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My SQL Interview Questions Answers

Question 1 : What's MySQL ?

Answer 1 : 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. ...

Question 2 : What is DDL, DML and DCL ?

Answer 2 : 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.

Question 3 : How do you get the number of rows affected by query?

Answer 3 : SELECT COUNT (user_id) FROM users would only return the number of user_id’s.

Question 4 : If the value in the column is repeatable, how do you find out the unique values?

Answer 4 : 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;

Question 5 : How do you return the a hundred books starting from 25th?

Answer 5 : SELECT book_title FROM books LIMIT 25, 100. The first number in LIMIT is the offset, the second is the number.

Question 6 : 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?

Answer 6 : 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.

Question 7 : How would you write a query to select all teams that won either 2, 4, 6 or 8 games?

Answer 7 : SELECT team_name FROM teams WHERE team_won IN (2, 4, 6, 8)

Question 8 : How would you select all the users, whose phone number is null?

Answer 8 : SELECT user_name FROM users WHERE ISNULL(user_phonenumber);

Question 9 : What does this query mean: SELECT user_name, user_isp FROM users LEFT JOIN isps USING (user_id) ?

Answer 9 : It’s equivalent to saying SELECT user_name, user_isp FROM users LEFT JOIN isps WHERE users.user_id=isps.user_id

Question 10 : How do you find out which auto increment was assigned on the last insert?

Answer 10 : 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.

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