Mysql

Jakub Romanowski

2024 Database Trends: Is SQL Still the King?

Want to know what database is most popular in 2024? In this article, I go through the 2024 Stack Overflow Developer Survey results. You’ll see why SQL is still top, PostgreSQL is on the rise and more developers are interested in SQLite. We’ll also talk about the benefits of learning SQL online, recommend resources, and how developers code outside of work for fun and professional growth. The annual Stack Overflow Developer Survey offers invaluable insights into the developer community, shedding light on trends, preferences, and emerging technologies.

Maria Durkin

SQL vs. MySQL: What’s the Difference?

Are SQL and MySQL the same thing? In this beginner-friendly guide, we'll explain the ddifferences of SQL and MySQL – and how they fit into the larger picture of relational database management systems. When you’re getting started with databases, it’s easy to feel overwhelmed by the terminology. SQL, MySQL, other acronyms… It can be a lot. And sometimes – like SQL and MySQL – the terminology can seem more similar than different.

Kamila Ostrowska

Mastering MySQL for Data Analysis

Mastering MySQL can significantly benefit anyone looking to advance in data science or database management. If you develop your data analysis skills in MySQL, you’re on the right path to positively influence your career. In this article, you’ll learn how to practice MySQL effectively and what kind of learning sources to choose. You’ll see for yourself that MySQL is not rocket science and that using it can be easy and intuitive.

Jakub Romanowski

Which Database Is Best for Small Projects?

If you're tackling a small project and need to pick the best database for managing and analyzing your data, this article will help. I'll compare PostgreSQL and MySQL, two top choices for data analysis, to make your decision easier. Read on! Are you just starting to explore the world of databases and feeling a bit uncertain about which one to choose for your small projects? I remember feeling the same way when I first started.

Kamila Ostrowska

How to Install MySQL on Ubuntu

Looking to install MySQL on Ubuntu? Delve into the world of efficient data management with this comprehensive guide. From updating your package list to securing your installation and ensuring smooth configurations, these step-by-step instructions will help you seamlessly set up MySQL on your Ubuntu system. If you’re reading this article, you’re seriously interested in working with data. What you need is a suitable tool to make it happen. If you’re considering installing MySQL on an Ubuntu Linux system, you may wonder if this combination is a good choice.

Gustavo du Mortier

MySQL Date Functions: Complete Analyst’s Guide

Master MySQL date functions and acquire a powerful tool to work with date and time information in your data analyses. Working with date and time data is an essential part of data analysis. Here are just three examples: Analyzing historical time series data is crucial for discovering trends and making reliable forecasts based on chronological information. Stock market analysis examines time series data extensively, especially when automated trading algorithms are employed.

Jill Thornhill

Integrating SQL with Python for Data Analysis

Integrating SQL with Python isn’t difficult. The two tools work together to combine the information-processing power of relational databases with the flexibility of a programming language. In this article, I will discuss the benefits of data analysis using SQL and Python, with real-world coding examples. Why do most of the best data analysis tools on the market – such as Tableau and Power BI – include both SQL and Python in their toolboxes?

Martyna Sławińska

MySQL Practice: Best Exercises for Beginners

These 15 MySQL exercises are especially designed for beginners. They cover essential topics like selecting data from one table, ordering and grouping data, and joining data from multiple tables This article showcases beginner-level MySQL practice exercises, including solutions and comprehensive explanations. If you need to practice … Selecting data from one table Ordering and grouping data Joining data from multiple tables … these 15 tasks are just for you!

Dominika Florczykowska

An Overview of MySQL Window Functions

MySQL window functions are very helpful when you want to create meaningful reports with SQL. In this article, we’ll demonstrate the most common window functions in MySQL and explain how to use them. MySQL 8.0 introduced a new feature: window functions. These functions are very useful to data analysts and anyone who creates reports using MySQL. Using them, you can easily compute moving averages, cumulative sums, and other calculations over specified subsets of your data.

LearnSQL.com Team

MySQL Cheat Sheet

Welcome to the MySQL cheat sheet! Whether you're just starting out with MySQL or you're a seasoned professional looking for a quick reference, this guide is tailor-made for you. This MySQL Cheat Sheet provides a concise and handy reference to the most commonly used MySQL commands and functionalities. It spans a range of topics, from connecting to a MySQL server and managing database contents, to the basic syntax for table creation and modification.

Kamila Ostrowska

Best MySQL Courses for Beginners

Updated on: May 4, 2024 SQL is among the most important languages when it comes to dealing with huge amounts of data. If your job includes analyzing or collecting data and you feel that an Excel file is not enough, you need to learn or develop your SQL skills. And if you learn SQL, you also need to know one of its most popular dialects - MySQL. If you've made the decision to learn SQL, you've come to the right place.

Martyna Sławińska

How to Export Data from MySQL into a CSV File

A database is a primary platform for working with data and storing data. But often, you must take your data out of the database. Read on to find out how to export data from MySQL database into a CSV file. In this article, we’ll demonstrate how to export data into a CSV file. We’ll start by introducing what a CSV file is and why we use it. Then, we’ll export data from a MySQL database into a CSV file.

Tihomir Babic

Top 10 MySQL Interview Questions And Answers

You learned SQL and now you want to use this valuable skill to get a better job. Now it’s time to prepare for the job interview. We’ve got ten MySQL interview questions and answers to help you with that. In today's hyperconnected world, knowing how to deal with it has become a necessary skill. That’s why you probably learned (or want to learn) SQL: either your job requires it or you realized that you need SQL to get a better job.

Tihomir Babic

An Overview of SQL Text Functions in MySQL

Do you work in MySQL as a data analyst? Then you should know how to use its text functions – data analysts don’t work only with numbers. Reporting requires computations, data classification, and label creation, and you can do all of that in MySQL.  In this article, I’ll explain several of the most common and useful MySQL text functions by discussing what they do and showing how they do it.

Kamila Ostrowska

Top 5 MySQL Query Tools (Updated for 2024)

Updated on: May 4, 2024 Are you on the hunt for a MySQL query tool that will make your database management a breeze? Let's dive into five of the best tools out there that promise to boost your productivity and make handling databases feel less like a chore. These tools come packed with features aimed at slashing the time you need to spend managing your data. Whether you're a developer, a database administrator, or just someone eager to get more out of your data management efforts, these MySQL query tools could be just what you need.

Dominika Florczykowska

An Overview of MySQL Data Types

What data types are available in a MySQL database? In this article, you’ll learn about numeric data types, text data types, and much more! You may have seen our article about data types in SQL. Here, we focus on the MySQL data types. We will discuss the most commonly used ones: numeric, text, and date and time. If you are interested in the topic, be sure to check out our Data Types in SQL course.

Kateryna Koidan

SQL Date and Time Functions in 5 Popular SQL Dialects

Are you confused by all the date and time functions used across different SQL dialects? In this article, I summarize the date and time data types used in PostgreSQL, Oracle, SQLite, MySQL, and T-SQL. I also provide examples with the key SQL date and time functions used across these dialects. It’s time to become date and time gurus! Do you want to calculate how often employees are running late for work?

How to Order by Date in MySQL

Problem: You want to sort the rows by date in MySQL. Example 1: The exam table has two columns, subject and exam_date. subjectexam_date Mathematics2019-12-19 English2020-01-08 Science2020-01-05 Health2020-01-05 ArtNULL You want to sort the rows by exam_date. Solution: SELECT * FROM exam ORDER BY exam_date; The result looks like this (the rows are sorted in ascending order by exam_date):

How to Calculate Timestamp Difference in MySQL

Problem: You have two columns of the type timestamp and you want to calculate the difference between them. Example: In the travel table, there are three columns: id, departure, and arrival. You'd like to calculate the difference between the arrival and the departure. The travel table looks like this: iddeparturearrival 12018-03-25 12:00:002018-04-05 07:30:00 22019-09-12 15:50:002019-10-23 10:30:30 32018-07-14 16:15:002018-07-14 20:40:30 42018-01-05 08:35:002019-01-08 14:00:00 Solution 1 (difference in days, hours, minutes, or seconds): SELECT id, departure, arrival, TIMESTAMPDIFF(SECOND, departure, arrival) AS difference FROM travel; The result is:

How to Group by Two Columns in SQL

Problem: You want to group your data by two columns so you can count some statistics. Example: In the order table, you have the columns order_date, product_id, customer_id, and number. You would like to count the number of products bought by each customer each day. The order table looks like this: order_dateproduct_idcustomer_idnumber 2023-11-25711 2023-11-251213 2023-11-265312 2023-11-26124 2023-11-26321 2023-11-261627 2023-11-26332 2023-11-27631 Solution: SELECT order_date, customer_id, SUM(number) AS products_number FROM order GROUP BY order_date, customer_id; The result is:

How to Delete a Foreign Key Constraint in SQL

Problem: You want to delete a foreign key from a table in a database. Example: We want to remove the foreign key named fk_student_city_id from the table student. Solution: ALTER TABLE student DROP CONSTRAINT fk_student_city_id; Discussion: To delete a foreign key from a table, use the ALTER TABLE clause with the name of the table (in our example, student) followed by the clause DROP CONSTRAINT with the name of the foreign key constraint.

How to Create a Table with a Foreign Key in SQL

Problem: You want to create a foreign key for a table in a database. Example: We would like to create a table named student that contains a foreign key that refers to the id column in the table city. Solution 1: Creating new table with single-column foreign key CREATE TABLE student ( id INT PRIMARY KEY, first_name VARCHAR(100) NOT NULL, last_name VARCHAR(100) NOT NULL, city_id INT FOREIGN KEY REFERENCES city(id) ); Discussion: To create a new table containing a foreign key column that references another table, use the keyword FOREIGN KEY REFERENCES at the end of the definition of that column.

How to Calculate the Difference Between Two Dates in MySQL

Problem: You have two columns of the date type and you want to calculate the difference between them. Example: In the travel table, there are three columns: id, departure, and arrival. You'd like to calculate the difference between arrival and departure, or the number of days from arrival to departure inclusively. The travel table looks like this: iddeparturearrival 12018-03-252018-04-05 22019-09-122019-09-23 32018-07-142018-07-14 42018-01-052018-01-08 Solution: SELECT id, departure, arrival, DATEDIFF(arrival, departure) AS date_difference, DATEDIFF(arrival, departure) + 1 AS days_inclusive FROM travel; The result is:

How to Make a Column Unique in SQL?

Problem: You would like to make a column unique in a given table in a database. Example: We would like to make the column name unique in the table product. The query below presents one way to do it. Solution 1: Creating new table with a UNIQUE constraint CREATE TABLE product ( id INT NOT NULL PRIMARY KEY, name VARCHAR(100) UNIQUE, producer VARCHAR(100), category VARCHAR(100) ); Discussion: In this example a given column (the column name) was made unique by adding the clause UNIQUE at the end of the column definition (name VARCHAR(100) UNIQUE).

How to Group by Year in SQL

Problem: You want to group your data by year in SQL. Example 1: One of the columns in your data is transaction_date. It contains a date. You would like to group all your data by year and calculate the total money earned each year. The data table looks like this: transaction_datemoney 2018-03-251700 2019-09-12100 2018-07-141200 2018-01-05400 2019-06-082000 2020-03-061500 Solution 1: Displaying the year and the money earned SELECT EXTRACT(year FROM transaction_date) AS year, SUM(money) AS money_earned FROM data GROUP BY EXTRACT(year FROM transaction_date); The result is:

How to Calculate a Square Root in SQL

Problem: You want to find the square root of a number in SQ:. Example: You want to compute the square root of all numbers in the column number from the table data. number 9 2 1 0.25 0 -4 Solution: SELECT number, SQRT(number) AS square_root FROM data; The result is:

How to Get Yesterday’s Date in MySQL

Problem: You would like to display yesterday's date (without time) in a MySQL database. Solution: SELECT DATE_SUB(CURDATE(), INTERVAL 1 DAY) AS yesterday_date; Assuming today is 2020-09-24, the result is: yesterday_date 2020-09-23 Discussion: To get yesterday's date, you need to subtract one day from today's date. Use CURDATE() to get today's date. In MySQL, you can subtract any date interval using the DATE_SUB() function.

How to Get the Year from a Datetime Column in MySQL

Problem: You’d like to get the year from a date/datetime column in a MySQL database. Example: Our database has a table named conference with data in the columns id, name, and start_datetime. idnamestart_datetime 1Social Media World2019-02-20 14:15:34 2Mobile World 20172017-08-31 20:10:14 3Electronics Show2018-04-03 10:05:45 4Tech Asia 20192019-01-01 12:47:54 For each conference, let’s get its name and year.

How to Get the Month from a Date in MySQL

Problem: You’d like to get the month from a date/datetime column in a MySQL database. Example: Our database has a table named apartment_rental with data in the columns id, address, floor, and start_date. idaddressfloorstart_date 1700 Oak Street22019-03-20 2295 Main Street32019-05-31 3506 State Road12019-01-03 43949 Route 3112019-02-01 For each available apartment, get the address, the floor, and the month when it’s available.

How to Get the Date from a Datetime Column in MySQL

Problem: You’d like to get the date from a date and time column in a MySQL database. Example: Our database has a table named travel with data in the columns id, first_name, last_name, and timestamp_of_booking. idfirst_namelast_nametimestamp_of_booking 1LisaWatson2019-04-20 14:15:34 2TomSmith2019-03-31 20:10:14 3AndyMarkus2019-08-03 10:05:45 4AliceBrown2019-07-01 12:47:54 For each traveler, let’s get their first and last name and the booking date only.

How to Get the Current Date and Time in MySQL

Problem: You’d like to get the current date and time for a MySQL database. Solution: We’ll use one of two functions, CURRENT_TIMESTAMP or NOW(), to get the current date and time. SELECT CURRENT_TIMESTAMP ; Here’s the result of the query: 2023-08-15 11:13:17 Discussion: The CURRENT_TIMESTAMP function in the MySQL database returns the current date and time (i.e. the time for the machine running that instance of MySQL). It is given as a value in the YYYY-MM-DD hh:mm:ss format.

How to Find the Number of Days Between Two Dates in MySQL

Problem: You’d like to get the difference, in days, between two dates in a MySQL database. Example: Our database has a table named food with data in the columns id, name, purchase_date, and expiration_date. idnamepurchase_dateexpiration_date 1bread 2019-07-202019-08-22 2butter2018-07-302019-08-10 3milk 2019-01-122019-01-13 4yogurt2019-02-252019-02-24 For each food product, let’s get the name of the product and the number of days between its expiration and purchase dates. Solution: We’ll use the DATEDIFF() function.

How to Find the Last Day of a Month in MySQL

Problem: You’d like to get the date of the last day of the month for a date in a MySQL database. Example: Our database has a table named car_sales with data in the columns id, make, model, and sale_date. idmakemodelsale_date 1FordKa2019-02-01 2ToyotaYaris2019-08-15 3OpelCorsa2019-06-22 For each car, let’s get the make and model and the last day of the month in which the car was sold.

How to Change Seconds to a Time Value in MySQL

Problem: You’d like to display a number of seconds as a time value in hours, minutes, and seconds in MySQL. Example: Our database has a table named athlete_score with data in the columns id, first_name, last_name, and score_seconds. idfirst_namelast_namescore_seconds 1EmilyWatson1124 2TomGarcia987 3GaryMartinez1003 4JamesAnderson1233 For each athlete, let’s get their first and last names and their record time from the seconds_record column.

How to Change Datetime Formats in MySQL

Problem: You’d like to change the format of date and time data in a MySQL database. Example: Our database has a table named student_platform with data in the columns id, first_name, last_name, and registration_datetime. idfirst_namelast_nameregistration_datetime 1LoraLorens2019-02-23 12:04:23 2AnneSmith2018-07-10 10:12:15 3TomJackson2019-03-09 08:20:33 4RichardWilliams2018-09-30 06:07:34 For each student, let’s get their first name, last name, and registration date and time.

How to Add Time to a Datetime Value in MySQL

Problem: You’d like to add a certain amount of time to a datetime value in a MySQL database. Example: Our database has a table named flight_schedule with data in the columns flight, aircraft, and arrival_datetime. flightaircraftarrival_datetime EK10L12012019-04-20 15:15:00 AY12K20012019-03-31 20:10:00 LA105F2052019-08-03 11:15:00 LH30K2562019-07-01 12:47:00 For each flight, let’s get the flight code, aircraft code, and a new arrival date and time.

How to Add Days to a Date in MySQL

Problem: You’d like to add days to a date in a MySQL database. Example: Our database has a table named trip with data in the columns id, city, and start_date. idcitystart_date 1Chicago2023-06-22 2Houston2023-07-15 3Dallas2023-08-30 4Austin2023-09-23 Let’s add two days to the start dates and get the trip cities with the new start dates.

How to Find Rows with Minimum Value

Problem: You want to find rows which store the smallest numeric value in a column. Example: Our database has a table named weather with data in the following columns: id, city, and temperature. You want to find cities with the lowest temperature. idcitytemperature 1Houston23 2Atlanta20 3Boston15 4Cleveland15 5Dallas34 6Austin28 Solution: SELECT id, city, temperature FROM weather WHERE temperature = (SELECT MIN(temperature) FROM weather); Here’s the result:

How to Find Rows with Maximum Value

Problem: You want to find rows which store the largest numeric value in a given column. Example: Our database has a table named student with data in the following columns: id, first_name, last_name, and grade. You want to find the students who have the highest grades. idfirst_namelast_namegrade 1LisaJackson3 2GaryLarry5 3TomMichelin2 4MartinBarker2 5EllieBlack5 6MarySimpson4 Solution: SELECT id, first_name, last_name, grade FROM student WHERE grade = (SELECT MAX(grade) FROM student); Here’s the result:

How to Find Records with NULL in a Column

Problem: You want to find records with NULL in a column. Example: Our database has a table named children with data in four columns: id, first_name, middle_name, and last_name. idfirst_namemiddle_namelast_name 1JohnCarlJackson 2TomNULLThomson 3LisaAliceNULL 4AnneNULLSmith Let’s find the id, first name, and last name of children without a middle name (NULL in column middle_name). We use the IS NULL operator for this. Solution: SELECT id, first_name, last_name FROM children WHERE middle_name IS NULL; Here’s the result:

How to Filter Rows without NULL in a column

Problem: You want to find records without a NULL in a column. Example: Our database has a table named product with data in three columns: id, name, and price. idnameprice 1butterNULL 2milk2.35 3bread3.25 4cheeseNULL Let’s find the names and prices of products that have a price (without a NULL). Do this by using the IS NOT NULL operator.

How to Filter Records with Aggregate Function SUM

Problem: You need to filter groups of rows by the sum of a given column. Example: Our database has a table named company with data in the following columns: id, department, first_name, last_name, and salary. iddepartmentfirst_namelast_namesalary 1marketingLoraBrown2300 2financeJohnJackson3200 3marketingMichaelThomson1270 4productionTonyMiller6500 5productionSallyGreen2500 6financeOlivierBlack3450 7productionJenifferMichelin2800 8marketingJeremyLorson3600 9marketingLouisSmith4200 Let’s find the names of departments that have sums of salaries of its employees less than 7000.

How to Filter Records with Aggregate Function COUNT

Problem: You want to find groups of rows with a specific number of entries in a group. Example: Our database has a table named product with data in the following columns: id, name and category. idnamecategory 1sofafurniture 2glovesclothing 3T-Shirtclothing 4chairfurniture 5deskfurniture 6watchelectronics 7armchairfurniture 8skirtclothing 9radio receiverelectronics Let’s find the category of products with more than two entries.

How to Filter Records with Aggregate Function AVG

Problem: You want to filter groups of rows in by the average value of a given column. Example: Our database has a table named product with data in the following columns: id, name, store and price. idnamestoreprice 1milkGreen Shop2.34 2breadClark’s Grocery3.56 3breadSuper Market4.15 4milkSuper Market1.80 5breadGrocery Amanda2.26 6milkViolet Grocery3.45 7milkClark’s Grocery2.10 8breadViolet Grocery2.

How to Concatenate Strings in SQL

Problem: You want to join strings from two columns of a table into one. Example: Our database has a table named student with data in the following columns: id, first_name and last_name. idfirst_namelast_name 1LoraSmith 2EmilBrown 3AlexJackson 4MartinDavis Let’s append the first name to the last name of the student in one string. Use a space between each name.

What’s the Difference Between RANK and DENSE_RANK in SQL?

Problem: You want to compare the rankings produced by RANK and DENSE_RANK and add them as new columns to a table. Example: Our database has a table named sales_assistant with data in the following columns: id (primary key), first_name, last_name, month, and sold products. idfirst_namelast_namemonthsold products 1LisaBlack 52300 2MaryJacobs52400 3LisaBlack 62700 4MaryJacobs62700 5AlexSmith 62900 6MaryJacobs71200 7LisaBlack 71200 8AlexSmith 71000 Let’s display each sales assistant’s first and last name and number of sold products.

How to Number Rows in SQL

Problem: You would like to give each row in the result table a separate number. Example: Our database has a table named furniture with data in the following columns: code (primary key) and name. codename 101bed 202sofa 333chair 123bookcase 235table 766desk The furniture table stores the name of pieces of furniture that we want to number.

How to Find the Minimum Value of a Column in SQL

Problem: You’d like to find the smallest numeric value in a column. Example: Our database has a table named employment with data in the following columns: id, first_name, last_name, department, and salary. idfirst_namelast_namedepartmentsalary 1EllieMartinesmarketing1200 2MartinJohnsonfinance2300 3MichaelJacobsproduction1100 4StephenKowalskimarketing4300 5StanleyMillermarketing3500 6JenyBrownfinance5000 7MargaretGreenmarketing1500 8LisaThomasproduction2800 Let’s find the lowest salary among all employees.

How to Find the Maximum Value of a Numeric Column in SQL

Problem: You’d like to find the maximum value of a numeric column. Example: Our database has a table named product with data in the following columns: id, name, year, and items. idnameyearitems 1bread roll2018345 2chocolate2017123 3butter201934 4bread roll2019456 5butter201856 6butter201778 7chocolate201987 8chocolate201876 Let’s find the maximum number of items sold over all years.

How to Find the Average of a Numeric Column in SQL

Problem: You’d like to calculate the average of numbers stored in a column. Example: Our database has a table named sale with data in the following columns: id, city, year, and amount. idcityyearamount 1Los Angeles20172345.50 2Chicago20181345.46 3Annandale2016900.56 4Annandale201723230.22 5Los Angeles201812456.20 6Chicago201789000.40 7Annandale201821005.77 8Chicago20162300.89 Let’s calculate the average sales, regardless of city or year.

How to Convert a String to Uppercase in SQL

Problem: You would like to convert a string to uppercase in SQL. Example: Our database has a table named questionnaire with data in the following columns: id, first_name, last_name, and favorite_car. idfirst_namelast_namefavorite_car 1AlanJacksonHonda Civic 2ElisaThomsonTOYOTA Camry 3MaryMartinesNissan rogue 4ChrisBrownford focus 5AdamSpringRam PICKUP Our table stores the make and model of the favorite car for each person who filled out our questionnaire.

How to Use LIKE in SQL

Problem: You need to search for specific patterns (certain combinations of letters or numbers) within text data in your database. We'll show you how to use the LIKE operator in SQL to: Find city names starting with S Find city names with exactly five characters Find city names starting with S and with o as the second-to-last character You'll also learn how to use LIKE and wildcard operators in SQL to find your own patterns in text data in your database.

How to Round Up a Number to the Nearest Integer in SQL

Problem: You want to round up a number to the nearest integer in SQL. Example: Our database has a table named rent with data in the following columns: id, city, area, and bikes_for_rent. idcityareabikes_for_rent 1Los Angeles1302.151000 2Phoenix1340.69500 3Fargo126.44101 Let’s show each city’s name along with the ratio of its area to the number of bikes for rent.

How to Round Numbers in SQL

Problem: You want to round a number to a specific number of decimal places in SQL. Example: Our database has a table named product with data in the following columns: id, name, and price_net. idnameprice_net 1bread2.34 2croissant1.22 3roll0.38 Suppose there’s a tax of 24% on each product, and you’d like to compute the gross price of each item (i.e., after taxes) and round the value to two decimal places.

How to Floor Numbers in SQL

Problem: You want to round a number down to the nearest integer. Example: Our database has a table named employee with data in the following columns: id, first_name, last_name, and hours_worked (for the current month). idfirst_namelast_namehours_worked 1AlanWatson95 2LisaBlack225 3LauraWilliams104 Let’s show the first and last name of each employee along with the number of days they worked.

How to Add Rankings with DENSE_RANK() in SQL

Problem: You want to add a ranking position to rows consecutively, even if the rows have the same values. Example: Our database has a table named competition with data in the following columns: id (primary key), first_name, last_name, and score. idfirst_namelast_namescore 11JohnThomas345 14MaryJohnson222 16LisaBrown154 23AlanBlake222 32ChrisTaylor154 Let’s display all details about students: their last name, first name, and scores sorted by score in descending order.

How to Trim Strings in SQL

Problem: You’d like to remove specific characters from the beginning and end of a string in SQL. Example: Our database has a table named post with data in two columns: id and title. idtitle 1' News about dogs ' 3'Eurovision 2019 ' 4'New political climate' Let’s trim the title of each post to remove the unnecessary space at the beginning and end.

How to Sum Values of a Column in SQL?

Problem: You’d like to sum the values of a column. Example: Our database has a table named game with data in the following columns: id, player, and score. idplayerscore 1John134 2Tom 146 3Lucy20 4Tom 118 5Tom 102 6Lucy90 7Lucy34 8John122 Let’s find the total score obtained by all players. Solution: SELECT SUM(score) as sum_score FROM game; Here’s the result:

How to Replace Part of a String in SQL

Problem: You’d like to replace part of a string with another string. Example: Our database has a table named investor with data in the following columns: id, company, and phone. idcompanyphone 1Big Market123–300-400 3The Sunny Restaurant123–222-456 4My Bank123-345-400 We’d like to change the phone number format for each company by replacing the hyphen character with a space.

How to Count the Number of Rows in a Table in SQL

Problem: You’d like to determine how many rows a table has. Example: Our database has a table named pet with data in the following columns: id, eID (electronic identifier), and name. ideIDname 123456sparky 223457mily 3NULLlessy 4NULLcarl 534545maggy Let’s count all rows in the table. Solution: COUNT(*) counts the total number of rows in the table:

How to Order by Count in SQL?

Problem: You aggregated data into groups, but you want to sort the records in descending order by the number of elements in the groups. Example: Our database has a table named user with data in the following columns: id, first_name, last_name, and country. idfirst_namelast_namecountry 1LisaWilliamsEngland 2GaryAndersPoland 3TomWilliamsPoland 4MichaelBrownFrance 5SusanSmithUSA 6AnneJonesUSA 7EllieMillerPoland Let’s create a report on our users.

How to Order By Two Columns in SQL?

Problem: You need to display records from a given table sorted by two columns. Example: Our database has a table named employee with the following columns: id, first_name, last_name, and salary. idfirst_namelast_namesalary 1LisaUlman3000 2AdaMuller2400 3ThomasGreen2400 4MichaelMuller3000 5MaryGreen2400 Let’s display all information for each employee but sort the records according to salary in descending order first and then by last name in ascending order.

How to Multiply Two Columns in SQL

Problem: You want to multiply values from two columns of a table. Example: Our database has a table named purchase with data in the following columns: id, name, price, quantity, and discount_id. idnamepricequantitydiscount_id 1pen731 2notebook582 3rubber1131 4pencil case2423 Let’s multiply the price by the quantity of the products to find out how much you paid for each item in your order.

How to Join on Multiple Columns

Problem: You want to join tables on multiple columns by using a primary compound key in one table and a foreign compound key in another. Example: Our database has three tables named student, enrollment, and payment. The student table has data in the following columns: id (primary key), first_name, and last_name. idfirst_namelast_name 1EllieWillson 2TomBrown 3SandraMiller The enrollment table has data in the following columns: primary key (student_id and course_code), is_active, and start_date.

How to Find Duplicate Rows in SQL?

Problem: You have duplicate rows in your table, with only the IDs being unique. How do you find those duplicate entries? Example: Our database has a table named product with data in the following columns: id, name, and category. idnamecategory 1steakmeat 2cakesweets 3steakmeat 4porkmeat 5cakesweets 6cakesweets Let’s find duplicate names and categories of products.

How to Eliminate Duplicate Rows in SQL

Problem: You’d like to eliminate any duplicate rows from the result set of a query so that each row appears only once. Example: Our database has a table named clothes with data in the following columns: id, name, color, and year_produced. idnamecoloryear_produced 1T-shirtyellow2015 2jacketblue2016 3jeansblack2015 4jacketblue2015 5jacketgreen2016 6jacketyellow2017 7hatyellow2017 Let’s get the non-repeated names and colors of clothes produced before 2017.

How to Count Distinct Values in SQL

Problem: You’d like to count how many different non-NULL values there are in a given column. Example: Our database has a table named customer with data in the following columns: id, first_name, last_name, and city. idfirst_namelast_namecity 1JohnWilliamsChicago 2TomBrownAustin 3LucyMillerChicago 4EllieSmithDallas 5BrianJonesAustin 6AllanDavisNULL Let’s find the number of different (and non-NULL) cities.

How to Order Alphabetically in SQL

Problem: You want to display records from a table in alphabetical or reverse-alphabetical order according to given column. Example: Our database has a table named customer. The customer table contains data in the id, first_name, and last_name columns. id first_name last_name 1 Susan Thomas 2 John Michael 3 Tom Muller Let’s display each customer’s information, sorted in ascending order by their last name.

Francisco Claria

Referential Constraints and Foreign Keys in MySQL

Foreign keys and referential constraints allow you to set relationships between tables and modify some of the database engine’s actions. This beginner’s guide explains referential integrity and foreign key use in MySQL. One of the most important aspects of database usage is being able to trust the information you store. Database engines provide several features that help you maintain the quality of your data, like defining required columns as NOT NULL and setting an exact data type for each column.

Agnieszka Kozubek-Krycuń

MySQL Collations

List Collations To list all collations available in MySQL, use SHOW COLLATION; +-------------------+----------+-----+---------+----------+---------+ | Collation | Charset | Id | Default | Compiled | Sortlen | +-------------------+----------+-----+---------+----------+---------+ | big5_chinese_ci | big5 | 1 | Yes | Yes | 1 | | big5_bin | big5 | 84 | | Yes | 1 | | dec8_swedish_ci | dec8 | 3 | Yes | Yes | 1 | | dec8_bin | dec8 | 69 | | Yes | 1 | | cp850_general_ci | cp850 | 4 | Yes | Yes | 1 | | cp850_bin | cp850 | 80 | | Yes | 1 | | hp8_english_ci | hp8 | 6 | Yes | Yes | 1 | | hp8_bin | hp8 | 72 | | Yes | 1 | | koi8r_general_ci | koi8r | 7 | Yes | Yes | 1 | | koi8r_bin | koi8r | 74 | | Yes | 1 | | latin1_german1_ci | latin1 | 5 | | Yes | 1 | | latin1_swedish_ci | latin1 | 8 | Yes | Yes | 1 | | latin1_danish_ci | latin1 | 15 | | Yes | 1 | | latin1_german2_ci | latin1 | 31 | | Yes | 2 | | latin1_bin | latin1 | 47 | | Yes | 1 | | latin1_general_ci | latin1 | 48 | | Yes | 1 | .

Patrycja Dybka

SQL vs. Mongo query

In recent times, NoSQL databases have become a hot topic and have gained a crowd of advocates. Indeed, when NoSQL database started to arise, SQL-to-NoSQL converters arose with them as well. But unfortunately, thanks to the code conversion possibility, SQL supporters gained unquestionable evidence that in many cases the good, old SQL is much easier to use. Especially when it comes to queries. I performed a quick Google search and found plenty of nice converter tools.

Agnieszka Kozubek-Krycuń

MySQL's group_concat Equivalents in PostgreSQL, Oracle, DB2, HSQLDB, and SQLite

The GROUP_CONCAT() function in MySQL MySQL has a very handy function which concatenates strings from a group into one string. For example, let's take a look at the children table with data about parents' and children's names. if (typeof VertabeloEmbededObject === 'undefined') {var VertabeloEmbededObject = "loading";var s=document.createElement("script");s.setAttribute("type","text/javascript");s.setAttribute("src", "https://my.vertabelo.com/js/public-model/v1/api.js");(document.getElementsByTagName("head")[0] || document.documentElement ).appendChild(s);} parent_name child_name John Tom Michael Sylvie John Anna Michael Sophie To get the names of children of each person as a comma-separated string, you use the GROUP_CONCAT() function as follows: