MYSQL Interview Questions and Answers
MySQL (pronounced "my ess cue el") is an open source relational database management system (RDBMS) that uses Structured Query Language (SQL), the most popular language for adding, accessing, and processing data in a database. Because it is open source, anyone can download MySQL and tailor it to their needs in accordance with the general public license. MySQL is noted mainly for its speed, reliability, and flexibility
MySQL Interviews are getting tough these days as the technology grows faster. To get through the MySQL interview one needs to update him/herself in a regular manner. Having said that, just before the interview, it is very important to have a quick glance of the reputed MySQL questions and answers to make yourself comfortable during the interview process. This is where DoAnswers.com helps you in renewing yourself on MySQL and various other technologies interview preparation.
31. How would you change a column from VARCHAR(10) to VARCHAR(50)?
ALTER TABLE techinterviews_questions CHANGE techinterviews_content techinterviews_CONTENT VARCHAR(50).
32. How would you change a table to InnoDB?
ALTER TABLE techinterviews_questions ENGINE innodb; When you create a table, and then run SHOW CREATE TABLE on it, you occasionally get different results than what you typed in.
33. How would you delete a column?
ALTER TABLE techinterviews_answers DROP answer_user_id.
34. How would you select all the users, whose phone number is null?
SELECT user_name FROM users WHERE ISNULL(user_phonenumber);
35. How would you write a query to select all teams that won either 2, 4, 6 or 8 games?
SELECT team_name FROM teams WHERE team_won IN (2, 4, 6, 8)
36. If I created a column with data type VARCHAR(3), what would I expect to see in MySQL table?
CHAR(3), since MySQL automatically adjusted the data type.
37. If the value in the column is repeatable, how do you find out the unique values?
Use DISTINCT in the query, such as SELECT DISTINCT user_firstname FROM users; You can also ask for a number of distinct values by saying SELECT COUNT (DISTINCT user_firstname) FROM users;
38. If you specify the data type as DECIMAL (5,2), what?s the range of values that can go in this table?
999.99 to -99.99. Note that with the negative number the minus sign is considered one of the digits.
39. On executing the DELETE statement I keep getting the error about foreign key constraint failing. What do I do?
What it means is that so of the data that you?re trying to delete is still alive in another table. Like if you have a table for universities and a table for students, which contains the ID of the university they go to, running a delete on a university table will fail if the students table still contains people enrolled at that university. Proper way to do it would be to delete the offending data first, and then delete the university in question. Quick way would involve running SET foreign_key_checks=0 before the DELETE command, and setting the parameter back to 1 after the DELETE is done. If your foreign key was formulated with ON DELETE CASCADE, the data in dependent tables will be removed automatically.
40. Use mysqldump to create a copy of the database?
mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql
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