DB & MySQL Interview Questions
Q: What is database?
A: A database is a logically coherent collection of data with some inherent meaning, representing some aspect of real world and which is designed, built and populated with data for a specific purpose.
Q: What is DBMS?
A: It is a collection of programs that enables user to create and maintain a database. In other words it is general-purpose software that provides the users with the processes of defining, constructing and manipulating the database for various applications.
Q: What is a Database system?
A: The database and DBMS software together is called as Database system.
Q: What are the advantages of DBMS?
A: Redundancy is controlled.
Unauthorised access is restricted.
Providing multiple user interfaces.
Enforcing integrity constraints.
Providing backup and recovery.
Q: What are the disadvantage in File Processing System?
A: Data redundancy and inconsistency.
Difficult in accessing data.
Data isolation.
Data integrity.
Concurrent access is not possible.
Security Problems.
Q: Describe the three levels of data abstraction?
A: The are three levels of abstraction:
Physical level: The lowest level of abstraction describes how data are stored.
Logical level: The next higher level of abstraction, describes what data are stored in database and what relationship among those data.
View level: The highest level of abstraction describes only part of entire database.
Q: Define the "integrity rules"?
A: There are two Integrity rules.
Entity Integrity: States that "Primary key cannot have NULL value"
Referential Integrity: States that "Foreign Key can be either a NULL value or should be Primary Key value of other relation.
Q: What is extension and intension?
A: Extension: It is the number of tuples present in a table at any instance. This is time dependent.
Intension: It is a constant value that gives the name, structure of table and the constraints laid on it.
Q: What is System R? What are its two major subsystems?
A: System R was designed and developed over a period of 1974-Q: At IBM San Jose Research Center. It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with performance at least comparable to that of existing system.
Its two subsystems are
Research Storage
System Relational Data System.
Q: How is the data structure of System R different from the relational structure?
A: Unlike Relational systems in System R
Domains are not supported
Enforcement of candidate key uniqueness is optional
Enforcement of entity integrity is optional
Referential integrity is not enforced
Q: What is Data Independence?
A: Data independence means that "the application is independent of the storage structure and access strategy of data". In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.
Two types of Data Independence:
Physical Data Independence: Modification in physical level should not affect the logical level.
Logical Data Independence: Modification in logical level should affect the view level.
NOTE: Logical Data Independence is more difficult to achieve
Q: What is a view? How it is related to data independence?
A: A view may be thought of as a virtual table, that is, a table that does not really exist in its own right but is instead derived from one or more underlying base table. In other words, there is no stored file that direct represents the view instead a definition of view is stored in data dictionary.
Growth and restructuring of base tables is not reflected in views. Thus the view can insulate users from the effects of restructuring and growth in the database. Hence accounts for logical data independence.
Q: What is Data Model?
A: A collection of conceptual tools for describing data, data relationships data semantics and constraints.
Q: What is E-R model?
A: This data model is based on real world that consists of basic objects called entities and of relationship among these objects. Entities are described in a database by a set of attributes.
Q: What is Object Oriented model?
A: This model is based on collection of objects. An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain same types of values and the same methods are grouped together into classes.
Q: What is an Entity?
A: It is a 'thing' in the real world with an independent existence.
Q: What is an Entity type?
A: It is a collection (set) of entities that have same attributes.
Q: What is an Entity set?
A: It is a collection of all entities of particular entity type in the database.
Q: What is an Extension of entity type?
A: The collections of entities of a particular entity type are grouped together into an entity set.
Q: What is Weak Entity set?
A: An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set.
Q: What is an attribute?
A: It is a particular property, which describes the entity.
Q: What is a Relation Schema and a Relation?
A: A relation Schema denoted by R(AQ: A2, ..., An) is made up of the relation name R and the list of attributes Ai that it contains. A relation is defined as a set of tuples. Let r be the relation which contains set tuples (t1, t2, t3, ..., tn). Each tuple is an ordered list of n-values t=(v1,v2, ..., vn).
Q: What is degree of a Relation?
A: It is the number of attribute of its relation schema.
Q: What is Relationship?
A: It is an association among two or more entities.
Q: What is Relationship set?
A: The collection (or set) of similar relationships.
Q: What is Relationship type?
A: Relationship type defines a set of associations or a relationship set among a given set of entity types.
Q: What is degree of Relationship type?
A: It is the number of entity type participating.
Q: What is DDL (Data Definition Language)?
A: A data base schema is specifies by a set of definitions expressed by a special language called DDL.
Q: What is VDL (View Definition Language)?
A: It specifies user views and their mappings to the conceptual schema.
Q: What is SDL (Storage Definition Language)?
A: This language is to specify the internal schema. This language may specify the mapping between two schemas.
Q: What is Relationship set?
A: The collection (or set) of similar relationships.
Q: What is Relationship type?
A: Relationship type defines a set of associations or a relationship set among a given set of entity types.
Q: What is degree of Relationship type?
A: It is the number of entity type participating.
Q: What is DDL (Data Definition Language)?
A: A data base schema is specifies by a set of definitions expressed by a special language called DDL.
Q: What is VDL (View Definition Language)?
A: It specifies user views and their mappings to the conceptual schema.
Q: What is SDL (Storage Definition Language)?
A: This language is to specify the internal schema. This language may specify the mapping between two schemas.
Q: What is Data Storage - Definition Language?
A: The storage structures and access methods used by database system are specified by a set of definition in a special type of DDL called data storage-definition language.
Q: What is DML (Data Manipulation Language)?
A: This language that enable user to access or manipulate data as organised by appropriate data model.
Procedural DML or Low level: DML requires a user to specify what data are needed and how to get those data.
Non-Procedural DML or High level: DML requires a user to specify what data are needed without specifying how to get those data.
Q: What is DML Compiler?
A: It translates DML statements in a query language into low-level instruction that the query evaluation engine can understand.
Q: What is Query evaluation engine?
A: It executes low-level instruction generated by compiler.
Q: What is DDL Interpreter?
A: It interprets DDL statements and record them in tables containing metadata.
Q: What is Record-at-a-time?
A: The Low level or Procedural DML can specify and retrieve each record from a set of records. This retrieve of a record is said to be Record-at-a-time.
Q: What is Set-at-a-time or Set-oriented?
A: The High level or Non-procedural DML can specify and retrieve many records in a single DML statement. This retrieve of a record is said to be Set-at-a-time or Set-oriented.
Q: What is Relational Algebra?
A: It is procedural query language. It consists of a set of operations that take one or two relations as input and produce a new relation.
Q: What is Relational Calculus?
A: It is an applied predicate calculus specifically tailored for relational databases proposed by E.F. Codd. E.g. of languages based on it are DSL ALPHA, QUEL.
Q: How does Tuple-oriented relational calculus differ from domain-oriented relational calculus?
A: The tuple-oriented calculus uses a tuple variables i.e., variable whose only permitted values are tuples of that relation. E.g. QUEL
The domain-oriented calculus has domain variables i.e., variables that range over the underlying domains instead of over relation. E.g. ILL, DEDUCE.
Q: What is normalization?
A: It is a process of analysing the given relation schemas based on their Functional Dependencies (FDs) and primary key to achieve the properties
(1).Minimizing redundancy, (2). Minimizing insertion, deletion and update anomalies.
Q: What is Functional Dependency?
A: A Functional dependency is denoted by X Y between two sets of attributes X and Y that are subsets of R specifies a constraint on the possible tuple that can form a relation state r of R. The constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X] then they have t1[Y] = t2[Y]. This means the value of X component of a tuple uniquely determines the value of component Y.
Q: What is Lossless join property?
A: It guarantees that the spurious tuple generation does not occur with respect to relation schemas after decomposition.
Q: What is 1 NF (Normal Form)?
A: The domain of attribute must include only atomic (simple, indivisible) values.
Q: What is Fully Functional dependency?
A: It is based on concept of full functional dependency. A functional dependency X Y is full functional dependency if removal of any attribute A from X means that the dependency does not hold any more.
Q: What is 2NF?
A: A relation schema R is in 2NF if it is in 1NF and every non-prime attribute A in R is fully functionally dependent on primary key.
Q: What is 3NF?
A: A relation schema R is in 3NF if it is in 2NF and for every FD X A either of the following is true
X is a Super-key of R.
A is a prime attribute of R.
In other words, if every non prime attribute is non-transitively dependent on primary key.
Q: What is BCNF (Boyce-Codd Normal Form)?
A: A relation schema R is in BCNF if it is in 3NF and satisfies an additional constraint that for every FD X A, X must be a candidate key.
Q: What is 4NF?
A: A relation schema R is said to be in 4NF if for every Multivalued dependency X Y that holds over R, one of following is true.
1.) X is subset or equal to (or) XY = R.
2.) X is a super key.
Q: What is 5NF?
A: A Relation schema R is said to be 5NF if for every join dependency {R1, R2, ..., Rn} that holds R, one the following is true
1.) Ri = R for some i.
2.) The join dependency is implied by the set of FD, over R in which the left side is key of R.
Q: What is Domain-Key Normal Form?
A: A relation is said to be in DKNF if all constraints and dependencies that should hold on the the constraint can be enforced by simply enforcing the domain constraint and key constraint on the relation.
Q: What are partial, alternate,, artificial, compound and natural key?
A: Partial Key: It is a set of attributes that can uniquely identify weak entities and that are related to same owner entity. It is sometime called as Discriminator.
Alternate Key: All Candidate Keys excluding the Primary Key are known as Alternate Keys.
Artificial Key: If no obvious key, either stand alone or compound is available, then the last resort is to simply create a key, by assigning a unique number to each record or occurrence. Then this is known as developing an artificial key.
Compound Key: If no single data element uniquely identifies occurrences within a construct, then combining multiple elements to create a unique identifier for the construct is known as creating a compound key.
Natural Key: When one of the data elements stored within a construct is utilized as the primary key, then it is called the natural key.
Q: What is indexing and what are the different kinds of indexing?
A: Indexing is a technique for determining how quickly specific data can be found.
Types:
Binary search style indexing
B-Tree indexing
Inverted list indexing
Memory resident table
Table indexing
Q: What is system catalog or catalog relation? How is better known as?
A: A RDBMS maintains a description of all the data that it contains, information about every relation and index that it contains. This information is stored in a collection of relations maintained by the system called metadata. It is also called data dictionary.
Q: What is meant by query optimization?
A: The phase that identifies an efficient execution plan for evaluating a query that has the least estimated cost is referred to as query optimization.
Q: What is durability in DBMS?
A: Once the DBMS informs the user that a transaction has successfully completed, its effects should persist even if the system crashes before all its changes are reflected on disk. This property is called durability.
Q: What do you mean by atomicity and aggregation?
A: Atomicity: Either all actions are carried out or none are. Users should not have to worry about the effect of incomplete transactions. DBMS ensures this by undoing the actions of incomplete transactions.
Aggregation: A concept which is used to model a relationship between a collection of entities and relationships. It is used when we need to express a relationship among relationships.
Q: What is a Phantom Deadlock?
A: In distributed deadlock detection, the delay in propagating local information might cause the deadlock detection algorithms to identify deadlocks that do not really exist. Such situations are called phantom deadlocks and they lead to unnecessary aborts.
Q: What is a checkpoint and When does it occur?
A: A Checkpoint is like a snapshot of the DBMS state. By taking checkpoints, the DBMS can reduce the amount of work to be done during restart in the event of subsequent crashes.
Q: What are the different phases of transaction?
A: Different phases are
1.) Analysis phase,
2.) Redo Phase,
3.) Undo phase.
Q: What do you mean by flat file database?
A: It is a database in which there are no programs or user access languages. It has no cross-file capabilities but is user-friendly and provides user-interface management.
Q: What is "transparent DBMS"?
A: It is one, which keeps its Physical Structure hidden from user.
Q: What is a query?
A: A query with respect to DBMS relates to user commands that are used to interact with a data base. The query language can be classified into data definition language and data manipulation language.
Q: What do you mean by Correlated subquery?
A: Subqueries, or nested queries, are used to bring back a set of rows to be used by the parent query. Depending on how the subquery is written, it can be executed once for the parent query or it can be executed once for each row returned by the parent query. If the subquery is executed for each row of the parent, this is called a correlated subquery.
A correlated subquery can be easily identified if it contains any references to the parent subquery columns in its WHERE clause. Columns from the subquery cannot be referenced anywhere else in the parent query. The following example demonstrates a non-correlated subquery.
Example: Select * From CUST Where '10/03/1990' IN (Select ODATE From ORDER Where CUST.CNUM = ORDER.CNUM)
Q: What are the primitive operations common to all record management systems?
A: Addition, deletion and modification.
Q: Name the buffer in which all the commands that are typed in are stored?
A: 'Edit' Buffer.
Q: What are the unary operations in Relational Algebra?
A: PROJECTION and SELECTION.
Q: Are the resulting relations of PRODUCT and JOIN operation the same?
A: No.
PRODUCT: Concatenation of every row in one relation with every row in another.
JOIN: Concatenation of rows from one relation and related rows from another.
Q: What is RDBMS KERNEL?
A: Two important pieces of RDBMS architecture are the kernel, which is the software, and the data dictionary, which consists of the system-level data structures used by the kernel to manage the database You might think of an RDBMS as an operating system (or set of subsystems), designed specifically for controlling data access; its primary functions are storing, retrieving, and securing data. An RDBMS maintains its own list of authorized users and their associated privileges; manages memory caches and paging; controls locking for concurrent resource usage; dispatches and schedules user requests; and manages space usage within its table-space structures.
Q: Name the sub-systems of a RDBMS.
A: I/O, Security, Language Processing, Process Control, Storage Management, Logging and Recovery, Distribution Control, Transaction Control, Memory Management, Lock Management.
Q: Which part of the RDBMS takes care of the data dictionary? How?
A: Data dictionary is a set of tables and database objects that is stored in a special area of the database and maintained exclusively by the kernel.
Q: What is the job of the information stored in data-dictionary?
A: The information in the data dictionary validates the existence of the objects, provides access to them, and maps the actual physical storage location.
Q: How do you communicate with an RDBMS?
A: You communicate with an RDBMS using Structured Query Language (SQL).
Q: Define SQL and state the differences between SQL and other conventional programming Languages.
A: SQL is a nonprocedural language that is designed specifically for data access operations on normalized relational database structures. The primary difference between SQL and other conventional programming languages is that SQL statements specify what data operations should be performed rather than how to perform them.
Q: Name the three major set of files on disk that compose a database in Oracle.
A: There are three major sets of files on disk that compose a database. All the files are binary. These are
1.) Database files
2.) Control files
3.) Redo logs
The most important of these are the database files where the actual data resides. The control files and the redo logs support the functioning of the architecture itself. All three sets of files must be present, open, and available to Oracle for any data on the database to be useable. Without these files, you cannot access the database, and the database administrator might have to recover some or all of the database using a backup, if there is one.
Q: What is database Trigger?
A: A database trigger is a PL/SQL block that can defined to automatically execute for insert, update, and delete statements against a table. The trigger can e defined to execute once for the entire statement or once for every row that is inserted, updated, or deleted. For any one table, there are twelve events for which you can define database triggers. A database trigger can call database procedures that are also written in PL/SQL.
Q: What are stored-procedures? And what are the advantages of using them?
A: Stored procedures are database objects that perform a user defined operation. A stored procedure can have a set of compound SQL statements. A stored procedure executes the SQL commands and returns the result to the client. Stored procedures are used to reduce network traffic.
Q: What is Storage Manager?
A: It is a program module that provides the interface between the low-level data stored in database, application programs and queries submitted to the system.
Q: What is Buffer Manager?
A: It is a program module, which is responsible for fetching data from disk storage into main memory and deciding what data to be cache in memory.
Q: What is Transaction Manager?
A: It is a program module, which ensures that database, remains in a consistent state despite system failures and concurrent transaction execution proceeds without conflicting.
Q: What is File Manager?
A: It is a program module, which manages the allocation of space on disk storage and data structure used to represent information stored on a disk.
Q: What is Authorization and Integrity manager?
A: It is the program module, which tests for the satisfaction of integrity constraint and checks the authority of user to access data.
Q: What are stand-alone procedures?
A: Procedures that are not part of a package are known as stand-alone because they independently defined. A good example of a stand-alone procedure is one written in a SQL*Forms application. These types of procedures are not available for reference from other Oracle tools. Another limitation of stand-alone procedures is that they are compiled at run time, which slows execution.
Q: What are cursors give different types of cursors?
A: PL/SQL uses cursors for all database information accesses statements. The language supports the use two types of cursors
1.) Implicit
2.) Explicit
Q: What is cold backup and hot backup (in case of Oracle)?
A: Cold Backup: It is copying the three sets of files (database files, redo logs, and control file) when the instance is shut down. This is a straight file copy, usually from the disk directly to tape. You must shut down the instance to guarantee a consistent copy. If a cold backup is performed, the only option available in the event of data file loss is restoring all the files from the latest backup. All work performed on the database since the last backup is lost.
Hot Backup: Some sites (such as worldwide airline reservations systems) cannot shut down the database while making a backup copy of the files. The cold backup is not an available option.
Q: What is meant by Proactive, Retroactive and Simultaneous Update.
A: Proactive Update: The updates that are applied to database before it becomes effective in real world.
Retroactive Update: The updates that are applied to database after it becomes effective in real world.
Simulatneous Update: The updates that are applied to database at the same time when it becomes effective in real world.
Q: What is MySQL?
A: MySQL is an open source DBMS which is built, supported and distributed by MySQL AB (now acquired by Oracle)
Q: What are the technical features of MySQL?
A: MySQL database software is a client or server system which includes
Multithreaded SQL server supporting various client programs and libraries
Different backend
Wide range of application programming interfaces and
Administrative tools.
Q: Why MySQL is used?
A: MySQL database server is reliable, fast and very easy to use. This software can be downloaded as freeware and can be downloaded from the internet.
Q: What are Heap tables?
A: HEAP tables are present in memory and they are used for high speed storage on temporary basis.
• BLOB or TEXT fields are not allowed
• Only comparison operators can be used =, <,>, = >,=<
• AUTO_INCREMENT is not supported by HEAP tables
• Indexes should be NOT NULL
Q: What is the default port for MySQL Server?
A: The default port for MySQL server is 3306.
Q: What are the advantages of MySQL when compard with Oracle?
A: MySQL is open source software which is available at any time and has no cost involved.
MySQL is portable
GUI with command prompt.
Administration is supported using MySQL Query Browser
Q: Differentiate between FLOAT and DOUBLE?
A: Following are differences for FLOAT and DOUBLE:
• Floating point numbers are stored in FLOAT with eight place accuracy and it has four bytes.
• Floating point numbers are stored in DOUBLE with accuracy of 18 places and it has eight bytes.
Q: Differentiate CHAR_LENGTH and LENGTH?
A: CHAR_LENGTH is character count whereas the LENGTH is byte count. The numbers are same for Latin characters but they are different for Unicode and other encodings.
Q: How to represent ENUMs and SETs internally?
A: ENUMs and SETs are used to represent powers of two because of storage optimizations.
Q: What is the usage of ENUMs in MySQL?
A: ENUM is a string object used to specify set of predefined values and that can be used during table creation.
Create table size(name ENUM('Small', 'Medium','Large');
Create table size(name ENUM('Small', 'Medium','Large');
Q: Define REGEXP?
A: REGEXP is a pattern match in which matches pattern anywhere in the search value.
Q: Difference between CHAR and VARCHAR?
A: Following are the differences between CHAR and VARCHAR:
CHAR and VARCHAR types differ in storage and retrieval
CHAR column length is fixed to the length that is declared while creating table. The length value ranges from 1 and 255
When CHAR values are stored then they are right padded using spaces to specific length. Trailing spaces are removed when CHAR values are retrieved.
Q: Give string types available for column?
A: The string types are:
SET
BLOB
ENUM
CHAR
TEXT
VARCHAR
Q: How to get current MySQL version?
A: SELECT VERSION ();
SELECT VERSION ();
is used to get the current version of MySQL.
Q: What storage engines are used in MySQL?
A: Storage engines are called table types and data is stored in files using various techniques.
Technique involves:
Storage mechanism
Locking levels
Indexing
Capabilities and functions.
Q: What are the drivers in MySQL?
A: Following are the drivers available in MySQL:
PHP Driver
JDBC Driver
ODBC Driver
C WRAPPER
PYTHON Driver
PERL Driver
RUBY Driver
CAP11PHP Driver
Ado.net5.mxj
Q: What does a TIMESTAMP do on UPDATE CURRENT_TIMESTAMP data type?
A: TIMESTAMP column is updated with Zero when the table is created. UPDATE CURRENT_TIMESTAMP modifier updates the timestamp field to current time whenever there is a change in other fields of the table.
Q: What is the difference between primary key and candidate key?
A: Every row of a table is identified uniquely by primary key. There is only one primary key for a table.
Primary Key is also a candidate key. By common convention, candidate key can be designated as primary and which can be used for any foreign key references.
Q: How do you login to MySql using Unix shell?
A: We can login through this command:
# [mysql dir]/bin/mysql -h hostname -u <UserName> -p <password>
Q: What does myisamchk do?
A: It compress the MyISAM tables, which reduces their disk or memory usage.
Q: How do you control the max size of a HEAP table?
A: Maximum size of Heal table can be controlled by MySQL config variable called max_heap_table_size.
Q: What is the difference between MyISAM Static and MyISAM Dynamic?
A: In MyISAM static all the fields will have fixed width. The Dynamic MyISAM table will have fields like TEXT, BLOB, etc. to accommodate the data types with various lengths.
MyISAM Static would be easier to restore in case of corruption.
Q: What are federated tables?
A: Federated tables which allow access to the tables located on other databases on other servers.
Q: What, if a table has one column defined as TIMESTAMP?
A: Timestamp field gets the current timestamp whenever the row gets altered.
Q: What happens when the column is set to AUTO INCREMENT and if you reach maximum value in the table?
A: It stops incrementing. Any further inserts are going to produce an error, since the key has been used already.
Q: How can we find out which auto increment was assigned on Last insert?
A: LAST_INSERT_ID will return the last value assigned by Auto_increment and it is not required to specify the table name.
Q: How can you see all indexes defined for a table?
A: Indexes are defined for the table by:
SHOW INDEX FROM <tablename>;
Q: What do you mean by % and _ in the LIKE statement?
A: % corresponds to 0 or more characters, _ is exactly one character in the LIKE statement.
Q: How can we convert between Unix & MySQL timestamps?
A: UNIX_TIMESTAMP is the command which converts from MySQL timestamp to Unix timestamp
FROM_UNIXTIME is the command which converts from Unix timestamp to MySQL timestamp.
Q: What are the column comparisons operators?
A: The = , <>, <=, <, >=, >,<<,>>, <=>, AND, OR, or LIKE operators are used in column comparisons in SELECT statements.
Q: How can we get the number of rows affected by query?
A: Number of rows can be obtained by
SELECT COUNT (user_id) FROM users;
SELECT COUNT (user_id) FROM users;
Q: Is Mysql query is case sensitive?
A: No.
SELECT VERSION(), CURRENT_DATE; SeLect version(), current_date; seleCt vErSiOn(), current_DATE;
SELECT VERSION(), CURRENT_DATE;
SeLect version(), current_date;
seleCt vErSiOn(), current_DATE;
All these examples are same. It is not case sensitive.
Q: What is the difference between the LIKE and REGEXP operators?
A: LIKE and REGEXP operators are used to express with ^ and %.
SELECT * FROM employee WHERE emp_name REGEXP "^b"; SELECT * FROM employee WHERE emp_name LIKE "%b";
SELECT * FROM employee WHERE emp_name REGEXP "^b";
SELECT * FROM employee WHERE emp_name LIKE "%b";
Q: What is the difference between BLOB AND TEXT?
A: A BLOB is a binary large object that can hold a variable amount of data. There are four types of BLOB –
TINYBLOB
BLOB
MEDIUMBLOB and
LONGBLOB
They all differ only in the maximum length of the values they can hold.
A TEXT is a case-insensitive BLOB. The four TEXT types
TINYTEXT
TEXT
MEDIUMTEXT and
LONGTEXT
They all correspond to the four BLOB types and have the same maximum lengths and storage requirements.
The only difference between BLOB and TEXT types is that sorting and comparison is performed in case-sensitive for BLOB values and case-insensitive for TEXT values.
Q: What is the difference between mysql_fetch_array and mysql_fetch_object?
A: Following are the differences between mysql_fetch_array and mysql_fetch_object:
mysql_fetch_array() -Returns a result row as an associated array or a regular array from database.
mysql_fetch_object – Returns a result row as object from database.
Q: How can we run batch mode in mysql?
A: Following commands are used to run in batch mode:
mysql ; mysql mysql.out
mysql ;
mysql mysql.out
Q: Where MyISAM table will be stored and also give their formats of storage?
A: Each MyISAM table is stored on disk in three formats:
The ‘.frm’ file stores the table definition
The data file has a ‘.MYD’ (MYData) extension
The index file has a ‘.MYI’ (MYIndex) extension
Q: What are the different tables present in MySQL?
A: Total 5 types of tables are present:
MyISAM
Heap
Merge
INNO DB
ISAM
MyISAM is the default storage engine as of MySQL.
Q: What is ISAM?
A: ISAM is abbreviated as Indexed Sequential Access Method.It was developed by IBM to store and retrieve data on secondary storage systems like tapes.
Q: What is InnoDB?
A: lnnoDB is a transaction safe storage engine developed by Innobase Oy which is a Oracle Corporation now.
Q: How MySQL Optimizes DISTINCT?
A: DISTINCT is converted to a GROUP BY on all columns and it will be combined with ORDER BY clause.
SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;
SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;
Q: How to enter Characters as HEX Numbers?
A: If you want to enter characters as HEX numbers, you can enter HEX numbers with single quotes and a prefix of (X), or just prefix HEX numbers with (Ox).
A HEX number string will be automatically converted into a character string, if the expression context is a string.
Q: How to display top 50 rows?
A: In MySql, top 50 rows are displayed by using this following query:
SELECT * FROM LIMIT 0,50;
SELECT * FROM
LIMIT 0,50;
Q: How many columns can be used for creating Index?
A: Maximum of 16 indexed columns can be created for any standard table.
Q: What is the different between NOW() and CURRENT_DATE()?
A: NOW () command is used to show current year,month,date with hours,minutes and seconds.
CURRENT_DATE() shows current year,month and date only.
Q: What are the objects can be created using CREATE statement?
A: Following objects are created using CREATE statement:
DATABASE
EVENT
FUNCTION
INDEX
PROCEDURE
TABLE
TRIGGER
USER
VIEW
Q: How many TRIGGERS are allowed in MySql table?
A: SIX triggers are allowed in MySql table. They are as follows:
BEFORE INSERT
AFTER INSERT
BEFORE UPDATE
AFTER UPDATE
BEFORE DELETE and
AFTER DELETE
Q: What are the nonstandard string types?
A: Following are Non-Standard string types:
TINYTEXT
TEXT
MEDIUMTEXT
LONGTEXT
Q: What are all the Common SQL Function?
A: CONCAT(A, B) – Concatenates two string values to create a single string output. Often used to combine two or more fields into one single field.
FORMAT(X, D) – Formats the number X to D significant digits.
CURRDATE(), CURRTIME() – Returns the current date or time.
NOW() – Returns the current date and time as one value.
MONTH(), DAY(), YEAR(), WEEK(), WEEKDAY() – Extracts the given data from a date value.
HOUR(), MINUTE(), SECOND() – Extracts the given data from a time value.
DATEDIFF(A, B) – Determines the difference between two dates and it is commonly used to calculate age
SUBTIMES(A, B) – Determines the difference between two times.
FROMDAYS(INT) – Converts an integer number of days into a date value.
Q: Explain Access Control Lists.
A: An ACL (Access Control List) is a list of permissions that is associated with an object. This list is the basis for MySQL server’s security model and it helps in troubleshooting problems like users not being able to connect.
MySQL keeps the ACLs (also called grant tables) cached in memory. When a user tries to authenticate or run a command, MySQL checks the authentication information and permissions against the ACLs, in a predetermined order.
Q: Define SQL?
A: Answer : SQL stands for Structured Query Language. SQL is a programming Language designed specially for managing data in Relational Database Management System (RDBMS).
Q: What is RDBMS? Explain its features?
A: Answer : A Relational Database Management System (RDBMS) is the most widely used database Management System based on the Relational Database model.
Features of RDBMS
Stores data in tables.
Tables have rows and column.
Creation and Retrieval of Table is allowed through SQL.
Q: What is Data Mining?
A: Answer : Data Mining is a subcategory of Computer Science which aims at extraction of information from set of data and transform it into Human Readable structure, to be used later.
Q: What is an ERD?
A: Answer : ERD stands for Entity Relationship Diagram. Entity Relationship Diagram is the graphical representation of tables, with the relationship between them.
Q: What is the difference between Primary Key and Unique Key?
A: Answer : Both Primary and Unique Key is implemented for Uniqueness of the column. Primary Key creates a clustered index of column where as an Unique creates unclustered index of column. Moreover, Primary Key doesn’t allow NULL value, however Unique Key does allows one NULL value.
Q: How to store picture file in the database. What Object type is used?
A: Answer : Storing Pictures in a database is a bad idea. To store picture in a database Object Type ‘Blob’ is recommended.
Q: What is Data Warehousing?
A: Answer : A Data Warehousing generally refereed as Enterprise Data Warehousing is a central Data repository, created using different Data Sources.
Q: What are indexes in a Database. What are the types of indexes?
A: Answer : Indexes are the quick references for fast data retrieval of data from a database. There are two different kinds of indexes.
Clustered Index
Only one per table.
Faster to read than non clustered as data is physically stored in index order.
Nonclustered Index
Can be used many times per table.
Quicker for insert and update operations than a clustered index.
Q: How many TRIGGERS are possible in MySql?
A: Answer : There are only six triggers are allowed to use in MySQL database and they are.
Before Insert
After Insert
Before Update
After Update
Before Delete
After Delete
Q: What is Heap table?
A: Answer : Tables that are present in the memory are called as HEAP tables. These tables are commonly known as memory tables. These memory tables never have values with data type like “BLOB” or “TEXT”. They use indexes which make them faster.
Q: Difference between LEFT and RIGHT OUTER Joins in SQL - MySQL Join example
A: There are two kinds of OUTER joins in SQL, LEFT OUTER join and RIGHT OUTER join. Main difference between RIGHT OUTER join and LEFT OUTER join, as there name suggest, is inclusion of non matched rows. Sine INNER join only include matching rows, where value of joining column is same, in final result set, but OUTER join extends that functionality and also include unmatched rows in final result. LEFT outer join includes unmatched rows from table written on left of join predicate. On the other hand RIGHT OUTER join, along with all matching rows, includes unmatched rows from right side of table.
In short result of LEFT outer join is INNER JOIN + unmatched rows from LEFT table and RIGHT OUTER join is INNER JOIN + unmatched rows from right hand side table. Similar to difference between INNER join and OUTER join, difference between LEFT and RIGHT OUTER JOIN can be better understand by a simple example, which we will see in next section. By the way joins are very popular in SQL interviews, and along with classic questions like finding second highest salary of employee, Inner join vs outer join or left outer join vs right outer join is commonly asked.
MySQL Scripts
CREATE USER 'jeffrey'@'localhost' IDENTIFIED BY 'mypass';
SHOW GRANTS FOR 'rfc'@'localhost';
GRANT SELECT, UPDATE, DELETE ON 'classicmodels'.* TO 'rfc'@'localhost'
REVOKE UPDATE, DELETE ON classicmodels.* FROM 'rfc'@'localhost';
SHOW GRANTS FOR 'rfc'@'localhost';
REVOKE ALL PRIVILEGES, GRANT OPTION FROM 'rfc'@'localhost';
GRANT ALL ON db1.* TO 'jeffrey'@'localhost';
GRANT SELECT ON db2.invoice TO 'jeffrey'@'localhost';
GRANT USAGE ON *.* TO 'jeffrey'@'localhost' WITH MAX_QUERIES_PER_HOUR 90;
GRANT ALL ON *.* TO 'someuser'@'somehost';
GRANT SELECT, INSERT ON *.* TO 'someuser'@'somehost';
CREATE TABLE `course` (
`id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(Q: NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB;
ALTER TABLE `user`
ADD CONSTRAINT `FK_course`
FOREIGN KEY (`course`) REFERENCES `course` (`id`)
ON UPDATE CASCADE;
INSERT INTO table_name ( field1, field2,...fieldN ) VALUES ( value1, value2,...valueN );
UPDATE table_name SET field1=new-value1, field2=new-value2 [WHERE Clause];
LOAD DATA INFILE '/tmp/test.txt' INTO TABLE test
FIELDS TERMINATED BY ',' LINES STARTING BY 'xxx';
SELECT user.name, course.name
FROM `user`
INNER JOIN `course` on user.course = course.id;
SELECT user.name, course.name
FROM `user`
LEFT JOIN `course` on user.course = course.id;
SELECT user.name, course.name
FROM `user`
RIGHT JOIN `course` on user.course = course.id;
SELECT user.name, course.name
FROM `course`
LEFT JOIN `user` on user.course = course.id;
SELECT course.name, COUNT(user.name)
FROM `course`
LEFT JOIN `user` ON user.course = course.id
GROUP BY course.id;
SELECT user.name, course.name
FROM `user`
LEFT JOIN `course` on user.course = course.id
UNION
SELECT user.name, course.name
FROM `user`
RIGHT JOIN `course` on user.course = course.id;
SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count
FROM tutorials_tbl a LEFT JOIN tcount_tbl b
ON a.tutorial_author = b.tutorial_author;
SELECT left_tbl.*
FROM { OJ left_tbl LEFT OUTER JOIN right_tbl ON left_tbl.id = right_tbl.id }
WHERE right_tbl.id IS NULL;
SELECT * FROM table1,table2 WHERE table1.id=table2.id;
SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id;
SELECT * FROM table1 LEFT JOIN table2 USING (id);
SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id
LEFT JOIN table3 ON table2.id=table3.id;
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