MySql Interview Questions Part-I

Q:1 How can we take a backup of a MySQL table and how can we restore it. ?
A:1 To backup:

BACKUP TABLE tbl_name[,tbl_name…] TO ‘/path/to/backup/directory’
RESTORE TABLE tbl_name[,tbl_name…] FROM ‘/path/to/backup/directory’

mysqldump: Dumping Table Structure and DataUtility to dump a database or a collection of database for backup or for transferring the data to another SQL server (not necessarily a MySQL server). The dump will contain SQL statements to create the table and/or populate the table.
-t, –no-create-info
Don’t write table creation information (the CREATE TABLE statement).
-d, –no-data
Don’t write any row information for the table. This is very useful if you just want to get a dump of the structure for a table!

Q:2 How can we optimize or increase the speed of a MySQL select query?
A:2 first of all instead of using select * from table1, use select
column1, column2, column3.. from table1
Look for the opportunity to introduce index in the table you are querying.
use limit keyword if you are looking for any specific number of rows from the result set.

Q:3 How can we repair a MySQL table?
A:3 The syntex for repairing a MySQL table is

REPAIR TABLENAME, [TABLENAME, ], [Quick],[Extended]

This command will repair the table specified if the quick is given the MySQL will do a repair of only the index tree if the extended is given it will create index row by row.

Q:4 What are the advantages of stored procedures, triggers, indexes?
A:4 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.

Q:5 How many values can the SET function of MySQL take?
A:5 MySQL set can take zero or more values but at the maximum it can take 64 values.

Q:6 What are the other commands to know the structure of table using MySQL commands except explain command?
A:6

describe Table-Name;

Q:7 How many tables will create when we create table, what are they?
A:7 The ‘.frm’ file stores the table definition.
The data file has a ‘.MYD’ (MYData) extension.
The index file has a ‘.MYI’ (MYIndex) extension,

Q:8 What is the purpose of the following files having extensions 1) .frm 2) .myd 3) .myi? What do these files contain?
A:8 In MySql, the default table type is MyISAM.
Each MyISAM table is stored on disk in three files. The files have names that begin with the table name and have an extension to indicate the file type.
The ‘.frm’ file stores the table definition.
The data file has a ‘.MYD’ (MYData) extension.
The index file has a ‘.MYI’ (MYIndex) extension,

Q:9 What is maximum size of a database in MySQL?
A:9 If the operating system or filesystem places a limit on the number of files in a directory, MySQL is bound by that constraint.The efficiency of the operating system in handling large numbers of files in a directory can place a practical limit on the number of tables in a database. If the time required to open a file in the directory increases significantly as the number of files increases, database performance can be adversely affected.
The amount of available disk space limits the number of tables.
MySQL 3.22 had a 4GB (4 gigabyte) limit on table size. With the MyISAM storage engine in MySQL 3.23, the maximum table size was increased to 65536 terabytes (2567 – 1 bytes). With this larger allowed table size, the maximum effective table size for MySQL databases is usually determined by operating system constraints on file sizes, not by MySQL internal limits.The InnoDB storage engine maintains InnoDB tables within a tablespace that can be created from several files. This allows a table to exceed the maximum individual file size. The tablespace can include raw disk partitions, which allows extremely large tables. The maximum tablespace size is 64TB.
The following table lists some examples of operating system file-size limits. This is only a rough guide and is not intended to be definitive.
For the most up-to-date information, be sure to check the documentation specific to your operating system.
Operating System File-size LimitLinux 2.2-Intel 32-bit 2GB (LFS: 4GB)
Linux 2.4+ (using ext3 filesystem) 4TB
Solaris 9/10 16TB
NetWare w/NSS filesystem 8TB
Win32 w/ FAT/FAT32 2GB/4GB
Win32 w/ NTFS 2TB (possibly larger)
MacOS X w/ HFS+ 2TB

Q:10 Give the syntax of Grant and Revoke commands?
A:10 The generic syntax for grant is as following

> GRANT [rights] on [database/s] TO [username@hostname] IDENTIFIED BY [password]

now rights can be
a) All privileges
b) combination of create, drop, select, insert, update and delete etc.We can grant rights on all databse by using *.* or some specific database by database.* or a specific table by database.table_name
username@hotsname can be either username@localhost, username@hostname
and username@%
where hostname is any valid hostname and % represents any name, the *.* any condition
password is simply the password of userThe generic syntax for revoke is as following

> REVOKE [rights] on [database/s] FROM [username@hostname]

now rights can be as explained above
a) All privileges
b) combination of create, drop, select, insert, update and delete etc.
username@hotsname can be either username@localhost, username@hostname
and username@%
where hostname is any valid hostname and % represents any name, the *.*
any condition

Q:11 Explain Normalization concept?
A:11 The normalization process involves getting our data to conform to three progressive normal forms, and a higher level of normalization cannot be achieved until the previous levels have been achieved (there are actually five normal forms, but the last two are mainly academic and will not be discussed).
First Normal FormThe First Normal Form (or 1NF) involves removal of redundant data from horizontal rows. We want to ensure that there is no duplication of data in a given row, and that every column stores the least amount of information possible (making the field atomic).
Second Normal FormWhere the First Normal Form deals with redundancy of data across a horizontal row, Second Normal Form (or 2NF) deals with redundancy of
data in vertical columns. As stated earlier, the normal forms are progressive, so to achieve Second Normal Form, your tables must already be in First Normal Form.
Third Normal Form I have a confession to make; I do not often use Third Normal Form. In Third Normal Form we are looking for data in our tables that is not fully dependant on the primary key, but dependant on another value in the table.

Q:12 How can we find the number of rows in a table using MySQL?
A:12 Use this for mysql

>SELECT COUNT(*) FROM table_name;

Q:13 How many ways we can we find the current date using MySQL?
A:13 SELECT CURDATE();
CURRENT_DATE() = CURDATE()
for time use
SELECT CURTIME();
CURRENT_TIME() = CURTIME()

Q:14 What is the difference between Primary Key and Unique key?
A:14 Primary Key: A column in a table whose values uniquely identify the rows in the table. A primary key value cannot be NULL.
Unique Key: Unique Keys are used to uniquely identify each row in the table. There can be one and only one row for each unique key value. So NULL can be a unique key. There can be only one primary key for a table but there can be more than one unique for a table.

Q:15 What is the difference between GROUP BY and ORDER BY in Sql?
A:15 ORDER BY [col1],[col2],…,[coln]; Tells DBMS according to what columns it should sort the result. If two rows will have the same value in col1 it will try to sort them according to col2 and so on.
GROUP BY [col1],[col2],…,[coln]; Tells DBMS to group results with same value of column col1. You can use COUNT(col1), SUM(col1), AVG(col1) with it, if you want to count all items in group, sum all values or view average.

Q:16 What is the difference between char and varchar data types?
A:16 Set char to occupy n bytes and it will take n bytes even if u r storing a value of n-m bytes
Set varchar to occupy n bytes and it will take only the required space and will not use the n bytes
eg. name char(15) will waste 10 bytes if we store ‘mizan’, if each char takes a byte
eg. name varchar(15) will just use 5 bytes if we store ‘mizan’, if each char takes a byte. rest 10 bytes will be free.

Q:17 How can I load data from a text file into a table?
A:17 you can use LOAD DATA INFILE file_name; syntax to load data from a text file. but you have to make sure that
a) data is delimited
b) columns and data matched correctly

Q:18 How can we know the number of days between two given dates using MySQL?
A:18

SELECT DATEDIFF(’2007-03-07',’2005-01-01');

Q:19 What is the maximum length of a table name, database name, and fieldname in MySQL?
A:19 The following table describes the maximum length for each type of identifier

Identifier Maximum Length
(bytes)
Database 64
Table 64
Column 64
Index 64
Alias 255

There are some restrictions on the characters that may appear in identifiers:



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