MySQL question & answer
What’s the default port for MySQL Server? – 3306
How do you change a password for an existing user via mysqladmin? – mysqladmin -u root -p password “newpassword”
Use mysqldump to create a copy of the database? – mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql
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.
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.
What does myisamchk do? – It compressed the MyISAM tables, which reduces their disk usage.
Explain advantages of InnoDB over MyISAM? – Row-level locking, transactions, foreign key constraints and crash recovery.
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.
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.
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.
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.
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 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
What do % and _ mean inside LIKE statement? – % corresponds to 0 or more characters, _ is exactly one character.
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.
