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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?
SHOW DATABASES LIKE ‘tech%’;
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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