Data Mastery: SQL — Case Function There are so many exciting projects out there in the Data World. Artificial Intelligence, Machine Learning, Neural Nets, Blockchain, and more are sweeping the technology industry. In order to get to the cutting-edge stuff, first and foremost, data needs to be stored, evaluated, and tested. The best place to do that is SQL (or a library that operates with SQL-like commands, ). see my article on Python’s Pandas library This series , will teach you the essential subjects. These are not exhaustive tutorials. Instead they are focused preparation guides — with brevity and efficiency in mind. It is meant for: Data Mastery: SQL Software Engineers who want to analyze their creation’s data Product Managers, Marketers, and others who want to be data-driven Beginning Data Scientists, Data Engineers, Data Analysts, or Business Intelligence Developers preparing for interviews See my resource list of the books I used to prepare for my big interview Each article will contain a brief technical explanation of the subject, an example question, and an answer. Follow up articles will contain challenging questions so you can evaluate your learning. This series does not come with accompanying data sets. The advantage to this is when you are on the drawing board, whether in an interview or project design, you do not have test data to play with. You have to think abstract. Case Function A CASE function checks a condition and changes a column’s value accordingly. The CASE function is used commonly in the SELECT statement. It follows this formula: CASE WHEN condition THEN value WHEN condition THEN value ELSE value END At least one WHEN/THEN condition is required. You can have as many WHEN/THEN conditions as you like. CASE and END are also required. Here is our daily_user_score table from earlier in the series: date | userid | sessionid | score ------------------------------------------ 2018–09–01 | 983489272 | 125 | 112 2018–09–01 | 234342423 | 34 | 112 2018–09–01 | 567584329 | 207 | 618 2018–09–02 | 983489272 | 126 | 410 2018–09–02 | 983489272 | 127 | 339 Say we know userid 983489272 is a bot and we want to add a user type to the table’s columns when we view the data. The query will look like this: SELECT date, userid, CASE WHEN userid = 983489272 THEN ‘bot’ ELSE ‘normal’ END AS user_type, sessionid, score FROM daily_user_score; These are the results of the above query: date | userid | user_type | sessionid | score ------------------------------------------------------ 2018–09–01 | 983489272 | bot | 125 | 112 2018–09–01 | 234342423 | normal | 34 | 112 2018–09–01 | 567584329 | normal | 207 | 618 2018–09–02 | 983489272 | bot | 126 | 410 2018–09–02 | 983489272 | bot | 127 | 339 Try it yourself Say the users 234342423 and 567584329 are fake accounts. Write a query that returns all the table’s columns and a user type column indicating if the corresponding userid is fake or real. Answer There are two correct possibilities, the first is a better answer than the second. SELECT date, userid, CASE WHEN userid = 234342423 OR userid = 567584329 THEN ‘fake’ ELSE ‘real’ END AS user_type, sessionid, score FROM daily_user_score; SELECT date, userid, CASE WHEN userid = 234342423 THEN ‘fake’ WHEN userid = 567584329 THEN ‘fake’ ELSE ‘real’ END AS user_type, sessionid, score FROM daily_user_score; This query returns: date | userid | user_type | sessionid | score ------------------------------------------------------ 2018–09–01 | 983489272 | real | 125 | 112 2018–09–01 | 234342423 | fake | 34 | 112 2018–09–01 | 567584329 | fake | 207 | 618 2018–09–02 | 983489272 | real | 126 | 410 2018–09–02 | 983489272 | real | 127 | 339 Thanks for reading! If you have questions feel free to comment & I will try to get back to you. Connect with me on Instagram @lauren__glass Connect with me on LinkedIn Check out my essentials list on Amazon Search for me using my nametag on Instagram!
