sql interview questions 1

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sql interview questions 1
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2012-02-15 20:57:22
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sql IQ 1
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  1. What are two methods of retrieving SQL?
    Select ,Cursor
  2. what is a cursor
    A cursor is a set of rows together with a pointer that identifies a current row. In other word, Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis, its like recordset in the ASP and visual basic.
  3. What cursor type do you use to retrieve multiple recordsets?
    Explicit cursor
  4. What is the difference between a "where" clause and a "having" clause?
    "Where" is a kind of restiriction statement. You use where clause to restrict all the data from DB.Where clause is using before result retrieving. But Having clause is using after retrieving the data.Having clause is a kind of filtering command.
  5. What is the basic form of a SQL statement to read data out of a table?
    basic form to read data out of table is ‘SELECT * FROM table_name; ‘ An answer: ‘SELECT * FROM table_name WHERE xyz= ‘whatever’;’ cannot be called basic form because of WHERE clause.
  6. What structure can you implement for the database to speed up table reads?
    Follow the rules of DB tuning we have to: 1] properly use indexes ( different types of indexes) 2] properly locate different DB objects across different tablespaces, files and so on.3] create a special space (tablespace) to locate some of the data with special datatype ( for example CLOB, LOB and …)
  7. What are the tradeoffs with having indexes?
    Extra storage space to store indexes. Updates are slower because in addition to updating the table you have to update the index.
  8. What is a "join"?
    ‘join’ used to connect two or more tables logically with or without common field.
  9. What is "normalization"? "Denormalization"? Why do you sometimes want to denormalize?
    Normalizing data means eliminating redundant information from a table and organizing the data so that future changes to the table are easier. Denormalization means allowing redundancy in a table. The main benefit of denormalization is improved performance with simplified data retrieval and manipulation. This is done by reduction in the number of joins needed for data processing.
  10. What is a "constraint"?
    A constraint allows you to apply simple referential integrity checks to a table. There are four primary types of constraints that are currently supported by SQL Server: PRIMARY/UNIQUE - enforces uniqueness of a particular table column. DEFAULT - specifies a default value for a column in case an insert operation does not provide one. FOREIGN KEY - validates that every value in a column exists in a column of another table. CHECK - checks that every value stored in a column is in some specified list. Each type of constraint performs a specific type of action. Default is not a constraint. NOT NULL is one more constraint which does not allow values in the specific column to be null. And also it the only constraint which is not a table level constraint.
  11. What types of index data structures can you have?
    An index helps to faster search values in tables. The three most commonly used index-types are: - B-Tree: builds a tree of possible values with a list of row IDs that have the leaf value. Needs a lot of space and is the default index type for most databases. - Bitmap: string of bits for each possible value of the column. Each bit string has one bit for each row. Needs only few space and is very fast.(however, domain of value cannot be large, e.g. SEX(m,f); degree(BS,MS,PHD) - Hash: A hashing algorithm is used to assign a set of characters to represent a text string such as a composite of keys or partial keys, and compresses the underlying data. Takes longer to build and is supported by relatively few databases.
  12. What is a "primary key"?
    A PRIMARY INDEX or PRIMARY KEY is something which comes mainly from database theory. From its behavior is almost the same as an UNIQUE INDEX, i.e. there may only be one of each value in this column. If you call such an INDEX PRIMARY instead of UNIQUE, you say something about your table design, which I am not able to explain in few words. Primary Key is a type of a constraint enforcing uniqueness and data integrity for each row of a table. All columns participating in a primary key constraint must possess the NOT NULL property.
  13. What is a "functional dependency"? How does it relate to database table design?
    Functional dependency relates to how one object depends upon the other in the database. for example, procedure/function sp2 may be called by procedure sp1. Then we say that sp1 has functional dependency on sp2.
  14. What is a "trigger"?
    Triggers are stored procedures created in order to enforce integrity rules in a database. A trigger is executed every time a data-modification operation occurs (i.e., insert, update or delete). Triggers are executed automatically on occurance of one of the data-modification operations. A trigger is a database object directly associated with a particular table. It fires whenever a specific statement/type of statement is issued against that table. The types of statements are insert,update,delete and query statements. Basically, trigger is a set of SQL statements A trigger is a solution to the restrictions of a constraint. For instance: 1.A database column cannot carry PSEUDO columns as criteria where a trigger can. 2. A database constraint cannot refer old and new values for a row where a trigger can.
  15. Why can a "group by" or "order by" clause be expensive to process?
    Processing of "group by" or "order by" clause often requires creation of Temporary tables to process the results of the query. Which depending of the result set can be very expensive.
  16. What is "index covering" of a query?
    Index covering means that "Data can be found only using indexes, without touching the tables"
  17. What types of join algorithms can you have?
    • there are 3 kinds of joins in sql server
    • nested table,merge and hash joins.all of these depend on the resource available,merge and hash consume a lot of memory.
    • nested table should have indexes
  18. What is a SQL view?
    An output of a query can be stored as a view. View acts like small table which meets our criterion. View is a precomplied SQL query which is used to select data from one or more tables. A view is like a table but it doesn’t physically take any space. View is a good way to present data in a particular format if you use that query quite often. View can also be used to restrict users from accessing the tables directly.
  19. What is the difference between oracle,sql and sql server ?
    Oracle is based on RDBMS.SQL is Structured Query Language.SQL Server is another tool for RDBMS provided by MicroSoft.
  20. why you need indexing ? where that is stroed and what you mean by schema object? For what purpose we are using view?
    We cant create an Index on Index.. Index is stoed in user_index table.Every object that has been created on Schema is Schema Object like Table,View etc.If we want to share the particular data to various users we have to use the virtual table for the Base table...So tht is a view.indexing is used for faster search or to retrieve data faster from various table. Schema containing set of tables, basically schema means logical separation of the database. View is crated for faster retrieval of data. It's customized virtual table. we can create a single view of multiple tables. Only the drawback is..view needs to be get refreshed for retrieving updated data.
  21. Difference between Store Procedure and Trigger?
    we can call stored procedure explicitly.but trigger is automatically invoked when the action defined in trigger is done.ex: create trigger after Insert onthis trigger invoked after we insert something on that table.Stored procedure can't be inactive but trigger can be Inactive.Triggers are used to initiate a particular activity after fulfilling certain condition.It need to define and can be enable and disable according to need.
  22. What the difference between UNION and UNIONALL?
    Union will remove the duplicate rows from the result set while Union all does'nt.
  23. What is the difference between TRUNCATE and DELETE commands?
    Both will result in deleting all the rows in the table .TRUNCATE call cannot be rolled back as it is a DDL command and all memory space for that table is released back to the server. TRUNCATE is much faster.Whereas DELETE call is an DML command and can be rolled back.
  24. Which system table contains information on constraints on all the tables created ?
    USER_CONSTRAINTS,system table contains information on constraints on all the tables created
  25. How to find out the database name from SQL*PLUS command prompt?
    Select * from global_name;
  26. What is the difference between SQL and SQL Server ?
    SQLServer is an RDBMS just like oracle,DB2 from Microsoftwhereas Structured Query Language (SQL), pronounced "sequel", is a language that provides an interface to relational database systems. It was developed by IBM in the 1970s for use in System R. SQL is a de facto standard, as well as an ISO and ANSI standard. SQL is used to perform various operations on RDBMS.
  27. What is diffrence between Co-related sub query and nested sub query?
    Correlated subquery runs once for each row selected by the outer query. It contains a reference to a value from the row selected by the outer query.Nested subquery runs only once for the entire nesting (outer) query. It does not contain any reference to the outer query row.For example,Correlated Subquery:select e1.empname, e1.basicsal, e1.deptno from emp e1 where e1.basicsal = (select max(basicsal) from emp e2 where e2.deptno = e1.deptno)Nested Subquery:select empname, basicsal, deptno from emp where (deptno, basicsal) in (select deptno, max(basicsal) from emp group by deptno)
  28. WHAT OPERATOR PERFORMS PATTERN MATCHING?
    Pattern matching operator is LIKE and it has to used with two attributes1. % and2. _ ( underscore )% means matches zero or more characters and under score means mathing exactly one character
  29. What is difference between Oracle and MS Access?2) What are disadvantages in Oracle and MS Access?3) What are feratures&advantages in Oracle and MS Access?
    Oracle's features for distributed transactions, materialized views and replication are not available with MS Access. These features enable Oracle to efficiently store data for multinational companies across the globe. Also these features increase scalability of applications based on Oracle.
  30. What is database?
    A database is a collection of data that is organized so that itscontents can easily be accessed, managed and updated.
  31. What is cluster.cluster index and non cluster index ?
    Clustered Index:- A Clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table may have only one clustered index.Non-Clustered Index:- A Non-Clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows in the disk. The leaf nodes of a non-clustered index does not consists of the data pages. instead the leaf node contains index rows.
  32. How can i hide a particular table name of our schema?
    you can hide the table name by creating synonyms.e.g) you can create a synonym y for table xcreate synonym y for x;
  33. What is difference between DBMS and RDBMS?
    The main difference of DBMS & RDBMS isRDBMS have Normalization. Normalization means to refining the redundant and maintain the stablization. the DBMS hasn't normalization concept.
  34. What are the advantages and disadvantages of primary key and foreign key in SQL?
    Primary keyAdvantages1) It is a unique key on which all the other candidate keys are functionally dependentDisadvantage1) There can be more than one keys on which all the other attributes are dependent on.Foreign KeyAdvantage1)It allows refrencing another table using the primary key for the other table
  35. Which date function is used to find the difference between two dates?
    datedifffor Eg: select datediff (dd,'2-06-2007','7-06-2007')output is 5

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