG(age) FROM users; -- Calculates average age, ignoring `NULL` values
```

The Importance of Understanding NULL.

Understanding how NULL works is critically important for effectively designing queries and handling data in databases.

Mistakes in handling NULL can lead to incorrect results or errors in application logic.

Proper use of NULL helps improve data quality and makes query results more predictable and reliable.
How to join tables in PostgreSQL using the WITH clause?
Tags: postgres, database, sql, howto
Published: 2024-04-26 17:50:14

To join tables in PostgreSQL using the WITH clause (also known as Common Table Expressions or CTEs), you first need to define these tables and the keys on which they will be joined. Let’s consider an example where we have five tables linked by a common key, such as id.

Suppose we have the following tables:

```
table1(id, data1)
table2(id, data2)
table3(id, data3)
table4(id, data4)
table5(id, data5)
```

Here, each table contains an id column that will be used for joining.

The join will be conducted using an inner join, which means that only those rows that have a corresponding match in all tables will be included in the result. Here’s how you can do this using WITH:

```
WITH cte1 AS (
 SELECT t1.id, t1.data1, t2.data2
 FROM table1 t1
 INNER JOIN table2 t2 ON t1.id = t2.id
),
cte2 AS (
 SELECT c1.id, c1.data1, c1.data2, t3.data3
 FROM cte1 c1
 INNER JOIN table3 t3 ON c1.id = t3.id
),
cte3 AS (
 SELECT c2.id, c2.data1, c2.data2, c2.data3, t4.data4
 FROM cte2 c2
 INNER JOIN table4 t4 ON c2.id = t4.id
),
cte4 AS (
 SELECT c3.id, c3.data1, c3.data2, c3.data3, c3.data4, t5.data5
 FROM cte3 c3
 INNER JOIN table5 t5 ON c3.id = t5.id
)
SELECT * FROM cte4;
```

Explanation:
cte1: The first CTE joins table1 and table2 by id. The result of this join, including id, data1, and data2, is used in the next CTE.
cte2: The second CTE takes the result from cte1 and joins it with table3 using the same key id. The result now includes data3.
cte3: The third CTE continues the process by adding data from table4 (data4).
cte4: The final CTE adds data from table5 (data5), completing the series of joins.
Final SELECT: Retrieves all data from the last CTE.

By using this structure, you can effectively manage and track each step of the joining process, which is particularly useful for debugging complex queries.

It also makes the code more readable and understandable for other developers.

`ask_dima@yahoo.com`
PostgreSQL. How to check connections that are not using SSL encryption?
Tags: security, database, postrges, devops
Published: 2024-04-26 17:48:29

This SQL query targets a PostgreSQL database and performs a join between two system views, pg_stat_ssl, and pg_stat_activity, using the process identifier (pid) as the join condition. The purpose of the query is to extract detailed information about database connections that are not using SSL (Secure Sockets Layer) encryption.

```
SELECT usename, datname, ssl, client_addr, query 
FROM pg_stat_ssl 
JOIN pg_stat_activity ON pg_stat_ssl.pid = pg_stat_activity.pid 
WHERE ssl!='t'
```

Let’s break down the components of this query:

SELECT Clause: The query selects the following columns:
usename: The name of the user connected to the session.
datname: The name of the database to which the session is connected.
ssl: A boolean indicating whether the connection is using SSL.
client_addr: The IP address of the client that made the connection.
query: The current or last query executed by the session.

FROM Clause: The query uses two PostgreSQL system views:
pg_stat_ssl: This view provides information about SSL usage for each connection, including whether the connection is encrypted (ssl column).
pg_stat_activity: This view provides a wide range of information about all current activities in the database server, including which queries are being executed and details about client connections.

JOIN Operation: The query performs an inner join on pg_stat_ssl and pg_stat_activity based on their pid (process identifier), which uniquely identifies each session or connection in the PostgreSQL server.

WHERE Clause: The condition specified in the query is ssl != ‘t’, which filters the results to include only those connections that are not using SSL. The character ‘t’ in PostgreSQL represents a boolean true.

By running this query, you will retrieve information about all non-SSL connections to the PostgreSQL server, including who is connected, from where they are connecting, and what query they are currently running or last ran.

This is particularly useful for security and compliance checks, as it identifies connections that are not encrypted and potentially exposes sensitive data to risks during transmission. If the intention is to ensure security protocols are being followed, this query provides essential insights into compliance with security policies regarding data transmission.
Statistical information about tables in a PostgreSQL Database.
Tags: postgres, database, sql, devops
Published: 2024-04-26 17:46:32

This query to the PostgreSQL database retrieves statistical information about tables located in the ‘public’ schema. The query helps assess the activity and state of these tables, which is useful for performance optimization and maintenance planning.

```
select 
 schemaname, 
 relname, 
 n_tup_ins, 
 n_tup_upd, 
 n_tup_del, 
 n_live_tup, 
 n_dead_tup, 
 last_vacuum, 
 last_autovacuum, 
 last_analyze, 
 last_autoanalyze 
from 
 pg_stat_all_tables 
where 
 schemaname = 'public'
order by 
 n_dead_tup desc;
```

Here is a detailed description of each column in the query:

schemaname: The name of the schema where the tables are located. In this case, it is filtered to ‘public’, which is the standard schema for user data in PostgreSQL.
relname: The name of the table within the schema.
n_tup_ins: The number of tuples (rows of data) that have been inserted into the table.
n_tup_upd: The number of tuples that have been updated in the table.
n_tup_del: The number of tuples that have been deleted from the table.
n_live_tup: The number of “live” tuples in the table. “Live” tuples are those that are available for current transactions or will be available for future transactions.
n_dead_tup: The number of “dead” tuples in the table. “Dead” tuples are those that have been marked as deleted or obsolete due to updates and need to be cleaned up in an automatic or manual VACUUM process.
last_vacuum: The date and time of the last manual VACUUM operation performed for this table.
last_autovacuum: The date and time of the last automatic VACUUM operation for this table.
last_analyze: The date and time of the last manual ANALYZE operation performed for this table, which helps update the statistics for the query optimizer.
last_autoanalyze: The date and time of the last automatic ANALYZE operation for this table.

Sorting the query results by n_dead_tup in descending order helps identify tables that may urgently need maintenance through a VACUUM operation. Tables with a high number of “dead” tuples may experience reduced performance due to the excessive volume of outdated data occupying storage and affecting the speed of query execution.

To collect DB statistics and to do vacuum (regular, not FULL) on all the objects of all the DBs of the PostgreSQL DB instance run the following command:

`vacuumdb -h <db_host> -p <db_port> -U <db_user> -j 4 -z -v -a`

To collect DB statistics and to do vacuum (regular, not FULL) on all the objects of some specific database of the PostgreSQL DB instance run the following command:

`vacuumdb -h <db_host> -p <db_port> -U <db_user> -j 4 -z -v <db_name>`
About transaction identifier (transaction_id) in PostgreSQL.
Tags: postgres, devops, database, sql
Published: 2024-04-26 17:44:54

In PostgreSQL, the transaction_id, or transaction identifier, is a crucial mechanism for ensuring data integrity and isolating transactions within the database. It is used to manage and track changes made by each transaction.

Every transaction in PostgreSQL is assigned a unique transaction identifier (XID), which marks all the data rows altered or created during that transaction. This identifier allows the system to determine which versions of rows should be visible to various transactions based on their start time.

PostgreSQL utilizes the MVCC (Multi-Version Concurrency Control) model to enable concurrent data access without blocking reads during writes. MVCC ensures that each transaction sees a consistent state of the database, excluding inconsistent changes made by other transactions. The transaction identifier plays a key role in this process by helping to determine which data versions are current for each transaction.

Once transactions are completed, the old versions of data that are no longer needed by any active or future transactions must be removed or reprocessed. PostgreSQL employs a process called “vacuuming” to clean up such obsolete data, which helps to free up space and maintain system performance.

Since transaction identifiers are limited in size (32 bits), they can exhaust their range and “wrap around” after reaching their maximum value (~4 billion transactions). PostgreSQL addresses this problem with a mechanism known as “wraparound protection,” which prevents the overflow of transaction identifiers through periodic cleaning and reuse of old identifiers.

Transaction identifiers are a fundamental aspect of transaction management and data integrity maintenance in PostgreSQL, ensuring the reliable operation of the database in multi-user and multitasking environments.

This query checks how long it has been since the PostgreSQL database underwent a VACUUM operation, which prevents the transaction counter from overflowing.

```
SELECT datname, age(datfrozenxid) 
FROM pg_database 
ORDER BY 2 DESC 
LIMIT 10;
```

To do a vacuum (regular, not FULL) and collect DB statistics on all the objects of all the DBs of the PostgreSQL DB instance, run the following command:

`vacuumdb -h <db_host> -p <db_port> -U <db_user> -j 4 -z -v -a`

To do a vacuum (regular, not FULL) and collect DB statistics on all the objects of some specific database of the PostgreSQL DB instance, run the following command:

`vacuumdb -h <db_host> -p <db_port> -U <db_user> -j 4 -z -v <db_name>`

`ask_dima@yahoo.com`
I'm a dad again 🤩🥳 dev.to pause
Tags: community
Published: 2024-04-26 17:26:01

I think this kind of message is unusual on dev.to, but I have to inform my followers.

My second daughter, Nina, was born yesterday. Everyone is fine if wondering.

With great power, there must also come great responsibility. ™️

As a consequence, I'll make a pause on my dev.to posts for a few moments.

I had already written a few posts that are now scheduled to be published in the next days.

See you, Christophe, proud and happy, father of 2.
Produce a News and Opinion Podcast for Your City
Published: 2024-04-26 17:24:36

Here's a business idea that you can take and run with especially it if nobody is doing this in your city or town. Produce a news and opinion podcast for your city. One of my favorite podcast is The Daily from The New York Times. They started this franchise in  2017 and now have millions of listeners for this daily podcast. In 2019 I began to produce my podcast called the “20 minute Morning Show” which I tried to keep up with the rigors of doing a daily podcast but after maybe 9 months I figured out I could not compete with the New York Times Newsroom with hundreds of reporters contributing daily content, audio engineers and producers and overwhelming production value; that it was just too much to try to duplicate.I kind of scaled things back and tried to figure out what could work for me. So one particular opportunity came up where some friends in Charlotte who have a barber shop franchise called No Grease opened a shop in South Park Mall which is a very ritzy upscale mall in Charlotte North Carolina. There was a controversy about the lease for their store and we found out that the mall wanted that barber shop out of South Park mall. We took action and produced a podcast talking about it.( https://spotifyanchor-web.app.link/e/UxDuhrv36Ib ) 

It went viral and got into the Charlotte Observer newspaper(Article from 3.2.2021
https://www.charlotteobserver.com/news/business/whats-in-store/article249628383.html?fbclid=IwAR1U-Jm-3GPBL-GFlouo0sOL2j79Bg7Ax8SPCho8Yz5OiCZrfbkOMiitj0A which made South Park Mall back down and actually offer them a new lease. I found the power of podcasting in that moment to say wow you can use this as a tool and cover stories that nobody else in your Market is covering just like on the radio. These are the type of things I use to do on my morning show in Charlotte on Power 98. 

So take the opportunity to do something that nobody else in your marketplace is doing.  I would do it as often as I could and you create the weekly schedule. Even if you want to be ambitious and try to put something out everyday, if it was three to five minutes it will be a value to the audience in that city. Podcasting is so exciting and you can make a living off of it if you're willing to be patient, because podcasting is indeed the long game in the New Media space. I still have that ambition for putting out a high quality daily news and opinion podcast.(smile)
How do you stay on the edge of tech? 🧑‍💻
Tags: webdev, ai, discuss
Published: 2024-04-26 17:21:53

_Twenty years ago, in 2004, the technological landscape was quite different from today. Here are some of the key advancements that were at the forefront of technology back then:_

- **Social Media Emergence**: Facebook was founded in February 2004, beginning its rollout to college students and starting a social media revolution.
- **Web Browsing Evolution**: Mozilla's Firefox browser emerged as a significant competitor to Microsoft's Internet Explorer, changing the way users interacted with the web.
- **Portable Music**: The iPod was at the peak of its popularity, with the iPod mini and iPod with video diversifying Apple's product line and leading the digital music market.
- **Wireless Connectivity**: Bluetooth technology was becoming more widespread, with over 250 million Bluetooth-enabled devices, ranging from computer mice to medical equipment.
- **High-Definition TV**: High-definition televisions (HDTVs) were offering crisp images at high prices, and Digital Video Recorders (DVRs) were changing TV viewing habits.
- **VoIP Communication**: Skype was establishing itself as a new form of telecommunication, showcasing the potential of Voice-over-Internet-Protocol (VoIP) services.

These innovations laid the groundwork for many of the technologies we consider standard today. It's fascinating to see how far we've come in just two decades!

----

I think it is interesting that years ago, when mobile phones were the “edge” of technology, people thought with awe:
“I wonder where phones will be in 20 years…”
The answer — they are now commonplace, taken for granted, everyday electronic devices (albeit they are still pretty amazing!).

The 'cutting edge' of technology right now seems to be AI. But in 20 years, will AI still be the “edge of tech”, or will it be commonplace?


**Where do you think AI will be in 20 years?** 

**What do you think the edge of technology will be in the future?** 

**Will AI become commonplace as technology moves on to something else?**

Let me know what _you_ think in the comments below. ⬇️

----

Thanks for reading!
_Article written by [BestCodes](https://the-best-codes.github.io/?dev.to=edge_of_tech)._
#752. Open the Lock
Tags: leetcode
Published: 2024-04-26 17:06:55

https://leetcode.com/problems/open-the-lock/description/?envType=daily-question&envId=2024-04-22

```javascript
function openLock(deadends, target) {
    let deadSet = new Set(deadends);

    let queue = ["0000"];
    let visited = new Set(["0000", ...deadends]);
    let turns = 0;

    while (queue.length > 0) {
        let size = queue.length;
        for (let i = 0; i < size; i++) {
            let current = queue.shift();
            if (current === target) return turns;
            if (deadSet.has(current)) continue;

            for (let j = 0; j < 4; j++) {
                let digit = parseInt(current[j]);
                let next1 = current.substring(0, j) + (digit === 9 ? 0 : digit + 1) + current.substring(j + 1);
                let next2 = current.substring(0, j) + (digit === 0 ? 9 : digit - 1) + current.substring(j + 1);

                if (!visited.has(next1) || !visited.has(next2)) {
                    queue.push(next1);
                    visited.add(next1);
                    queue.push(next2);
                    visited.add(next2);
                }
            }
        }
        turns++;
    }

    return -1;
}


```
Demystifying the Spring Framework: A Beginner's Guide
Tags: spring, java, learning
Published: 2024-04-26 16:53:00

Hey Dev.to community! Today, I delved into the Spring Framework, a cornerstone of Java development. Here's a beginner-friendly overview of what I learned:

What is Spring?: Explored the basics of the Spring Framework, understanding its role in Java development and its core principles.
Key Features: Discussed the key features of Spring, including dependency injection, aspect-oriented programming, and inversion of control. 
Explored how these features simplify application development and improve maintainability.
Getting Started: Shared resources and tutorials for beginners looking to get started with Spring development. 
Discussed how to set up a Spring project and start building applications.
Overall, my exploration of the Spring Framework was eye-opening, and I'm excited to continue learning and experimenting with its capabilities. Stay tuned for more insights and discoveries!
Software Development: Driving Innovation in the Modern Era
Tags: webdev, softwaredevelopment
Published: 2024-04-26 16:47:14

In the contemporary age, software development stands as an essential pillar for business progress. It goes beyond being merely a technological tool, serving as a catalyst for innovation that enables companies to adapt, optimize, and deliver exceptional experiences to their customers.


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/q8hzf0r4203w1mqb6ykj.png)

Optimization and Efficiency:

Custom software automates tasks, eliminates redundancies, and streamlines processes, resulting in a palpable increase in efficiency and productivity. This frees up valuable resources for strategic activities that drive business growth.

Personalized Experiences:

Through software development, companies can create applications and platforms that provide personalized experiences to their customers. By deeply understanding user needs and preferences through data collection and analysis, companies can develop solutions that effectively meet their customers' expectations.

Key Benefits of Software Development:

Flexibility and Adaptability: Custom software adapts to each company's unique needs, allowing greater flexibility to respond to market changes.
Efficiency and Productivity: Automation and optimization increase efficiency and productivity, freeing up resources for more strategic activities.
Innovation and Differentiation: Software development enables the creation of distinctive solutions that differentiate companies, positioning them as leaders in their respective sectors.
Customer Experience: Customized applications and platforms offer satisfying experiences that foster customer loyalty and business growth.

In Summary:

Software development is an essential investment for thriving in today's business environment. It facilitates adaptation to changing market demands, optimization of internal processes, and delivery of exceptional experiences to customers, paving the way for business success in a competitive and dynamic world.