MySQL面试题（English）

1.What are the differences between Get and post methods in form submitting. give the case where we can use get and we can use post methods?

GET and POST basically allow information to be sent back to the webserver from a browser (or other HTTP client for that matter).

for form Submitt

Choosing GET as the "method" will append all of the data to the URL and it will show up in the URL bar of your browser. The amount of information you can send back using a GET is restricted as URLs can only be 1024 characters.

A POST on the other hand will (typically) send the information through a socket back to the webserver and it won't show up in the URL bar. You can send much more information to the server this way - and it's not restricted to textual data either. It is possible to send files and even binary data such as serialized Java objects!

• If the method is "get" - -, the user agent takes the value of action, appends a ? to it, then appends the form data set, encoded using the application/x-www-form-urlencoded content type. The user agent then traverses the link to this URI. In this scenario, form data are restricted to ASCII codes.
• If the method is "post" --, the user agent conducts an HTTP post transaction using the value of the action attribute and a message created according to the content type specified by the enctype attribute.

2. In how many ways we can retrieve the data in the result set of MySQL using PHP?

• 1. mysql_fetch_row.
• 2. mysql_fetch_array
• 3. mysql_fetch_object
• 4. mysql_fetch_assoc

3.  What is the difference between mysql_fetch_object and mysql_fetch_array?
mysql_fetch_object() is similar tomysql_fetch_array(), with one difference -an object is returned, instead of an array. Indirectly, that means that
you can only access the data by the field names, and not by their offsets (numbers are illegal property names).

4. How can we create a database using PHP and MySQL?
mysql_create_db(“Database Name”)

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

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

7.Explain about Stored Procedure?

A stored procedure is a named group of SQL statements that have been previously created and stored in the server database. Stored procedures accept input parameters so that a single procedure can be used over the network by several clients using different input data. And when the procedure is modified, all clients automatically get the new version. Stored procedures reduce network traffic and improve performance. Stored procedures can be used to help ensure the integrity of the database. e.g. sp_helpdb, sp_renamedb, sp_depends etc.

8.Explain about Trigger?

A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE) occurs. Triggers are stored in and managed by the DBMS. Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed; DBMS automatically fires the trigger as a result of a data modification to the associated table. Triggers can be viewed as similar to stored procedures in that both consist of procedural logic that is stored at the database level. Stored procedures, however, are not event-drive and are not attached to a specific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to the procedure while triggers are implicitly executed. In addition, triggers can also execute stored procedures. Nested Trigger: A trigger can also contain INSERT, UPDATE and DELETE logic within itself, so when the trigger is fired because of data modification it can also cause another data modification, thereby firing another trigger. A trigger that contains data modification logic within itself is called a nested trigger.

9.Explain about View?

A simple view can be thought of as a subset of a table. It can be used for retrieving data, as well as updating or deleting rows. Rows updated or deleted in the view are updated or deleted in the table the view was created with. It should also be noted that as data in the original table changes, so does data in the view, as views are the way to look at part of the original table. The results of using a view are not permanently stored in the database. The data accessed through a view is actually constructed using standard T-SQL select command and can come from one to many different base tables or even other views.

10.Explain about Index?

An index is a physical structure containing pointers to the data. Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes; they are just used to speed up queries. Effective indexes are one of the best ways to improve performance in a database application. A table scan happens when there is no index available to help a query. In a table scan SQL Server examines every row in the table to satisfy the query results. Table scans are sometimes unavoidable, but on large tables, scans have a terrific impact on performance.

11.Explain about a Linked Server?

Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and query both the SQL Server dbs using T-SQL Statements. With a linked server, you can create very clean, easy to follow, SQL statements that allow remote data to be retrieved, joined and combined with local data. Stored Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new Linked Server.

12.Explain about Cursor?

Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis, instead of the typical SQL commands that operate on all the rows in the set at one time. In order to work with a cursor we need to perform some steps in the following order: • Declare cursor • Open cursor • Fetch row from the cursor • Process fetched row • Close cursor • Deallocate cursor

13.Explain about Collation?

Collation refers to a set of rules that determine how data is sorted and compared. Character data is sorted using rules that define the correct character sequence, with options for specifying case sensitivity, accent marks, kana character types and character width.

14.Explain about Difference between Function and Stored Procedure?

UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section where as Stored procedures cannot be. UDFs that return tables can be treated as another rowset. This can be used in JOINs with other tables. Inline UDF’s can be thought of as views that take parameters and can be used in JOINs and other Rowset operations.

15. What is normalization? What are different type of normalization?

There is set of rules that has been established to aid in the design of tables that are meant to be connected through relationships. This set of rules is known as Normalization.  Benefits of Normalizing your database include:

• =>Avoiding repetitive entries
• =>Reducing required storage space
• =>Preventing the need to restructure existing tables to accommodate new data.
• =>Increaed speed and flexibility of queries, sorts, and summaries.

Following are the three normal forms :-
First Normal Form
For a table to be in first normal form, data must be broken up into the smallest un possible.In addition to breaking data up into the smallest meaningful values, tables first normal form should not contain repetitions groups of fields.
Second Normal form
The second normal form states that each field in a multiple field primary keytable must be directly related to the entire primary key. Or in other words,each non-key field should be a fact about all the fields in the primary key.
Third normal form
A non-key field should not depend on other Non-key field.

16. we have an employee salary table how do we find the second highest from it?
SELECT * FROM pcdsEmployeeSalary a WHERE (2=(SELECT COUNT(DISTINCT(b.salary)) FROM pcdsEmployeeSalary b WHERE b.salary>=a.salary))
    
17. What are different types of joins in SQL? 
INNER JOIN
Inner join shows matches only when they exist in both tables. Example in the below SQL there are two tables Customers and Orders and the inner join in made on Customers.Customerid and Orders.Customerid. So this SQL will only give you result with customers who have orders. If the customer does not have order it will not display that record.
SELECT Customers.*, Orders.* FROM Customers INNER JOIN Orders ON Customers.CustomerID =Orders.CustomerID
LEFT OUTER JOIN
Left join will display all records in left table of the SQL statement. In SQL below customers with or without orders will be displayed. Order data for customers without orders appears as NULL values. For example, you want to determine the amount ordered by each customer and you need to see who has not ordered anything as well. You can also see the LEFT OUTER JOIN as a mirror image of the RIGHT OUTER JOIN (Is covered in the next section) if you switch the side of each table.
SELECT Customers.*, Orders.* FROM Customers LEFT OUTER JOIN Orders ON Customers.CustomerID =Orders.CustomerID
RIGHT OUTER JOIN
Right join will display all records in right table of the SQL statement. In SQL below all orders with or without matching customer records will be displayed. Customer data for orders without customers appears as NULL values. For example, you want to determine if there are any orders in the data with undefined CustomerID values (say, after a conversion or something like it). You can also see the RIGHT OUTER JOIN as a mirror image of the LEFT OUTER JOIN if you switch the side of each table.
SELECT Customers.*, Orders.* FROM Customers RIGHT OUTER JOIN Orders ON Customers.CustomerID =Orders.CustomerID
    
18.  What is “CROSS JOIN”? or What is Cartesian product?
“CROSS JOIN” or “CARTESIAN PRODUCT” combines all rows from both tables. Number of rows will be product of the number of rows in each table. In real life scenario I can not imagine where we will want to use a Cartesian product. But there are scenarios where we would like permutation and combination probably Cartesian would be the easiest way to achieve it.
    
19.how to select the first record in a given set of rows?
Select top 1 * from sales.salesperson

20. What is a View? 
View is a virtual table which is created on the basis of the result set returned by the select statement.
CREATE VIEW [MyView] AS SELECT * from pcdsEmployee where LastName = 'singh'
In order to query the view
SELECT * FROM [MyView]

21. How to get an accurate count of the number of records in a table?

SELECT * FROM table1 SELECT COUNT(*) FROM table1 SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2

22.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.

23. 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;

24. 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.

25. 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.

26. 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)

27. How would you select all the users, whose phone number is null?
SELECT user_name FROM users WHERE ISNULL(user_phonenumber);

28. 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

29.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.

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

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

参考资料：

• http://booleandreams.wordpress.com/php-mysql-interview-question/
• http://www.techinterviews.com/29-mysql-interview-questions
• http://techpreparation.com/mysql-interview-questions-answers1.htm
• http://www.calsql.com/2009/10/sql-server-database-design-interview.html?m=1
• http://www.techiebird.com/DBInterview1.html
• http://mcamady.blogspot.ca/2012/03/database-dbms-interview-questions-and.html?m=1
• http://iforinterview.blogspot.ca/2010/12/frequently-asked-mysql-interview.html?m=1
• http://rajeshstutorials.blogspot.ca/?m=0