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My SQL Interview Questions and Answers

How would you change a table to InnoDB?
ALTER TABLE techpreparation_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. What does MySQL modify in your newly created tables?
1. VARCHARs with length less than 4 become CHARs
2. CHARs with length more than 3 become VARCHARs.
3. NOT NULL gets added to the columns declared as PRIMARY KEYs
4. Default values such as NULL are specified for each column

How do I find out all databases starting with ‘tech’ to which I have access to?

How do you concatenate strings in MySQL?
CONCAT (string1, string2, string3)

How do you get a portion of a string?
SELECT SUBSTR(title, 1, 10) from techpreparation_questions;

What’s the difference between CHAR_LENGTH and LENGTH?
The first is, naturally, the character count. The second is byte count. For the Latin characters the numbers are the same, but they’re not the same for Unicode and other encodings.

How do you convert a string to UTF-8?


SELECT (techpreparation_question USING utf8);

What do % and _ mean inside LIKE statement?
% corresponds to 0 or more characters, _ is exactly one character. 

What does + mean in REGEXP?
At least one character. Appendix G. Regular Expressions from MySQL manual is worth perusing before the interview.

How do you get the month from a timestamp?
SELECT MONTH(techpreparation_timestamp) from techpreparation_questions;

How do you offload the time/date handling to MySQL?
SELECT DATE_FORMAT(techpreparation_timestamp, ‘%Y-%m-%d’) from techpreparation_questions; A similar TIME_FORMAT function deals with time.

How do you add three minutes to a date?
ADDDATE(techpreparation_publication_date, INTERVAL 3 MINUTE)

What’s the difference between Unix timestamps and MySQL timestamps?
Internally Unix timestamps are stored as 32-bit integers, while MySQL timestamps are stored in a similar manner, but represented in readable YYYY-MM-DD HH:MM:SS format.

How do you convert between Unix timestamps and MySQL timestamps?
UNIX_TIMESTAMP converts from MySQL timestamp to Unix timestamp, FROM_UNIXTIME converts from Unix timestamp to MySQL timestamp.

What are ENUMs used for in MySQL?
You can limit the possible values that go into the table. CREATE TABLE months (month ENUM ‘January’, ‘February’, ‘March’,…); INSERT months VALUES (’April’); 

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