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My SQL Interview Questions and Answers
What's MySQL ?
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. ...
What is DDL, DML and DCL ?
If you look at the large variety of SQL commands, they
can be divided into three large subgroups. Data Definition Language deals with
database schemas and descriptions of how the data should reside in the database,
therefore language statements like CREATE TABLE or ALTER TABLE belong to DDL.
DML deals with data manipulation, and therefore includes most common SQL
statements such SELECT, INSERT, etc. Data Control Language includes commands
such as GRANT, and mostly concerns with rights, permissions and other controls
of the database system.
How do
you get the number of rows affected by query?
SELECT COUNT (user_id) FROM users would only return the
number of user_id’s.
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;
How do
you return the a hundred books starting from 25th?
SELECT book_title FROM books LIMIT 25, 100. The first
number in LIMIT is the offset, the second is the number.
You
wrote a search engine that should retrieve 10 results at a time, but at the same
time you’d like to know how many rows there’re total. How do you display that to
the user?
SELECT SQL_CALC_FOUND_ROWS page_title FROM web_pages
LIMIT 1,10; SELECT FOUND_ROWS(); The second query (not that COUNT() is never
used) will tell you how many results there’re total, so you can display a phrase
"Found 13,450,600 results, displaying 1-10". Note that FOUND_ROWS does not pay
attention to the LIMITs you specified and always returns the total number of
rows affected by query.
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)
How
would you select all the users, whose phone number is null?
SELECT user_name FROM users WHERE ISNULL(user_phonenumber);
What does this query mean: SELECT user_name, user_isp
FROM users LEFT JOIN isps USING (user_id) ?
It’s equivalent to saying SELECT user_name, user_isp
FROM users LEFT JOIN isps WHERE users.user_id=isps.user_id
How do you find out which auto increment was assigned on
the last insert?
SELECT LAST_INSERT_ID() will return the last value
assigned by the auto_increment function. Note that you
don’t have to specify the table name.
What does –i-am-a-dummy flag to do when starting MySQL?
Makes the MySQL engine refuse UPDATE and DELETE commands
where the WHERE clause is not present.
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.
When would you use ORDER BY in DELETE statement?
When you’re not deleting by row ID. Such as in DELETE
FROM techpreparation_com_questions ORDER BY timestamp
LIMIT 1. This will delete the most recently posted
question in the table techpreparation_com_questions.
How can you see all indexes defined for a table?
SHOW INDEX FROM techpreparation_questions;
How would you change a column from VARCHAR(10) to
VARCHAR(50)?
ALTER TABLE techpreparation_questions CHANGE
techpreparation_content techpreparation_CONTENT
VARCHAR(50).
How would you delete a column?
ALTER TABLE techpreparation_answers DROP answer_user_id.
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