PHP Interview Questions and Answers

what is the use of mysql_fetch_array() function in php ?

This function returns a row from the table as an associative array or numeric array.

Write down the code for upload a file in php..

0) { echo “Error: ” . $_FILES["file"]["error"] . “
”; } else { echo “Upload: ” . $_FILES["file"]["name"] . “
”; echo “Type: ” . $_FILES["file"]["type"] . “
”; echo “Size: ” . ($_FILES["file"]["size"] / 1024) . ” Kb
”; echo “Stored in: ” . $_FILES["file"]["tmp_name"]; } ?>

what is the use of the function " explode() " in php?

This function is used to split a string by special character or symbol in the string, we must be pass the string and splitting character as parameter into the function.

What is use of in_array() function in php ?

in_array used to checks if a value exists in an array

How to retrieve the data from MySQL result set ?

using the methods given below 1. mysql_fetch_row. 2. mysql_fetch_array 3. mysql_fetch_object 4. mysql_fetch_assoc

differences between GET and POST methods ?

We can send 1024 bytes using GET method but POST method can transfer large amount of data and POST is the secure method than GET method .

What is SSL ?

SSL stands for Secure Sockets Layer. This is a cryptographic protocols which provide secure communications on the Internet

How to calculate the sum of values in an array ?

"array_sum" method used for calculate sum of values in an array

What is the use of "ksort" in php?

it is used for sort an array by key in reverse order. 

PHP Interview Questions and Answers

what is the use of mysql_fetch_array() function in php ?

This function returns a row from the table as an associative array or numeric array.

Write down the code for upload a file in php..

0) { echo “Error: ” . $_FILES["file"]["error"] . “
”; } else { echo “Upload: ” . $_FILES["file"]["name"] . “
”; echo “Type: ” . $_FILES["file"]["type"] . “
”; echo “Size: ” . ($_FILES["file"]["size"] / 1024) . ” Kb
”; echo “Stored in: ” . $_FILES["file"]["tmp_name"]; } ?>

what is the use of the function " explode() " in php?

This function is used to split a string by special character or symbol in the string, we must be pass the string and splitting character as parameter into the function.

What is use of in_array() function in php ?

in_array used to checks if a value exists in an array

How to retrieve the data from MySQL result set ?

using the methods given below 1. mysql_fetch_row. 2. mysql_fetch_array 3. mysql_fetch_object 4. mysql_fetch_assoc

differences between GET and POST methods ?

We can send 1024 bytes using GET method but POST method can transfer large amount of data and POST is the secure method than GET method .

What is SSL ?

SSL stands for Secure Sockets Layer. This is a cryptographic protocols which provide secure communications on the Internet

How to calculate the sum of values in an array ?

"array_sum" method used for calculate sum of values in an array

What is the use of "ksort" in php?

it is used for sort an array by key in reverse order. 

mySQL Interview Questions and Answers

My SQL Interview Questions and Answers

What's MySQL ?
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. ...

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.

How do you get the number of rows affected by query? 
SELECT COUNT (user_id) FROM users would only return the number of user_id’s.

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

How do you return the a hundred books starting from 25th? 
SELECT book_title FROM books LIMIT 25, 100. The first number in LIMIT is the offset, the second is the number.

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 write a query to select all teams that won either 2, 4, 6 or 8 games? 
SELECT team_name FROM teams WHERE team_won IN (2, 4, 6, 8)

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

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

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.

What does –i-am-a-dummy flag to do when starting MySQL? 
Makes the MySQL engine refuse UPDATE and DELETE commands where the WHERE clause is not present.

On executing the DELETE statement I keep getting the error about foreign key constraint failing. What do I do? 
What it means is that so of the data that you’re trying to delete is still alive in another table. Like if you have a table for universities and a table for students, which contains the ID of the university they go to, running a delete on a university table will fail if the students table still contains people enrolled at that university. Proper way to do it would be to delete the offending data first, and then delete the university in question. Quick way would involve running SET foreign_key_checks=0 before the DELETE command, and setting the parameter back to 1 after the DELETE is done. If your foreign key was formulated with ON DELETE CASCADE, the data in dependent tables will be removed automatically.

When would you use ORDER BY in DELETE statement? 
When you’re not deleting by row ID. Such as in DELETE FROM techpreparation_com_questions ORDER BY timestamp LIMIT 1. This will delete the most recently posted question in the table techpreparation_com_questions.

How can you see all indexes defined for a table? 
SHOW INDEX FROM techpreparation_questions;

How would you change a column from VARCHAR(10) to VARCHAR(50)? 
ALTER TABLE techpreparation_questions CHANGE techpreparation_content techpreparation_CONTENT VARCHAR(50).

How would you delete a column? 
ALTER TABLE techpreparation_answers DROP answer_user_id.




How would you change a table to InnoDB?
ALTER TABLE techpreparation_questions ENGINE innodb;

When you create a table, and then run SHOW CREATE TABLE on it, you occasionally get different results than what you typed in. What does MySQL modify in your newly created tables? 
1. VARCHARs with length less than 4 become CHARs
2. CHARs with length more than 3 become VARCHARs.
3. NOT NULL gets added to the columns declared as PRIMARY KEYs
4. Default values such as NULL are specified for each column

How do I find out all databases starting with ‘tech’ to which I have access to? 
SHOW DATABASES LIKE ‘tech%’;

How do you concatenate strings in MySQL? 
CONCAT (string1, string2, string3) 
How do you get a portion of a string? 
SELECT SUBSTR(title, 1, 10) from techpreparation_questions;

What’s the difference between CHAR_LENGTH and LENGTH? 
The first is, naturally, the character count. The second is byte count. For the Latin characters the numbers are the same, but they’re not the same for Unicode and other encodings.

How do you convert a string to UTF-8?

SELECT (techpreparation_question USING utf8);

What do % and _ mean inside LIKE statement? 
% corresponds to 0 or more characters, _ is exactly one character. 

What does + mean in REGEXP? 
At least one character. Appendix G. Regular Expressions from MySQL manual is worth perusing before the interview.

How do you get the month from a timestamp? 
SELECT MONTH(techpreparation_timestamp) from techpreparation_questions;

How do you offload the time/date handling to MySQL? 
SELECT DATE_FORMAT(techpreparation_timestamp, ‘%Y-%m-%d’) from techpreparation_questions; A similar TIME_FORMAT function deals with time.

How do you add three minutes to a date? 
ADDDATE(techpreparation_publication_date, INTERVAL 3 MINUTE)

What’s the difference between Unix timestamps and MySQL timestamps? 
Internally Unix timestamps are stored as 32-bit integers, while MySQL timestamps are stored in a similar manner, but represented in readable YYYY-MM-DD HH:MM:SS format.

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.

What are ENUMs used for in MySQL? 
You can limit the possible values that go into the table. CREATE TABLE months (month ENUM ‘January’, ‘February’, ‘March’,…); INSERT months VALUES (’April’); 


How are ENUMs and SETs represented internally?
As unique integers representing the powers of two, due to storage optimizations.

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

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

Explain the difference between mysql and mysql interfaces in PHP? 
mysqli is the object-oriented version of mysql library functions. 
What’s the default port for MySQL Server? 
3306

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.

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.

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

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

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 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.

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

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

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

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

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 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.

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.

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.

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

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. 
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.

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.

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.

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.


General Information About MySQL
MySQL is a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server. 

MySQL is free software. It is licensed with the GNU GENERAL PUBLIC LICENSE http://www.gnu.org/.

What Is MySQL
MySQL, the most popular Open Source SQL database, is provided by MySQL AB. MySQL AB is a commercial company that builds is business providing services around the MySQL database. See section 1.2 What Is MySQL AB.

ySQL is a database management system. 
A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL. Since computers are very good at handling large amounts of data, database management plays a central role in computing, as stand-alone utilities, or as parts of other applications.

MySQL is a relational database management system. 
A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The tables are linked by defined relations making it possible to combine data from several tables on request. The SQL part of MySQL stands for "Structured Query Language" - the most common standardized language used to access databases.

MySQL is Open Source Software. 
Open source means that it is possible for anyone to use and modify. Anybody can download MySQL from the Internet and use it without paying anything. Anybody so inclined can study the source code and change it to fit their needs. MySQL uses the GPL (GNU General Public License) http://www.gnu.org, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL into a commercial application you can buy a commercially licensed version from us.

Why use MySQL? 
MySQL is very fast, reliable, and easy to use. If that is what you are looking for, you should give it a try. MySQL also has a very practical set of features developed in very close cooperation with our users. You can find a performance comparison of MySQL to some other database managers on our benchmark page. See section 12.7 Using Your Own Benchmarks. MySQL was originally developed to handle very large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Though under constant development, MySQL today offers a rich and very useful set of functions. The connectivity, speed, and security make MySQL highly suited for accessing databases on the Internet.

The technical features of MySQL 
For advanced technical information, see section 7 MySQL Language Reference. MySQL is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a programming interface. We also provide MySQL as a multi-threaded library which you can link into your application to get a smaller, faster, easier to manage product. MySQL has a lot of contributed software available. 

It is very likely that you will find that your favorite application/language already supports MySQL. The official way to pronounce MySQL is ``My Ess Que Ell'' (not MY-SEQUEL). But we try to avoid correcting people who say MY-SEQUEL.

The Main Features of MySQL
The following list describes some of the important characteristics of MySQL: 

Fully multi-threaded using kernel threads. That means it can easily use multiple CPUs if available. 
C, C++, Eiffel, Java, Perl, PHP, Python and Tcl APIs. 
Works on many different platforms. 
Many column types: signed/unsigned integers 1, 2, 3, 4, and 8 bytes long, FLOAT, DOUBLE, CHAR, VARCHAR, TEXT, BLOB, DATE, TIME, DATETIME, TIMESTAMP, YEAR, SET, and ENUM types. 
Very fast joins using an optimized one-sweep multi-join. 
Full operator and function support in the SELECT and WHERE parts of queries. Example: 
mysql> SELECT CONCAT(first_name, " ", last_name) FROM tbl_name
WHERE income/dependents > 10000 AND age > 30;

SQL functions are implemented through a highly optimized class library and should be as fast as they can get! Usually there shouldn't be any memory allocation at all after query initialization. 
Full support for SQL GROUP BY and ORDER BY clauses. Support for group functions (COUNT(), COUNT(DISTINCT), AVG(), STD(), SUM(), MAX() and MIN()). 
Support for LEFT OUTER JOIN and RIGHT OUTER JOIN with ANSI SQL and ODBC syntax. 
You can mix tables from different databases in the same query (as of Version 3.22). 
A privilege and password system that is very flexible and secure and allows host-based verification. Passwords are secure because all password traffic is encrypted when you connect to a server. 
ODBC (Open-DataBase-Connectivity) support for Win32 (with source). All ODBC 2.5 functions and many others. For example, you can use MS Access to connect to your MySQL server. See section 18 MySQL ODBC Support. 
Very fast B-tree disk tables with index compression. 
Up to 32 indexes per table are allowed. Each index may consist of 1 to 16 columns or parts of columns. The maximum index length is 500 bytes (this may be changed when compiling MySQL). An index may use a prefix of a CHAR or VARCHAR field. Fixed-length and variable-length records. 
In-memory hash tables which are used as temporary tables. 
Handles large databases. We are using MySQL with some databases that contain 50,000,000 records and we know of users that uses MySQL with 60,000 tables and about 5,000,000,000 rows 
All columns have default values. You can use INSERT to insert a subset of a table's columns; those columns that are not explicitly given values are set to their default values. 
Uses GNU Automake, Autoconf, and libtool for portability. 
Written in C and C++. Tested with a broad range of different compilers. 
A very fast thread-based memory allocation system. 
No memory leaks. Tested with a commercial memory leakage detector (purify). 
Includes myisamchk, a very fast utility for table checking, optimization, and repair. See section 15 Maintaining a MySQL Installation. 
Full support for several different character sets, including ISO-8859-1 (Latin1), big5, ujis, and more. For example, the Scandinavian characters `@ringaccent{a}', `@"a' and `@"o' are allowed in table and column names. 
All data are saved in the chosen character set. All comparisons for normal string columns are case insensitive. 
Sorting is done according to the chosen character set (the Swedish way by default). It is possible to change this when the MySQL server is started up. To see an example of very advanced sorting, look at the Czech sorting code. MySQL supports many different character sets that can be specified at compile and run time. 
Aliases on tables and columns are allowed as in the SQL92 standard. 
DELETE, INSERT, REPLACE, and UPDATE return how many rows were changed (affected). It is possible to return the number of rows matched instead by setting a flag when connecting to the server. 
Function names do not clash with table or column names. For example, ABS is a valid column name. The only restriction is that for a function call, no spaces are allowed between the function name and the `(' that follows it. See section 7.39 Is MySQL Picky About Reserved Words?. 
All MySQL programs can be invoked with the --help or -? options to obtain online assistance. 
The server can provide error messages to clients in many languages. See section 10.1 What Languages Are Supported by MySQL?. Clients may connect to the MySQL server using TCP/IP Sockets, Unix Sockets (Unixes), or Named Pipes (NT). The MySQL-specific SHOW command can be used to retrieve information about databases, tables, and indexes. The EXPLAIN command can be used to determine how the optimizer resolves a query. 

MySQL optimization

1) The sql query becomes faster if you use the actual columns names in SELECT statement instead of than '*'.

For Example: Write the query as

SELECT id, first_name, last_name, age, subject FROM student_details;

Instead of:

SELECT * FROM student_details;

 

2) HAVING clause is used to filter the rows after all the rows are selected. It is just like a filter. Do not use HAVING clause for any other purposes. 
For Example: Write the query as

SELECT subject, count(subject) 
FROM student_details 
WHERE subject != 'Science' 
AND subject != 'Maths' 
GROUP BY subject;

Instead of:

SELECT subject, count(subject) 
FROM student_details 
GROUP BY subject 
HAVING subject!= 'Vancouver' AND subject!= 'Toronto';

 

3) Sometimes you may have more than one subqueries in your main query. Try to minimize the number of subquery block in your query. 
For Example: Write the query as

SELECT name 
FROM employee 
WHERE (salary, age ) = (SELECT MAX (salary), MAX (age) 
FROM employee_details) 
AND dept = 'Electronics'; 

Instead of:

SELECT name 
FROM employee
WHERE salary = (SELECT MAX(salary) FROM employee_details) 
AND age = (SELECT MAX(age) FROM employee_details) 
AND emp_dept = 'Electronics';

 

4) Use operator EXISTS, IN and table joins appropriately in your query. 
a) Usually IN has the slowest performance. 
b) IN is efficient when most of the filter criteria is in the sub-query. 
c) EXISTS is efficient when most of the filter criteria is in the main query.

For Example: Write the query as

Select * from product p 
where EXISTS (select * from order_items o 
where o.product_id = p.product_id)

Instead of:

Select * from product p 
where product_id IN 
(select product_id from order_items

 

5) Use EXISTS instead of DISTINCT when using joins which involves tables having one-to-many relationship. 
For Example: Write the query as

SELECT d.dept_id, d.dept 
FROM dept d 
WHERE EXISTS ( SELECT 'X' FROM employee e WHERE e.dept = d.dept);

Instead of:

SELECT DISTINCT d.dept_id, d.dept 
FROM dept d,employee e 
WHERE e.dept = e.dept;

 

6) Try to use UNION ALL in place of UNION. 
For Example: Write the query as

SELECT id, first_name 
FROM student_details_class10 
UNION ALL 
SELECT id, first_name 
FROM sports_team;

Instead of:

SELECT id, first_name, subject 
FROM student_details_class10 
UNION 
SELECT id, first_name 
FROM sports_team;

 

7) Be careful while using conditions in WHERE clause. 
For Example: Write the query as

SELECT id, first_name, age FROM student_details WHERE age > 10;

Instead of:

SELECT id, first_name, age FROM student_details WHERE age != 10;

Write the query as

SELECT id, first_name, age 
FROM student_details 
WHERE first_name LIKE 'Chan%';

Instead of:

SELECT id, first_name, age 
FROM student_details 
WHERE SUBSTR(first_name,1,3) = 'Cha';

Write the query as

SELECT id, first_name, age 
FROM student_details 
WHERE first_name LIKE NVL ( :name, '%');

Instead of:

SELECT id, first_name, age 
FROM student_details 
WHERE first_name = NVL ( :name, first_name);

Write the query as

SELECT product_id, product_name 
FROM product 
WHERE unit_price BETWEEN MAX(unit_price) and MIN(unit_price)

Instead of:

SELECT product_id, product_name 
FROM product 
WHERE unit_price >= MAX(unit_price) 
and unit_price <= MIN(unit_price)

Write the query as

SELECT id, name, salary 
FROM employee 
WHERE dept = 'Electronics' 
AND location = 'Bangalore';

Instead of:

SELECT id, name, salary 
FROM employee 
WHERE dept || location= 'ElectronicsBangalore';

Use non-column expression on one side of the query because it will be processed earlier.

Write the query as

SELECT id, name, salary 
FROM employee 
WHERE salary < 25000;

Instead of:

SELECT id, name, salary 
FROM employee 
WHERE salary + 10000 < 35000;

Write the query as

SELECT id, first_name, age 
FROM student_details 
WHERE age > 10;

Instead of:

SELECT id, first_name, age 
FROM student_details 
WHERE age NOT = 10;

8) Use DECODE to avoid the scanning of same rows or joining the same table repetitively. DECODE can also be made used in place of GROUP BY or ORDER BY clause. 
For Example: Write the query as

SELECT id FROM employee 
WHERE name LIKE 'Ramesh%' 
and location = 'Bangalore';

Instead of:

SELECT DECODE(location,'Bangalore',id,NULL) id FROM employee 
WHERE name LIKE 'Ramesh%';

9) To store large binary objects, first place them in the file system and add the file path in the database.

10) To write queries which provide efficient performance follow the general SQL standard rules.

a) Use single case for all SQL verbs
b) Begin all SQL verbs on a new line
c) Separate all words with a single space 
d) Right or left aligning verbs within the initial SQL verb 

mySQL tips

1. What is MySQL? 
MySQL is an open source DBMS which is built, supported and distributed by MySQL AB (now acquired by Oracle)
2. What are the technical features of MySQL? 
MySQL database software is a client or server system which includes
Multithreaded SQL server supporting various client programs and libraries
Different backend
Wide range of application programming interfaces and
Administrative tools.
3. Why MySQL is used?
MySQL database server is reliable, fast and very easy to use.  This software can be downloaded as freeware and can be downloaded from the internet.
4. What are Heap tables?
HEAP tables are present in memory and they are used for high speed storage on temporary
basis.
• BLOB or TEXT fields are not allowed
• Only comparison operators can be used =, <,>, = >,=<
• AUTO_INCREMENT is not supported by HEAP tables
• Indexes should be NOT NULL
5. What is the default port for MySQL Server?
The default port for MySQL server is 3306.
6.  What are the advantages of MySQL when compared with Oracle? 
MySQL is open source software which is available at any time and has no cost involved.
MySQL is portable
GUI with command prompt.
Administration is supported using MySQL Query Browser
7. Differentiate between FLOAT and DOUBLE? 
Following are differences for FLOAT and DOUBLE:
• Floating point numbers are stored in FLOAT with eight place accuracy and it has four bytes.
• Floating point numbers are stored in DOUBLE with accuracy of 18 places and it has eight bytes.
 8. Differentiate CHAR_LENGTH and LENGTH?
CHAR_LENGTH  is character count whereas the LENGTH is byte count. The numbers are same for Latin characters but they are different for Unicode and other encodings.
9. How to represent ENUMs and SETs internally? 
ENUMs and SETs are used to represent powers of two because of storage optimizations.
10. What is the usage of ENUMs in MySQL?
ENUM is a string object used to specify set of predefined values and that can be used during table creation.
?
1
Create table size(name ENUM('Small', 'Medium','Large')
11. Define REGEXP? 
REGEXP is a pattern match in which  matches pattern anywhere in the search value.
12. Difference between CHAR and VARCHAR? 
Following are the differences between CHAR and VARCHAR:
CHAR and VARCHAR types differ in storage and retrieval
CHAR column length is fixed to the length that is declared while creating table. The length value ranges from 1 and 255
When CHAR values are stored then they are right padded using spaces to specific length. Trailing spaces are removed when CHAR values are retrieved.
 13. Give string types available for column?
The string types are:
SET
BLOB
ENUM
CHAR
TEXT
VARCHAR
14. How to get current MySQL version?
SELECT VERSION (); is used to get the current version of MySQL.
 15. What storage engines are used in MySQL? 
Storage engines are called table types and data is stored in files using various techniques.
Technique involves:
Storage mechanism
Locking levels
Indexing
Capabilities and functions.
16. What are the drivers in MySQL?
Following are the drivers available in MySQL:
PHP Driver
JDBC Driver
ODBC Driver
C WRAPPER
PYTHON Driver
PERL Driver
RUBY Driver
CAP11PHP Driver
Ado.net5.mxj
17. What does a TIMESTAMP do on UPDATE CURRENT_TIMESTAMP data type?
TIMESTAMP column is updated with Zero when the table is created.  UPDATE CURRENT_TIMESTAMP modifier updates the timestamp field to  current time whenever there is a 

change in other fields of the table.
18. What is the difference between primary key and candidate key?
Every row of a table is identified uniquely by primary key. There is only one primary key for a table.
Primary Key is also a candidate key. By common convention, candidate key can be designated as primary and which can be used for any foreign key references.
19. How do you login to MySql using Unix shell?
We can login through this command:
# [mysql dir]/bin/mysql -h hostname -u  -p 
20. What does myisamchk do?
It compress the MyISAM tables, which reduces their disk or memory usage.
21. How do you control the max size of a HEAP table?
Maximum size of Heal table can be controlled by MySQL config variable called max_heap_table_size.
22. What is the difference between MyISAM Static and MyISAM Dynamic?
In MyISAM static all the fields will have fixed width. The Dynamic MyISAM table will have fields like TEXT, BLOB, etc. to accommodate the data types with various 

lengths.
MyISAM Static would be easier to restore in case of corruption.
23. What are federated tables?
Federated tables which allow access to the tables located on other databases on other servers.
24. What, if a table has one column defined as TIMESTAMP?
Timestamp field gets the current timestamp whenever the row gets altered.
25. What happens when the column is set to AUTO INCREMENT and if you reach maximum value in the table?
It stops incrementing. Any further inserts are going to produce an error, since the key has been used already.
26. How can we find out which auto increment was assigned on Last insert?
LAST_INSERT_ID will return the last value assigned by Auto_increment and it is not required to specify the table name.
27. How can you see all indexes defined for a table?
Indexes are defined for the table by:
SHOW INDEX FROM ;
28. What do you mean by % and _ in the LIKE statement?
% corresponds to 0 or more characters, _ is exactly one character in the LIKE statement.
29. How can we convert between Unix & MySQL timestamps?
UNIX_TIMESTAMP is the command which converts from MySQL timestamp to Unix timestamp
FROM_UNIXTIME is the command which converts from Unix timestamp to MySQL timestamp.
30. What are the column comparisons operators?
The = , <>, <=, <, >=, >,<<,>>, <=>, AND, OR, or LIKE operators are used in column comparisons in SELECT statements.
 31. How can we get the number of rows affected by query?
Number of rows can be obtained by
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SELECT COUNT (user_id) FROM users;
32.  Is Mysql query is case sensitive?
No.
?
1
2
3
SELECT VERSION(), CURRENT_DATE;
SeLect version(), current_date;
seleCt vErSiOn(), current_DATE;
All these examples are same. It is not case sensitive.
33. What is the difference between the LIKE and REGEXP operators?  
LIKE and REGEXP operators are used to express with ^ and %.
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SELECT * FROM employee WHERE emp_name REGEXP "^b";
SELECT * FROM employee WHERE emp_name LIKE "%b";
34. What is the difference between BLOB AND TEXT?
A BLOB is a binary large object that can hold a variable amount of data. There are four types of BLOB –
TINYBLOB
BLOB
MEDIUMBLOB and
LONGBLOB
They all differ only in the maximum length of the values they can hold.
A TEXT is a case-insensitive BLOB. The four TEXT types
TINYTEXT
TEXT
MEDIUMTEXT and
LONGTEXT
They all correspond to the four BLOB types and have the same maximum lengths and storage requirements.
The only difference between BLOB and TEXT types is that sorting and comparison is performed in case-sensitive for BLOB values and case-insensitive for TEXT values.
35. What is the difference between mysql_fetch_array and mysql_fetch_object?
Following are the differences between mysql_fetch_array and mysql_fetch_object:
mysql_fetch_array() -Returns a result row as an associated array or a regular array from database.
mysql_fetch_object –  Returns a result row as object from database.
36. How can we run batch mode in mysql?
Following commands are used to run in batch mode:
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mysql ;
mysql  mysql.out
37. Where MyISAM table will be stored and also give their formats of storage?
Each MyISAM table is stored on disk in three formats:
The ‘.frm’ file stores the table definition
The data file has a ‘.MYD’ (MYData) extension
The index file has a ‘.MYI’ (MYIndex) extension
38. What are the different tables present in MySQL?
Total 5 types of tables are present:
MyISAM
Heap
Merge
INNO DB
ISAM
MyISAM is the default storage engine as of MySQL .
39. What is ISAM?
ISAM  is abbreviated as Indexed Sequential Access Method.It was developed by IBM to store and retrieve data on secondary storage systems like tapes.
 40. What is InnoDB?
lnnoDB is a transaction safe storage engine developed by Innobase Oy which is a Oracle Corporation now.
41. How MySQL Optimizes DISTINCT?
DISTINCT is converted to a GROUP BY on all columns and it will be combined with ORDER BY clause.
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SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;
42. How to enter Characters as HEX Numbers?
If you want to enter characters as HEX numbers, you can enter HEX numbers with single quotes and a prefix of (X), or just prefix HEX numbers with (Ox).
A HEX number string will be automatically converted into a character string, if the expression context is a string.
43. How to display top 50 rows?
In MySql, top 50 rows are displayed by using this following query:
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2
SELECT * FROM
LIMIT 0,50
44. How many columns can be used for creating Index?
Maximum of 16 indexed columns can be created for any standard table.
45. What is the different between NOW() and CURRENT_DATE()?
NOW () command is used to show current year,month,date with hours,minutes and seconds.
CURRENT_DATE() shows current year,month and date only.
46. What are the objects can be created using CREATE statement?
Following objects are created using CREATE statement:
DATABASE
EVENT
FUNCTION
INDEX
PROCEDURE
TABLE
TRIGGER
USER
VIEW
47. How many TRIGGERS are allowed in MySql table?
SIX triggers are allowed in MySql table. They are as follows:
BEFORE INSERT
AFTER INSERT
BEFORE UPDATE
AFTER UPDATE
BEFORE DELETE and
AFTER DELETE
48. What are the nonstandard string types?
Following are Non-Standard string types:
TINYTEXT
TEXT
MEDIUMTEXT
LONGTEXT
49. What are all the Common SQL Function?
CONCAT(A, B) – Concatenates two string values to create a single string output. Often used to combine two or more fields into one single field.
FORMAT(X, D) – Formats the number X to D significant digits.
CURRDATE(), CURRTIME() – Returns the current date or time.
NOW() – Returns the current date and time as one value.
MONTH(), DAY(), YEAR(), WEEK(), WEEKDAY() – Extracts the given data from a date value.
HOUR(), MINUTE(), SECOND() – Extracts the given data from a time value.
DATEDIFF(A, B) – Determines the difference between two dates and it is commonly used to calculate age
SUBTIMES(A, B) – Determines the difference between two times.
FROMDAYS(INT) – Converts an integer number of days into a date value.
50. Explain Access Control Lists.
An ACL (Access Control List) is a list of permissions that is associated with an object. This list is the basis for MySQL server’s security model and it helps in 

troubleshooting problems like users not being able to connect.
MySQL keeps the ACLs (also called grant tables) cached in memory. When a user tries to authenticate or run a command, MySQL checks the authentication information and 

permissions against the ACLs, in a predetermined order.

Title: 1- OPTIMIZE YOUR CODE

Imagine if you have 100 records, 10 columns.Your working with a database of your clients details, and need to select their name, phone number and address. You’d write:

SELECT * FROM clients;
What’s up with that? Nothing. You’d then simply reference the columns you want later. Wrong!

That single line of code is highly inefficient. It’s fetching 1000 pieces of information from the database! However, if you wrote this:

SELECT name, phone, address FROM clients;
You’ve now cut down to only 300 calls. That’s a 700 saving. Probably not noticeable on a database that size, but you should always think about what your code is actually doing. That way, you can code efficiently in the first place, and make your app much more scalable.

Super Tip: In some cases, you can cut down to simply 3, by using a well thought out ‘WHERE’ clause.