Top Mysql interview questions and answers
1. What is MySql ?
MySql is a multi-threading, multi-user open source SQL database management system .
It is typically used for web application development, and often accessed using PHP.
2. In which language it is written ?
MySql is writen in C and C++ and SQL parser writen in yacc.
3. What is MySql default port number ?
MySql default port number is 3306
4. How do you start MySQL on Linux?
/etc/init.d/mysql start command is used for start MySQL on Linux
5. Explain the difference between mysql and mysql interfaces in PHP?
mysqli is the object-oriented version of mysql library functions.
6. 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 note.
7. How do you change a password for an existing user via mysqladmin?
mysqladmin -u root -p password “newpassword”
8. How to use mysqldump to create a copy of the database?
mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql
9. List some advantages and disadvantages of MySql ?
Advantages of using Mysql in compared to Orcale
MySQL is a free, fast, reliable, open source relational database while Oracle is expensive, although they have provided Oracle free edition to attract MySQL users.
MySQL uses only just under 1 MB of RAM on your laptop while Oracle 9i installation uses 128 MB.
MySQL is great for database enabled websites while Oracle is made for enterprises.
MySQL is portable.
Disadvantages of MySql.
MySQL is not so efficient for large scale databases.
It does not support COMMIT and STORED PROCEDURES functions version less than 5.0.
Transactions are not handled very efficiently.
10. What is mysqlcheck do ?
mysqlcheck is a client program that check the integrity of database tables.
11. What is mysqldump ?
mysqldump is a client program that creates logical backups of database.
12. What mysql -u john -p command does ?
mysql -u john -p command will prompt for the password for user john before allowing access to the database management system.
If your database server requires a username and password to gain access the -u and -p command-line options.
13. Explain Timestamp ?
Timestamp is a data type that exposes automatically generated binary numbers, which are guaranteed to be unique within a database. timestamp is used typically as a mechanism for version-stamping table rows. The storage size is 8 bytes.
14. Command to list all Databases in MySql ?
15. What is “i_am_a_dump” flag in MySql ?
“i_am_a_dump” enables MySQL engine to refuse any UPDATE or DELETE statement to execute if the WHERE clause is not present.
16. What is difference between UNIX timestamp and mysql timestamp ?
Both Unix timestamp and MySQL timestamp are stored as 32-bit integers but MySQL timestamp is represented in readable format of YYYY-MM-DD HH:MM:SS format.
17. What are the advantages of stored procedures, triggers, indexes?
A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don’t need to keep re-issuing the entire query but can refer to the stored procedure.
This provides better overall performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server. However, stored procedures of course do increase the load on the database server system, as more of the work is done on the server side and less on the client (application) side. Triggers will also be implemented. A trigger is effectively a type of stored procedure, one that is invoked when a particular event occurs.
For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that stored procedure automatically deletes the corresponding customer from a customer table when all his transactions are deleted. Indexes are used to find rows with specific column values quickly.
Without an index, MySQL must begin with the first row and then read through the entire table to find the relevant rows. The larger the table, the more this costs. If the table has an index for the columns in question,
MySQL can quickly determine the position to seek to in the middle of the data file without having to look at all the data. If a table has 1,000 rows, this is at least 100 times faster than reading sequentially. If you need to access most of the rows, it is faster to read sequentially, because this minimizes disk seeks.
18. 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.
19. Explain advantages of InnoDB over MyISAM?
Advantages of InnoDB over MyISAM
- Row-level locking
- foreign key constraints
- crash recovery
20. What is the maximum length of a table name, database name, and fieldname in MySQL?
The following table describes the maximum length for each type of identifier.
- Database 64 bytes
- Table 64 bytes
- Column 64 bytes
- Index 64 bytes
- Alias 255 bytes
There are some restrictions on the characters that may appear in identifiers.
21. How many values can the SET function of MySQL take?
MySQL set can take zero or more values but at the maximum it can take 64 values.
22. 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.