Mysql Interview Questions

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.

MySql is written in C and C++ programming and SQL parser written in yacc..

MySql default port number is 3306

/etc/init.d/mysql start command is used for start MySQL on Linux

mysqli is the object-oriented version of mysql library functions.

Tee followed by a filename turns on MySQL logging to a specified file. It can be stopped by command note.

mysqladmin -u root -p password “newpassword”

mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql

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.

mysqlcheck is a client program that check the integrity of database tables.

mysqldump is a client program that creates logical backups of database.

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.

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.

Show Databases; command is used to list all Mysql Databases.

“i_am_a_dump” enables MySQL engine to refuse any UPDATE or DELETE statement to execute if the WHERE clause is not present.

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.

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.

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.

Advantages of InnoDB over MyISAM

• Row-level locking
• transactions
• foreign key constraints
• crash recovery

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.

MySQL set can take zero or more values but at the maximum it can take 64 values.

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.

Run below command on the terminal to import a MySQL database from the file

mysql -u username -p database_name < file.sql 

If you have to store the short piece of data like password hashes then it is advised to store it in VARCHAR.

If you need to store arbitrary chunks of binary data then BLOB is the desired product to store the data.

Both GROUP and ORDER BY clauses are used for organizing data in MYSQL.

• ORDER BY clause is used to sort the query result by specific columns.
• GROUP BY clause is used to summarize unique combinations of columns values.

You can use subquery and IN clause to find the Second highest salary from a table in MySQL

SELECT max(salary) FROM Employee WHERE salary NOT IN (SELECT max(salary) FROM Employee);

A trigger can be defined as that stored procedure that executes when some particular event happens in SQL such as update, insert, delete, etc. On the other hand, a stored procedure is a piece of code purely defined by the user and is written in several local versions of PL or SQL. A stored procedure may return a value if it is made a function and can be invoked by calling it explicitly.

Inner Join: Inner Join can also be called the simple join. It allows the coder to filter the outcomes of the cartesian product by some predicate.

Self Join: It is used to join one single table with itself as there were two different tables.

Cross Join: This is the basic join, it is nothing but a cartesian product. This join method compares every single row of a table with every single row of the other table.

Major differences between INNODB and MYISAM are

Different types of data storage engines available in Mysql are:

• MyISAM
• InnoDB
• MERGE
• MEMORY (HEAP)
• ARCHIVE
• CSV
• FEDERATED

Indexes can be defined as a process used to speed up the retrieval of data. These are a special kind of lookup tables that the search engine of the database can be used for the purpose of improving the speed of the data. It can be done by putting a pointer named Index to data in a table. It works similar to that of the index of any book.

Difference between clustered index and non clustered index

• Cluster index is an index type that is used to sort table data rows on the basis of their key values. In RDBMS primary key allows us to create a clustered index based on that specific column.
• A non-clustered index (or regular b-tree index) is an index where the order of the rows does not match the physical order of the actual data. It is instead ordered by the columns that make up the index.

In MySQL, a candidate key can be any column or a combination of columns that can qualify as a unique key in the database. There can be multiple candidate keys in one table. Each candidate key can qualify as the primary key. Candidate keys can take null values whereas primary keys can never be null.