My SQL Interview Questions and Answers

Answer - 41 : - A default value is used on initialization, a current timestamp is inserted on update of the row.

Answer - 42 : - 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/.

Answer - 43 : - 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. Database A repository to store data. Table The part of a database that stores the data. A table has columns or attributes, and the data stored in rows. 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. 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. Relational model A model that uses tables to store data and manage the relationship between tables. 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. SQL A query language that interacts with a DBMS. SQL is a set of statements to manage databases, tables, and data. 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. 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. Index A data structure

Answer - 44 : - 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.

Answer - 45 : - The MySQL command interpreter is commonly used to create databases and tables in web database applications and to test queries. Throughout the remainder of this chapter we discuss the SQL statements for managing a database. All these statements can be directly entered into the command interpreter and executed. The statements can also be included in server-side PHP scripts, as discussed in later chapters. Once the MySQL DBMS server is running, the command interpreter can be used. The command interpreter can be run using the following command from the shell, assuming you've created a user hugh with a password shhh: % /usr/local/bin/mysql -uhugh -pshhh The shell prompt is represented here as a percentage character, %. Running the command interpreter displays the output: Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 36 to server version: 3.22.38 Type 'help' for help. mysql> The command interpreter displays a mysql> prompt and, after executing any command or statement, it redisplays the prompt. For example, you might issue the statement: mysql> SELECT NOW( ); This statement reports the time and date by producing the following output: +---------------------+ | NOW( ) | +---------------------+ | 2002-01-01 13:48:07 | +---------------------+ 1 row in set (0.00 sec) mysql> After running a statement, the interpreter redisplays the mysql> prompt. We discuss the SELECT statement later in this chapter. As with all other SQL statements, the SELECT statement ends in a semicolon. Almost all SQL command interpreters permit any amount of whitespace—spaces, tabs, or carriage returns—in SQL statements, and they check syntax and execute statements only after encountering a semicolon that is followed by a press of the Enter key. We have used uppercase for the SQL statements throughout this book. However, any mix of upper- and lowercase is equivalent. On startup, the command interpreter encourages the use of the help command. Typing help produces a list of com

Answer - 46 : - MySQL binary distributions are provided as compressed tar archives and have names like `mysql-VERSION-OS.tar.gz', where VERSION is a number (for example, 3.21.15), and OS indicates the type of operating system for which the distribution is intended (for example, pc-linux-gnu-i586). Add a user and group for mysqld to run as: shell> groupadd mysql shell> useradd -g mysql mysql These commands add the mysql group and the mysql user. The syntax for useradd and groupadd may differ slightly on different Unixes. They may also be called adduser and addgroup. You may wish to call the user and group something else instead of mysql. Change into the intended installation directory: shell> cd /usr/local> Unpack the distribution and create the installation directory: shell> gunzip < /path/to/mysql-VERSION-OS.tar.gz | tar xvf - shell> ln -s mysql-VERSION-OS mysql The first command creates a directory named `mysql-VERSION-OS'. The second command makes a symbolic link to that directory. This lets you refer more easily to the installation directory as `/usr/local/mysql'. Change into the installation directory: shell> cd mysql You will find several files and subdirectories in the mysql directory. The most important for installation purposes are the `bin' and `scripts' subdirectories. `bin' This directory contains client programs and the server You should add the full pathname of this directory to your PATH environment variable so that your shell finds the MySQL programs properly. `scripts' This directory contains the mysql_install_db script used to initialize the server access permissions. If you would like to use mysqlaccess and have the MySQL distribution in some nonstandard place, you must change the location where mysqlaccess expects to find the mysql client. Edit the `bin/mysqlaccess' script at approximately line 18. Search for a line that looks like this: $MYSQL = '/usr/local/bin/mysql'; # path to mysql executable Change the path to reflect the location where mysql actually is stored on your system. If you do not do this, you will get a Broken pipe error when you run mysqlaccess. Create the MySQL grant tables (necessary only if you haven't installed MySQL before): shell>

Answer - 47 : - MySQL includes some extensions that you probably will not find in other SQL databases. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the form /*! ... */. In this case, MySQL will parse and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example: SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ... If you add a version number after the '!', the syntax will only be executed if the MySQL version is equal to or newer than the used version number: CREATE /*!32302 TEMPORARY */ TABLE (a int); The above means that if you have Version 3.23.02 or newer, then MySQL will use the TEMPORARY keyword. MySQL extensions are listed below: The field types MEDIUMINT, SET, ENUM, and the different BLOB and TEXT types. The field attributes AUTO_INCREMENT, BINARY, NULL, UNSIGNED, and ZEROFILL. All string comparisons are case insensitive by default, with sort ordering determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, you should declare your columns with the BINARY attribute or use the BINARY cast, which causes comparisons to be done according to the ASCII order used on the MySQL server host. MySQL maps each database to a directory under the MySQL data directory, and tables within a database to filenames in the database directory. This has a few implications: Database names and table names are case sensitive in MySQL on operating systems that have case-sensitive filenames (like most Unix systems). Database, table, index, column, or alias names may begin with a digit (but may not consist solely of digits). You can use standard system commands to backup, rename, move, delete, and copy tables. For example, to rename a table, rename the `.MYD', `.MYI', and `.frm' files to which the table corresponds. In SQL statements, you can access tables from different databases with the db_name.tbl_name syntax. Some SQL servers provide the same functionality but call this User space. MySQL doesn't support tablespaces as in: create table ral

Answer - 48 : - The following functionality is missing in the current version of MySQL. For a prioritized list indicating when new extensions may be added to MySQL, you should consult the online MySQL TODO list. That is the latest version of the TODO list in this manual. The following functionality is missing in the current version of MySQL. For a prioritized list indicating when new extensions may be added to MySQL, you should consult the online MySQL TODO list. That is the latest version of the TODO list in this manual. MySQL - Sub-selects The following will not yet work in MySQL: SELECT * FROM table1 WHERE id IN (SELECT id FROM table2); SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2); SELECT * FROM table1 WHERE NOT EXISTS (SELECT id FROM table2 where table1.id=table2.id); However, in many cases you can rewrite the query without a sub-select: SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id; SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL For more complicated subqueries you can often create temporary tables to hold the subquery. In some cases, however this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in sub-selects). For this situation there are two options available until subqueries are supported by MySQL. The first option is to use a procedural programming language (such as Perl or PHP) to submit a SELECT query to obtain the primary keys for the records to be deleted, and then use these values to construct the DELETE statement (DELETE FROM ... WHERE ... IN (key1, key2, ...)). The second option is to use interactive SQL to contruct a set of DELETE statements automatically, using the MySQL extension CONCAT() (in lieu of the standard || operator). For example: SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', tab1.pkid, ';') FROM tab1, tab2 WHERE tab1.col1 = tab2.col2; You can place this query in a script file and redirect input from it to the mysql command-line interpreter, piping its output back to a second instance of the interpreter: prompt> mysql --skip-column-names mydb > myscript.sql | mysql mydb MySQL only support

Answer - 49 : - There are so many problems with foreign key constraints that we don't know where to start: Foreign key constraints make life very complicated, because the foreign key definitions must be stored in a database and implementing them would destroy the whole ``nice approach'' of using files that can be moved, copied, and removed. The speed impact is terrible for INSERT and UPDATE statements, and in this case almost all FOREIGN KEY constraint checks are useless because you usually insert records in the right tables in the right order, anyway. There is also a need to hold locks on many more tables when updating one table, because the side effects can cascade through the entire database. It's MUCH faster to delete records from one table first and subsequently delete them from the other tables. You can no longer restore a table by doing a full delete from the table and then restoring all records (from a new source or from a backup). If you use foreign key constraints you can't dump and restore tables unless you do so in a very specific order. It's very easy to do ``allowed'' circular definitions that make the tables impossible to re-create each table with a single create statement, even if the definition works and is usable. It's very easy to overlook FOREIGN KEY ... ON DELETE rules when one codes an application. It's not unusual that one loses a lot of important information just because a wrong or misused ON DELETE rule. The only nice aspect of FOREIGN KEY is that it gives ODBC and some other client programs the ability to see how a table is connected and to use this to show connection diagrams and to help in building applicatons. MySQL will soon store FOREIGN KEY definitions so that a client can ask for and receive an answer about how the original connection was made. The current `.frm' file format does not have any place for it. At a later stage we will implement the foreign key constraints for application that can't easily be coded to avoid them.

Answer - 50 : - MySQL doesn't support views. Some other SQL databases use `--' to start comments. MySQL has `#' as the start comment character, even if the mysql command-line tool removes all lines that start with `--'. You can also use the C comment style /* this is a comment */ with MySQL. MySQL Version 3.23.3 and above supports the `--' comment style only if the comment is followed by a space. This is because this degenerate comment style has caused many problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!: UPDATE tbl_name SET credit=credit-!payment! What do you think will happen when the value of payment is negative? Because 1--1 is legal in SQL, we think it is terrible that `--' means start comment. In MySQL Version 3.23 you can, however, use: 1-- This is a comment The following discussion only concerns you if you are running a MySQL version earlier than Version 3.23: If you have a SQL program in a text file that contains `--' comments you should use: shell> replace " --" " #" < text-file-with-funny-comments.sql \ | mysql database instead of the usual: shell> mysql database < text-file-with-funny-comments.sql You can also edit the command file ``in place'' to change the `--' comments to `#' comments: shell> replace " --" " #" -- text-file-with-funny-comments.sql Change them back with this command: shell> replace " #" " --" -- text-file-with-funny-comments.sql