Automatically reconnect to mysql database if connection is dropped

Automatically reconnect to mysql database if connection is dropped

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linux awk for occurance count

filename | cut -f24 --delimiter="\""  | awk -F"|" '{if (NF > 50) print NF}'

MySQL Basic Details

1 :: What Is MySQL?

MySQL is an open source database management system developed by MySQL AB, http://www.mysql.com.

2 :: What Is mSQL?

Mini SQL (mSQL) is a light weight relational database management system capable of providing rapid access to your data with very little overhead. mSQL is developed by Hughes Technologies Pty Ltd.

MySQL was started from mSQL and shared the same API.

3 :: What Is SQL?

SQL, SEQUEL (Structured English Query Language), is a language for RDBMS (Relational Database Management Systems). SQL was developed by IBM Corporation.

4 :: What Is Table?

A table is a data storage structure with rows and columns.

5 :: What Is Column?

A column defines one piece of data stored in all rows of the table. 

6 :: What Is Row?

A row is a unit of data with related data items stored as one item in one column in a table.

7 :: What Is Primary Key?

A primary key is a single column or multiple columns defined to have unique values that can be used as row identifications.

8 :: What Is Foreign Key?

A foreign key is a single column or multiple columns defined to have values that can be mapped to a primary key in another table.

9 :: What Is Index?

An index is a single column or multiple columns defined to have values pre-sorted to speed up data retrieval speed.

10 :: What Is View?

A view is a logical table defined by a query statement.

11 :: What Is Join?

Join is data retrieval operation that combines rows from multiple tables under certain matching conditions to form a single row.

12 :: What Is Union?

Join is data retrieval operation that combines multiple query outputs of the same structure into a single output.

13 :: What Is ISAM?

ISAM (Indexed Sequential Access Method) was developed by IBM to store and retrieve data on secondary storage systems like tapes.

14 :: What Is MyISAM?

MyISAM is a storage engine used as the default storage engine for MySQL database. MyISAM is based on the ISAM (Indexed Sequential Access Method) concept and offers fast data storage and retrieval. But it is not transaction safe.

15 :: What Is InnoDB?

InnoDB is a transaction safe storage engine developed by Innobase Oy (an Oracle company now).

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16 :: What Is BDB (BerkeleyDB)?

BDB (BerkeleyDB) is transaction safe storage engine originally developed at U.C. Berkeley. It is now developed by Sleepycat Software, Inc. (an Oracle company now).

17 :: What Is CSV?

CSV (Comma Separated Values) is a file format used to store database table contents, where one table row is stored as one line in the file, and each data field is separated with comma.

18 :: What Is Transaction?

A transaction is a logical unit of work requested by a user to be applied to the database objects. MySQL server introduces the transaction concept to allow users to group one or more SQL statements into a single transaction, so that the effects of all the SQL statements in a transaction can be either all committed (applied to the database) or all rolled back (undone from the database).

19 :: What Is Commit?

Commit is a way to terminate a transaction with all database changes to be saved permanently to the database server.

20 :: What Is Rollback?

Rollback is a way to terminate a transaction with all database changes not saving to the database server.

29 MySQL interview questions

1. How do you start and stop MySQL on Windows? - net start MySQL, net stop MySQL
2. How do you start MySQL on Linux? - /etc/init.d/mysql start
3. Explain the difference between mysql and mysqli interfaces in PHP? - mysqli is the object-oriented version of mysql library functions.
4. What’s the default port for MySQL Server? - 3306
5. 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 notee.
6. 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.
7. How do you change a password for an existing user via mysqladmin? - mysqladmin -u root -p password "newpassword"
8. Use mysqldump to create a copy of the database? - mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql
9. Have you ever used MySQL Administrator and MySQL Query Browser? Describe the tasks you accomplished with these tools.
10. 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.
11. 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.
12. What does myisamchk do? - It compressed the MyISAM tables, which reduces their disk usage.
13. Explain advantages of InnoDB over MyISAM? - Row-level locking, transactions, foreign key constraints and crash recovery.
14. 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.
15. 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.
16. How do you control the max size of a HEAP table? - MySQL config variable max_heap_table_size.
17. What are CSV tables? - Those are the special tables, data for which is saved into comma-separated values files. They cannot be indexed.
18. Explain federated tables. - Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.
19. What is SERIAL data type in MySQL? - BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT
20. 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.
21. 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.
22. 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.
23. 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.
24. What happens if a table has one column defined as TIMESTAMP? - That field gets the current timestamp whenever the row gets altered.
25. 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.
26. 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.
27. 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.
28. 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.
29. 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.

Select Random Records in Database Table

In this tip, you will learn various techniques to select random items from a database table in MySQL.
MySQL does not have any automatic way to select random items from a database table. In some programming tasks, it is required and very useful to select random items from a result set such as:

• You'll need to select a random picture to display it for a category, so each time when the visitor come to your website, they will see different pictures displaying on the categories.
• You'll need to select a random banner to display to visitors.
• You'll need to pick a row in "quote of the day" application.
• And more and more cases you can think of...

In order to achieve this result, MySQL allows to do that via RAND function. To select a random value from a result set you perform the following query:

SELECT * FROM table  ORDER BY RAND()  LIMIT 1 

The key technique used above is randomizing the returned records and then pick the first one. If you want to select n random items just change the parameter after LIMIT as follows:

SELECT * FROM table  ORDER BY RAND()  LIMIT n

The technique as demonstrated above work very well with the small table. With the table which has many records, it could be very slow because we have to sort the entire table to pick random items. To work around this problem, we will use another technique as demonstrated below:

1. First we select a random ID(s) of a column. This column should be the primary key and the value is in sequential range.
2. Then pick the rows based on the ID(s) we selected

The SQL script for doing this is as follows:

SET @ID = FLOOR(RAND( )* N) + 1; 
 SELECT *  FROM table  WHERE ID >= @ID LIMIT 1  

This technique work faster because the ID column is indexed and we don't have to order the whole table as previous one.

MySQL Copy Database Table

In this tutorial, you will learn how to copy data from one table into a new table by using SQL CREATE TABLE and SELECT statement. Copying data from an existing table to a new one is useful in some cases such as backing up data, create a copying of real data for testing. In order to copy data from one table to a new one you can use the following command:

CREATE TABLE new_table 
 SELECT *  FROM existing_table 

MySQL will first create a new table with name as indicated after CREATE TABLE statement, new_table in this case. Then it will fill the new table with all the data from an existing table (existing_table).
To copy a part of data from an existing table, you can use WHERE clause to filter the selected data base on conditions. The command is as follows:

CREATE TABLE new_table 
SELECT *  FROM existing_table WHERE conditions

It is very important to check whether table you want to create is existed or not, you should use IF NOT EXIST after CREATE TABLE statement. The full sql command of copying data from an existing table to a new one will be as follows:

CREATE TABLE IF NOT EXISTS new_table 
 SELECT *  FROM existing_table 
 WHERE conditions 

Here is the example of using copying data command. We have Office data table, now we can copy the table from this table into a new one by using the following command:

CREATE TABLE IF NOT EXISTS offices_bk 
 SELECT * FROM offices 

If we need only copy all offices in US, so we can use WHERE condition for it as follows:

CREATE TABLE IF NOT EXISTS offices_usa 
 SELECT *  FROM offices 
 WHERE country = 'USA'