MySQL Question and Answers

1. How do you start and stop MySQL on Windows? - net start MySQL, net stop MySQL


2. How do you start MySQL on Linux? - /etc/init.d/mysql start


3. Explain the difference between mysql and mysqli interfaces in PHP? - mysqli is the object-oriented version of mysql library functions.


4. What’s the default port for MySQL Server? - 3306


5. What does tee command do in MySQL? - tee followed by a filename turns on MySQL logging to a specified file. It can be stopped by command notee.


6. Can you save your connection settings to a conf file? - Yes, and name it ~/.my.conf. You might want to change the permissions on the file to 600, so that it’s not readable by others.


7. How do you change a password for an existing user via mysqladmin? - mysqladmin -u root -p password "newpassword"


8. Use mysqldump to create a copy of the database? - mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql


9. Have you ever used MySQL Administrator and MySQL Query Browser? Describe the tasks you accomplished with these tools.


10. What are some good ideas regarding user security in MySQL? - There is no user without a password. There is no user without a user name. There is no user whose Host column contains % (which here indicates that the user can log in from anywhere in the network or the Internet). There are as few users as possible (in the ideal case only root) who have unrestricted access.


11. Explain the difference between MyISAM Static and MyISAM Dynamic. - In MyISAM static all the fields have fixed width. The Dynamic MyISAM table would include fields such as TEXT, BLOB, etc. to accommodate the data types with various lengths. MyISAM Static would be easier to restore in case of corruption, since even though you might lose some data, you know exactly where to look for the beginning of the next record.


12. What does myisamchk do? - It compressed the MyISAM tables, which reduces their disk usage.


13. Explain advantages of InnoDB over MyISAM? - Row-level locking, transactions, foreign key constraints and crash recovery.


14. Explain advantages of MyISAM over InnoDB? - Much more conservative approach to disk space management - each MyISAM table is stored in a separate file, which could be compressed then with myisamchk if needed. With InnoDB the tables are stored in tablespace, and not much further optimization is possible. All data except for TEXT and BLOB can occupy 8,000 bytes at most. No full text indexing is available for InnoDB. TRhe COUNT(*)s execute slower than in MyISAM due to tablespace complexity.


15. What are HEAP tables in MySQL? - HEAP tables are in-memory. They are usually used for high-speed temporary storage. No TEXT or BLOB fields are allowed within HEAP tables. You can only use the comparison operators = and <=>. HEAP tables do not support AUTO_INCREMENT. Indexes must be NOT NULL.


16. How do you control the max size of a HEAP table? - MySQL config variable max_heap_table_size.


17. What are CSV tables? - Those are the special tables, data for which is saved into comma-separated values files. They cannot be indexed.


18. Explain federated tables. - Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.


19. What is SERIAL data type in MySQL? - BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT


20. What happens when the column is set to AUTO INCREMENT and you reach the maximum value for that table? - It stops incrementing. It does not overflow to 0 to prevent data losses, but further inserts are going to produce an error, since the key has been used already.


21. Explain the difference between BOOL, TINYINT and BIT. - Prior to MySQL 5.0.3: those are all synonyms. After MySQL 5.0.3: BIT data type can store 8 bytes of data and should be used for binary data.


22. Explain the difference between FLOAT, DOUBLE and REAL. - FLOATs store floating point numbers with 8 place accuracy and take up 4 bytes. DOUBLEs store floating point numbers with 16 place accuracy and take up 8 bytes. REAL is a synonym of FLOAT for now.


23. 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.


24. What happens if a table has one column defined as TIMESTAMP? - That field gets the current timestamp whenever the row gets altered.


25. But what if you really want to store the timestamp data, such as the publication date of the article? - Create two columns of type TIMESTAMP and use the second one for your real data.


26. Explain data type TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP - The column exhibits the same behavior as a single timestamp column in a table with no other timestamp columns.


27. What does TIMESTAMP ON UPDATE CURRENT_TIMESTAMP data type do? - On initialization places a zero in that column, on future updates puts the current value of the timestamp in.


28. Explain TIMESTAMP DEFAULT ‘2006:09:02 17:38:44′ ON UPDATE CURRENT_TIMESTAMP. - A default value is used on initialization, a current timestamp is inserted on update of the row.


29. 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.


30. Explain MySQL architecture. - The front layer takes care of network connections and security authentications, the middle layer does the SQL query parsing, and then the query is handled off to the storage engine. A storage engine could be either a default one supplied with MySQL (MyISAM) or a commercial one supplied by a third-party vendor (ScaleDB, InnoDB, etc.)


31. Explain MySQL locks. - Table-level locks allow the user to lock the entire table, page-level locks allow locking of certain portions of the tables (those portions are referred to as tables), row-level locks are the most granular and allow locking of specific rows.


32. Explain multi-version concurrency control in MySQL. - Each row has two additional columns associated with it - creation time and deletion time, but instead of storing timestamps, MySQL stores version numbers.


33. What are MySQL transactions? - A set of instructions/queries that should be executed or rolled back as a single atomic unit.


34. What’s ACID? - Automicity - transactions are atomic and should be treated as one in case of rollback. Consistency - the database should be in consistent state between multiple states in transaction. Isolation - no other queries can access the data modified by a running transaction. Durability - system crashes should not lose the data.


35. Which storage engines support transactions in MySQL? - Berkeley DB and InnoDB.


36. How do you convert to a different table type? - ALTER TABLE customers TYPE = InnoDB


37. How do you index just the first four bytes of the column? - ALTER TABLE customers ADD INDEX (business_name(4))


38. What’s the difference between PRIMARY KEY and UNIQUE in MyISAM? - PRIMARY KEY cannot be null, so essentially PRIMARY KEY is equivalent to UNIQUE NOT NULL.


39. How do you prevent MySQL from caching a query? - SELECT SQL_NO_CACHE …


40. What’s the difference between query_cache_type 1 and 2? - The second one is on-demand and can be retrieved via SELECT SQL_CACHE … If you’re worried about the SQL portability to other servers, you can use SELECT /* SQL_CACHE */ id FROM … - MySQL will interpret the code inside comments, while other servers will ignore it.