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MySQL Interview Questions – Part 3

67) Explain Database Basics
Databases are managed by a relational database management system (RDBMS). An RDBMS supports a database language to create and delete databases and to manage and search data. The database language used in almost all DBMSs is SQL, a set of statements that define and manipulate data. After creating a database, the most common SQL statements used are INSERT, UPDATE, DELETE, and SELECT, which add, change, remove, and search data in a database, respectively.

68) what is Database?
A repository to store data.

69) What is Table?
The part of a database that stores the data. A table has columns or attributes, and the data stored in rows.

70) Define Attributes?
The columns in a table. All rows in table entities have the same attributes. For example, a customer table might have the attributes name, address, and city. Each attribute has a data type such as string, integer, or date.

71) What are Rows?
The data entries in a table. Rows contain values for each attribute. For example, a row in a customer table might contain the values “Matthew Richardson,” “Punt Road,” and “Richmond.” Rows are also known as records.

72) Explain Relational model?
A model that uses tables to store data and manage the relationship between tables.

73) Define Relational database management system?
A software system that manages data in a database and is based on the relational model. DBMSs have several components described in detail in Chapter 1.

74) What is SQL?
A query language that interacts with a DBMS. SQL is a set of statements to manage databases, tables, and data.

75) Define Constraints?
Restrictions or limitations on tables and attributes. For example, a wine can be produced only by one winery, an order for wine can’t exist if it isn’t associated with a customer, having a name attribute could be mandatory for a customer.

76) What is Primary key?
One or more attributes that contain values that uniquely identify each row. For example, a customer table might have the primary key of cust ID. The cust ID attribute is then assigned a unique value for each customer. A primary key is a constraint of most tables.

77) Define Index?
A data structure used for fast access to rows in a table. An index is usually built for the primary key of each table and can then be used to quickly find a particular row. Indexes are also defined and built for other attributes when those attributes are frequently used in queries.

78) Define Entity-relationship modeling?
A technique used to describe the real-world data in terms of entities, attributes, and relationships.

79) What is Normalized database?
A correctly designed database that is created from an ER model. There are different types or levels of normalization, and a third-normal form database is generally regarded as being an acceptably designed relational database.

MySQL doesn’t yet support the Oracle SQL extension: SELECT … INTO TABLE …. MySQL supports instead the ANSI SQL syntax INSERT INTO … SELECT …, which is basically the same thing.

Alternatively, you can use SELECT INTO OUTFILE… or CREATE TABLE … SELECT to solve your problem.

81) MySQL – Stored Procedures and Triggers
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 reissuing the entire query but can refer to the stored procedure. This provides better 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.
A trigger is a stored procedure 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 automatically deletes the corresponding customer from a customer table when all his transactions are deleted.
The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.

82) MySQL – What the Privilege System Does
The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as select, insert, update and delete.
Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

83) MySQL User Names and Passwords
There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows:
User names, as used by MySQL for authentication purposes, have nothing to do with Unix user names (login names) or Windows user names. Most MySQL clients by default try to log in using the current Unix user name as the MySQL user name, but that is for convenience only. Client programs allow a different name to be specified with the -u or –user options. This means that you can’t make a database secure in any way unless all MySQL user names have passwords. Anyone may attempt to connect to the server using any name, and they will succeed if they specify any name that doesn’t have a password. MySQL user names can be up to 16 characters long; Unix user names typically are limited to 8 characters. MySQL passwords have nothing to do with Unix passwords. There is no necessary connection between the password you use to log in to a Unix machine and the password you use to access a database on that machine. MySQL encrypts passwords using a different algorithm than the one used during the Unix login process.
Note that even if the password is stored ‘scrambled’, and knowing your ‘scrambled’ password is enough to be able to connect to the MySQL server!

84) MySQL – When Privilege Changes Take Effect
When mysqld starts, all grant table contents are read into memory and become effective at that point.
Modifications to the grant tables that you perform using GRANT, REVOKE, or SET PASSWORD are noticed by the server immediately.

If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges or mysqladmin reload to tell the server to reload the grant tables. Otherwise your changes will have no effect until you restart the server. If you change the grant tables manually but forget to reload the privileges, you will be wondering why your changes don’t seem to make any difference!

When the server notices that the grant tables have been changed, existing client connections are affected as follows:

Table and column privilege changes take effect with the client’s next request.
Database privilege changes take effect at the next USE db_name command.
Global privilege changes and password changes take effect the next time the client connects.

85) How do I rotate replication logs?
In Version 3.23.28 you should use PURGE MASTER LOGS TO command after determining which logs can be deleted, and optionally backing them up first. In earlier versions the process is much more painful, and cannot be safely done without stopping all the slaves in the case that you plan to re-use log names. You will need to stop the slave threads, edit the binary log index file, delete all the old logs, restart the master, start slave threads,and then remove the old log files.

86) How do I upgrade on a hot replication setup?
If you are upgrading pre-3.23.26 versions, you should just lock the master tables, let the slave catch up, then run FLUSH MASTER on the master, and FLUSH SLAVE on the slave to reset the logs, then restart new versions of the master and the slave. Note that the slave can stay down for some time – since the master is logging all the updates, the slave will be able to catch up once it is up and can connect.
After 3.23.26, we have locked the replication protocol for modifications, so you can upgrade masters and slave on the fly to a newer 3.23 version and you can have different versions of MySQL running on the slave and the master, as long as they are both newer than 3.23.26.

87) How can I use replication to improve performance of my system?
You should set up one server as the master, and direct all writes to it, and configure as many slaves as you have the money and rackspace for, distributing the reads among the master and the slaves. You can also start the slaves with –skip-bdb, –low-priority-updates and –delay-key-write-for-all-tables to get speed improvements for the slave. In this case the slave will use non-transactional MyISAM tables instead of BDB tables to get more speed.

88) MySQL – Drawbacks to Creating Large Numbers of Tables in the Same Database
If you have many files in a directory, open, close, and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.

89) MySQL – Why So Many Open tables?
When you run mysqladmin status, you’ll see something like this:

Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12

This can be somewhat perplexing if you only have 6 tables.

MySQL is multithreaded, so it may have many queries on the same table simultaneously. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads.

90) How MySQL Optimizes DISTINCT ?
DISTINCT is converted to a GROUP BY on all columns, DISTINCT combined with ORDER BY will in many cases also need a temporary table.

When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.

If you don’t use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match.

SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;

In the case, assuming t1 is used before t2 (check with EXPLAIN), then MySQL will stop reading from t2 (for that particular row in t1) when the first row in t2 is found.

91) How MySQL Optimizes LIMIT ?
In some cases MySQL will handle the query differently when you are using LIMIT # and not using HAVING:

If you are selecting only a few rows with LIMIT, MySQL will use indexes in some cases when it normally would prefer to do a full table scan.
If you use LIMIT # with ORDER BY, MySQL will end the sorting as soon as it has found the first # lines instead of sorting the whole table.
When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.
In some cases a GROUP BY can be resolved by reading the key in order (or do a sort on the key) and then calculate summaries until the key value changes. In this case LIMIT # will not calculate any unnecessary GROUP BY’s.
As soon as MySQL has sent the first # rows to the client, it will abort the query.
LIMIT 0 will always quickly return an empty set. This is useful to check the query and to get the column types of the result columns.
The size of temporary tables uses the LIMIT # to calculate how much space is needed to resolve the query.

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