SQL Interview Questions For Software Testers

Software testing - Questions and Answers - SQL Interview Questions

SQL recognizes 4 general type of Data Character Strings

These are sentences, symbols or both VARCHAR2 and CHAR Math functions can not be done at the

character string data. NUMERIC Data

DATE, Timestamp, Numbers, and Integer BOOLEANS

Boolean values are either True or False. NULLS

A NULL Value indicates that nothing exists in that field

Allowing NULL values can be decided at the time of creating the table.

What is an Operator? Operators are a means by which SQL can manipulate

numbers and strings or test for equality

Four types of Operator Arithmetic Range Equality and Logical

RDBMS provides a lot of built in Functions to perform an operation. And it is an excellent tool inside a query.

SELECT COUNT(*) FROM table one;

An Expression is a special statement that returns a value

4 types of Expressions Boolean

The Boolean Expression returns the True/False Result

Numeric It generally returns a Number

AVG() SUM()

Character Character expressions are used to test for

values of a string Date Expressions

Date and Timestamps

Q. What is a primary key? A. Primary key : Each row of the data in a table uniquely identified by a Primary Key The column (columns) that has completely unique data throughout the table is known as the primary key field.

primary key, also called a primary keyword, is a key in a relational database that is unique for each record. It is a unique identifier, such as a driver license number, telephone number (including area code), or vehicle identification number (VIN). A relational database must always have one and only one primary key. Primary keys typically appear as columns in relational database tables.

Primary Key

A primary key is a property given to a table column that distinguishes that record apart from each

There are 3 Types of Primary Key Simple Primary Key Composite Primary Key Surrogate Primary Key

Simple Primary Key It uses only one field to identify a record.

Composite Primary Key Multiple fields joined together to identify a

record in a table. Surrogate Key

Unique running sequence number is generated to identify a record.

Q. What is the main role of a primary key in a table?

A. The main role of a primary key in a data table is to maintain the internal integrity of a data table.

Q. What are foreign keys?

A.Foreign key, also called a foreign keyword, in a database table is a key from another table that refers to (or targets) a specific key, usually the primary key , in the table being used. A primary key can be targeted by multiple foreign keys from other tables. But a primary key does not necessarily have to be the target of any foreign keys.

A. Foreign Key : You can logically relate data from multiple tables using Foreign Keys

Q. Can a table have more than one foreign key defined?

A. A table can have any number of foreign keys defined. It can have only one primary key defined.

Q. What is difference between UNIQUE and PRIMARY KEY constraints?

A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE KEY can have null values.

Q. Can a primary key contain more than one columns?

Yes. Primary key created on more than one column is called composite primary key.

Constraints

The Oracle Server uses constraints to prevent invalid data entry into tables.You can use constraints to do the following:• Enforce rules on the data in a table whenever a row is inserted, updated, or deleted from thattable. The constraint must be satisfied for the operation to succeed.• Prevent the deletion of a table if there are dependencies from other tables• Provide rules for Oracle tools, such as Oracle Developer

CONSTRAINTS

PRIMARY CONSTRAINTS : PRIMARY,UNIQUE,CHECK,REFERENCES)SECONDARY CONSTRAINTS : NOT NULL,DEFAULT)

CONSTRAINTS BASED ON 2 LEVELS COLUMN LEVEL and TABLE LEVEL

Eg. For SECONDARY CONSTRAINTS

~~~~~~~~~~~~~~~~~~~~~~~

1. NOT NULL CONSTRAINT

CREATE TABLE EMP9( ENO NUMBER(3) NOT NULL, ENAME VARCHAR2(10));

2. DEFAULT CONSTRAINT

CREATE TABLE EMP9(ENO NUMBER(3) NOT NULL, ENAME VARCHAR2(10),DOJ DATE DEFAULT SYSDATE);

3. PRIMARY CONSTRAINT(COLUMN LEVEL)

UNIQUE

CREATE TABLE EMP9( ENO NUMBER(3) NOT NULL CONSTRAINT UNIEMP UNIQUE,ENAME VARCHAR2(10));

PRIMARY KEY

CREATE TABLE EMP9( ENO NUMBER(3) CONSTRAINT PKEMP9 PRIMARY KEY,ENAME VARCHAR2(10));

4. CHECK CONSTRAINT

CREATE TABLE BANK( ACNO NUMBER(2) CONSTRAINT PKBANK PRIMARY KEY,ACTYPE VARCHAR2(2) CONSTRAINT CKBANK CHECK (ACTYPE IN

('SB','CA','RD')),ACNAME VARCHAR2(10),AMOUNT NUMBER(4));

5. REFERENCES

CREATE TABLE EMP9( ENO NUMBER(3) CONSTRAINT PKE9 PRIMARY KEY,JOB VARCHAR2(10),ENAME VARCHAR2(10),MGR NUMBER(4) REFERENCES EMP9(ENO));

6. REFERENCES(REFERING TO DIFFERENT TABLE)

CREATE TABLE DEPT9(DEPTNO NUMBER(2) CONSTRAINT PKDNO PRIMARY KEY, DNAME VARCHAR2(10), LOC VARCHAR2(10));

CREATE TABLE EMP9( EMPNO NUMBER(4),ENAME VARCHAR2(10), SAL NUMBER(7,2),DEPTNO NUMBER(2) CONSTRAINT FKDNO

REFERENCES DEPT9(DEPTNO));

7. TABLE LEVEL CONSTRAINTS

UNIQUE TABLE LEVEL

CREATE TABLE BANK( ACNO NUMBER(3),ACTYPE VARCHAR2(10), BAL NUMBER(7,2),PLACE VARCHAR2(10),CONSTRAINT

UNIBANK UNIQUE(ACNO,ACTYPE));

PRIMARY KEY(TABLE LEVEL)

CREATE TABLE BANK( ACNO NUMBER(2), ACTYPE VARCHAR2(2) CONSTRAINT CKBANK CHECK (ACTYPE IN ('SB','CA','RD')),AMOUNT NUMBER)

DATA Integrity Constraints

Constraint DescriptionDescribe the different type of Integrity Constraints supported by ORACLE ? NOT NULL Constraint - Disallows NULLs in a table's column.UNIQUE Constraint - Disallows duplicate values in a column or set of columns.PRIMARY KEY Constraint - Disallows duplicate values and NULLs in a column or set of columns.FOREIGN KEY Constrain - Require each value in a column or set of columns match a value in a related table's UNIQUE or PRIMARY KEY.CHECK Constraint - Disallows values that do not satisfy the logical expression of the constraint.

What is difference between UNIQUE constraint and PRIMARY KEY constraint?

A column defined as UNIQUE can contain NULLs while a column defined as PRIMARY KEY can't contain Nulls.

What are the Limitations of a CHECK Constraint ?

The condition must be a Boolean expression evaluated using the values in the row being inserted or updated and can't contain subqueries, sequence, the SYSDATE,UID,USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.

What is the maximum number of CHECK constraints that can be defined on a column ?

No Limit.

Q What is an Index ?An Index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. Index can be created on one or more columns of a table.

Indexes are automatically maintained and used by ORACLE. Changes to table data are automatically incorporated into all relevant indexes.

Indexes

Index will improve the throughput of the SQL Query. The Index should be created based on the frequently

used columns in the WHERE clause.

CREATE INDEX emp_idx ON emp(emp_name);

SELECT ename,dept_id,sal,mgrFROM EMPWHERE ename like ‘Sun%’;

Q What is the Subquery ?A Subquery is a query whose return values are used in filtering conditions of the main query.

Q. What is correlated sub-query ?A Correlated sub_query is a sub_query which has reference to the main query.

Q. What is an Integrity Constraint ?A Integrity constraint is a rule that restricts values to a column in a table.

Q. What is Referential Integrity ?A Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.

Q. what is Case Function Case facilitates conditional inquires by doing the work of an if-then-else statement

CASE TATEMENT

In SQL case works with either the select or update clauses. It provides when-then-else functionality (WHEN this_happens THEN do_this) also known as nested IF-THEN-ELSE - IF conditional statements

SELECT ename, CASE WHEN sal > 0 AND SAL <= 100000 THEN 1WHEN sal > 100000 AND SAL < 250000 THEN 2 WHEN sal > 250000 AND SAL < 5000000 THEN 3ELSE 99END AS emp_categoryFROM EMP

Q. Decode function

Decode : facilitates conditional inquires by doing the work of a case or if then else statement

SELECT supplier_name, decode(supplier_id, 10000, 'IBM', 10001, 'Microsoft', 10002, 'Hewlett Packard', 'Gateway') result FROM suppliers;

Q. How you will avoid duplicating records in a query? A By using DISTINCT

Q. What is difference between Rename and Alias? Rename is a permanent name given to a table or column whereas Alias is a temporary name given to a table or column which do not exist once the SQL statement is executed. Q. What is a view ? A view is a virtual table based on one or more tables.

Why Use views ?

To restrict data access• To make complex queries easy• To provide data independence• To present different views of the same data

Q. What are the advantages of Views ?• Views restrict access to the data because the view can display selective columns from the table.• Views can be used to make simple queries to retrieve the results of complicated queries. For example, views can be used to query information from multiple tables without the user knowing how to write a join statement.• Views provide data independence for ad hoc users and application programs. One view can be used to retrieve data from several tables.• Views provide groups of users access to data according to their particular criteria.Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.Hide data complexity.Simplify commands for the user.Present the data in a different perpecetive from that of the base table.Store complex queries.

Q. What are various privileges that a user can grant to another user?

SELECTCONNECTRESOURCES

Q. What is schema?A schema is collection of database objects of a User.

Q. what is Table ?

A table is the basic unit of data storage in an ORACLE database. The tables of a database hold all of the user accessible data. Table data is stored in rows and columns.

Q. Do View contain Data?Views do not contain or store data.

Q. Can a View based on another View ?Yes.

Q. What is a Sequence ?A sequence generates a serial list of unique numbers for numerical columns of a database's tables.

Q. What is a Synonym ?A synonym is an alias for a table, view, sequence or program unit.

There are two types of Synonyms Private and Public.A Private Synonyms can be accessed only by the owner.A Public synonyms can be accessed by any user on the database.

Synonyms are used to : Mask the real name and owner of an object.Provide public access to an objectProvide location transparency for tables,views or program units of a remote database.Simplify the SQL statements for database users.

Q. What is difference between TRUNCATE & DELETE ?

TRUNCATE commits after deleting entire table i.e., can not be rolled back. Database triggers do not fire on TRUNCATE

DELETE allows the filtered deletion. Deleted records can be rolled back or committed.Database triggers fire on DELETE.

Advantages of COMMIT and ROLLBACK StatementsWith COMMIT and ROLLBACK statements, you can:• Ensure data consistency• Preview data changes before making changes permanent• Group logically related operations

Q. Difference between SUBSTR and INSTR ?

INSTR (String1,String2(n,(m)),INSTR returns the position of the mth occurrence of the string 2 instring1. The search begins from nth position of string1.

SUBSTR (String1 n,m)SUBSTR returns a character string of size m in string1, starting from nth postion of string1.

Q. Explain UNION, MINUS, UNION ALL, INTERSECT ?

INTERSECT returns all distinct rows selected by both queries.MINUS - returns all distinct rows selected by the first query but not by the second.UNION - returns all distinct rows selected by either queryUNION ALL - returns all rows selected by either query, including all duplicates.

Union

The union clause places two separate queries together forming one table. A union works best when using two tables with similar columns because each column must have the same data type

SELECT dno FROM empUNIONSELECT dno FROM dept;

UNION ALL selects all rows from each table and combines them into a single table

The Difference between UNION and UNION ALL, The difference between Union and Union all is that

Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table

Q. What is ROWID ?

ROWID is a pseudo column attached to each row of a table. It is 18 character long, blockno, rownumber are the components of ROWID.

Q. What is the fastest way of accessing a row in a table ?

Using ROWID.

Q. What is difference between CHAR and VARCHAR2 ? , What is the maximum SIZE allowed for each type ?

CHAR pads blank spaces to the maximum length. VARCHAR2 does not pad blank spaces. For CHAR it is 255 and 2000 for VARCHAR2.

Q. How many LONG columns are allowed in a table ? Is it possible to use LONG columns in WHERE clause or ORDER BY ?

A Only one LONG columns is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.

Q. What is Database Link ?

A database link is a named object that describes a "path" from one database to another.

Private Database Link, Public Database Link & Network Database Link.

Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner's views or procedures.

Public database link is created for the special user group PUBLIC. A public database link can be used when any user in the associated database specifies a global object name in a SQL statement or object definition.

Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.

Q. Which is more faster - IN or EXISTS? EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.

Q. What is a join?

A. Join is a process of retrieve pieces of data from different sets (tables) and returns them to the user or program as one “joined†collection of data.�

Join & Union

JOIN The join clause combines columns of one table to

that of another to create a single table

A join query does not alter either table, but temporarily combines data from each table to be viewed as a single table

3 different types of Join Inner Left Right

Inner join

An inner join returns all rows that result in a match such as the example above.SELECT a.ename,b.dname,e.sal,e.mgrFROM emp a,

dept bWHERE a.dno = b.dno

Types of Joins• Equijoins• Non-equijoins• Outer joins• Self joins• Cross joins• Natural joins• Full or outer joins

EquijoinsTo determine an employee’s department name, you compare the value in the DEPARTMENT_IDcolumn in the EMPLOYEES table with the DEPARTMENT_ID values in the DEPARTMENTS table. The relationship between the EMPLOYEES and DEPARTMENTS tables is an equijoin—that is, values in the DEPARTMENT_ID column on both tables must be equal. Frequently, this type of join involves primary and foreign key complements.

Note: Equijoins are also called simple joins or inner joins.

Non-EquijoinsA non-equijoin is a join condition containing something other than an equality operator.The relationship between the EMPLOYEES table and the JOB_GRADES table has anexample of a non-equijoin. A relationship between the two tables is that the SALARYcolumn in the EMPLOYEES table must be between the values in the LOWEST_SALARYand HIGHEST_SALARY columns of the JOB_GRADES table. The relationship isobtained using an operator other than equals (=).

SELECT e.last_name, e.salary, j.grade_levelFROM employees e, job_grades jWHERE e.salaryBETWEEN j.lowest_sal AND j.highest_sal;

Outer join : to also see rows that do not meet the join condition

Returning Records with No Direct Match with Outer JoinsIf a row does not satisfy a join condition, the row will not appear in the query result. For example, in the equijoin condition of EMPLOYEES and DEPARTMENTS tables, employee Grant does not appear because there is no department ID recorded for her in the EMPLOYEES table. Instead of seeing 20 employees in the result set, you see 19 records.

SELECT e.last_name, e.department_id, d.department_nameFROM employees e, departments dWHERE e.department_id = d.department_id;

SELECT e.last_name, e.department_id, d.department_nameFROM employees e, departments dWHERE e.department_id(+) = d.department_id ;

Self Join :Joining a Table to ItselfSometimes you need to join a table to itself. To find the name of each employee’s manager, you need to join the EMPLOYEES table to itself, or perform a self join. For example, to find the name of Whalen’s manager, you need to:

• Find Whalen in the EMPLOYEES table by looking at the LAST_NAME column.• Find the manager number for Whalen by looking at the MANAGER_ID column. Whalen’s manager number is 101.• Find the name of the manager with EMPLOYEE_ID 101 by looking at the LAST_NAME column. Kochhar’s employee number is 101, so Kochhar is Whalen’s manager.In this process, you look in the table twice. The first time you look in the table to find Whalen in the LAST_NAME column and MANAGER_ID value of 101. The second time you look in the EMPLOYEE_ID column to find 101 and the LAST_NAME column to find Kochhar.

SELECT worker.last_name || ’ works for ’|| manager.last_nameFROM employees worker, employees managerWHERE worker.manager_id = manager.employee_id ;

Left Outer Join : This query retrieves all rows in the EMPLOYEES table, which is the left table even if there is no match in the DEPARTMENTS table.This query was completed in earlier releases as follows:SELECT e.last_name, e.department_id, d.department_nameFROM employees e, departments dWHERE d.department_id (+) = e.department_id;

SELECT e.last_name, e.department_id, d.department_nameFROM employees eLEFT OUTER JOIN departments dON (e.department_id = d.department_id) ;

Left Outer Join Syntax

A Left join returns all rows of the left of the conditional even if there is no right column to match

SELECT a.eno, a.ename,d.dnameFROM emp a LEFT OUTER JOIN dept bON a.dno = b.dno;

Right Outer Join :

SELECT e.last_name, e.department_id, d.department_nameFROM employees eRIGHT OUTER JOIN departments dON (e.department_id = d.department_id) ;

Example of RIGHT OUTER JOINThis query retrieves all rows in the DEPARTMENTS table, which is the right table even if there is no match in the EMPLOYEES table.This query was completed in earlier releases as follows:SELECT e.last_name, e.department_id, d.department_nameFROM employees e, departments dWHERE d.department_id = e.department_id (+);

Right Outer Join

A Right join returns all rows of the Right of the conditional even if there is no right column to match

SELECT a.eno, a.ename,b.dno,b.dnameFROM emp a RIGHT OUTER JOIN dept bON a.dno = b.dno;

Full outer join SELECT e.last_name, e.department_id, d.department_nameFROM employees eFULL OUTER JOIN departments dON (e.department_id = d.department_id) ;

Example of FULL OUTER JOINThis query retrieves all rows in the EMPLOYEES table, even if there is no match in theDEPARTMENTS table. It also retrieves all rows in the DEPARTMENTS table, even if there is no match in the EMPLOYEES table.

Cartesian ProductsA Cartesian product results in all combinations of rows displayed. This is done by either omitting the WHERE clause or specifying the CROSS JOIN clause.Table Aliases• Table aliases speed up database access.• Table aliases can help to keep SQL code smaller, by conserving memory.

Q. What kinds of joins do you know? Give examples.A. We have self join, outer joint (LEFT, RIGHT), , cross-join ( Cartesian product n*m rows returned)Exp:outer joint

SELECT Employee.Name, Department. DeptName FROM Employee, DepartmentWHERE Employee.Employee_ID = Department.Employee_ID;

cross-joinSELECT * FROM table1, table2;

self joinSELECT e1.name | |’ ‘ | | e2.ename FROM emp e1, emp e2 WHERE e1. emp_no = e2.emp_no;

Q. How do you add record to a table? A. INSERT into table_name VALUES (‘ALEX’ , 33 , ‘M’);

Q. How do you add a column to a table? A. ALTER TABLE Department ADD (AGE, NUMBER);

Q. How do you change value of the field?

A. UPDATE EMP_table set number = 200 where item_munber = ‘CD’;

update name_table set status = 'enable' where phone = '4161112222';

update SERVICE_table set REQUEST_DATE = to_date ('2006-03-04 09:29', 'yyyy-mm-dd hh24:MI') where phone = '4161112222';

What does COMMIT do ?COMMIT makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions transactions that start only after transaction is committed.

What does ROLLBACK do ?ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction

Q. What is the highest value that can be stored in a BYTE data field?

A. The highest value that can be stored in a BYTE field is 255. or from -128 to 127. Byte is a set of Bits that represent a single character. Usually there are 8 Bits in a

Byte, sometimes more, depending on how the measurement is being made. Each Char requires one byte of memory and can have a value from 0 to 255 (or 0 to 11111111 in binary).

What is a Procedure ?A Procedure consist of a set of SQL and PL/SQL statements that are grouped together as a unit to solve a specific problem or perform a set of related tasks.

Q. What is a stored procedure?A. A procedure is a group of PL/SQL statements that can be called by a name. Procedures do not return values they perform tasks.

Q. Describe how NULLs work in SQL?A. The NULL is how SQL handles missing values. Arifthmetic operation with NULL in SQL will return a NULL.

What is a SNAPSHOT ?Snapshots are read-only copies of a master table located on a remote node which is periodically refreshed to reflect changes made to the master table.

135. What is a SNAPSHOT LOG ?A snapshot log is a table in the master database that is associated with the master table. ORACLE uses a snapshot log to track the rows that have been updated in the master table. Snapshot logs are used in updating the snapshots based on the master table.

Q. What is Normalization?A. The process of table design is called normalization.

Q. What is referential integrity constraints?A. Referential integrity constraints are rules that are partnof the table in a database schema.

What is Database Trigger ?A Database Trigger is procedure (set of SQL and PL/SQL statements) that is automatically executed as a result of an insert in,update to, or delete from a table.

Q. What are the uses of Database Trigger ?Database triggers can be used to automatic data generation, audit data modifications, enforce complex Integrity constraints, and customize complex security authorizations.

Q. What are the differences between Database Trigger and Integrity constraints ?A declarative integrity constraint is a statement about the database that is always true. A constraint applies to existing data in the table and any statement that manipulates the table.

A trigger does not apply to data loaded before the definition of the trigger, therefore, it does not guarantee all data in a table conforms to the rules established by an associated trigger.

A trigger can be used to enforce transitional constraints where as a declarative integrity constraint cannot be used.

Q. Which of the following WHERE clauses will return only rows that have a NULL in the PerDiemExpenses column?A. WHERE PerDiemExpenses <> B. WHERE PerDiemExpenses IS NULLC. WHERE PerDiemExpenses = NULLD. WHERE PerDiemExpenses NOT IN (*)

A. B is correct � When searching for a NULL value in a column, you must use the keyword IS. No quotes are required around the keyword NULL.

Q. You issue the following query:SELECT FirstName FROM StaffListWHERE FirstName LIKE'_A%'Which names would be returned by this query? Choose all that apply.

A. AllenB. CLARKC. JACKSOND. David

A. C is correct � Two wildcards are used with the LIKE operator. The underscore (_) stands for any one character of any case, and the percent sign (%) stands for any number of characters of any case including none. Because this string starts with an underscore rather than a percent sign, it won't return Allen or Clark because they represent zero and two characters before the "A". If the LIKE string had been "%A%", both of these values would have been returned. David was not returned because all non-wild card characters are case sensitive. Therefore, only strings with an uppercase "A" as their second letter are returned

Q. Write a SQL SELECT query that only returns each city only once from Students table?Do you need to order this list with an ORDER BY clause?

A. SELECT DISTINCT City FROM Students;

The Distinct keyword automatically sorts all data in ascending order. However, if you want the data sorted in descending order, you have to use an ORDER BY clause

Q. Write a SQL SELECT sample of the concatenation operator.

A. SELECT LastName ||',' || FirstName, City FROM Students;

Q. How to rename column in the SQL SELECT query?

A. SELECT LastName ||',' || FirstName AS "Student Name", City AS "Home City" "FROM StudentsORDER BY "Student Name"

Q. Write SQL SELECT example how you limiting the rows returned with a WHERE clause.

A. SELECT InstructorID, Salary FROM Instructors WHERE Salary > 5400 AND Salary < 6600;

Q. Write SQL SELECT query that returns the first and last name of each instructor, the Salary, and gives each of them a number.

A. SELECT FirstName, LastName, Salary, ROWNUM FROM Instructors;

Q. Which of the following functions can be used only with numeric values? (Choose all that apply.)A. AVGB. MINC. LENGTHD. SUME. ROUND

A. A and D � Only A and D are correct. The MIN function works with any character, numeric, or date datatype. The LENGTH function is a character function that returns the number of letters in a character value. The ROUND function works with both numeric and date values.

Q. Which function do you use to remove all padded characters to the right of a character value in a column with a char datatype?A. RTRIMB. RPADC. TRIM

A. C � The TRIM function is used to remove padded spaces. LTRIM and RTRIM functions were included in earlier versions of Oracle, but Oracle 8i has replaced them with a single TRIM function

Q. Which statement do you use to eliminate padded spaces between the month and day values in a function TO_CHAR(SYSDATE,'Month, DD, YYYY') ?

A. To remove padded spaces, you use the "fm" prefix before the date element that contains the spaces. TO_CHAR(SYSDATE,'fmMonth DD, YYYY')

Q. Is the WHERE clause must appear always before the GROUP BY clause in SQL SELECT ?

A. Yes.

The proper order for SQL SELECT clauses is: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY. Only the SELECT and FROM clause are mandatory.

Q. How Oracle executes a statement with nested subqueries?

A. When Oracle executes a statement with nested subqueries,it always executes the innermost query first. This query passes its results to the next query and so on until it reaches the outermost query. It is the outermost query that returns a result set.

Q. Which operator do you use to return all of the rows from one query except rows are returned in a second query?

A. You use the MINUS operator to return all rows from one query except where duplicate rows are found in a second query. The UNION operator returns all rows from both queries minus duplicates. The UNION ALL operator returns all rows from both queries including duplicates. The INTERSECT operator returns only those rows that exist in both queries.

Q. Which of the following statements are Data Manipulation Language commands?A. INSERTB. UPDATEC. GRANTD. TRUNCATEE. CREATE

A. A and B � The INSERT and UPDATE statements are Data Manipulation Language (DML) commands. GRANT is a Data Control Language (DCL) command. TRUNCATE and CREATE are Data Definition Language (DDL) commands

Q. What is Oracle locking?A. Oracle uses locking mechanisms to protect data from being destroyed by concurrent transactions.

Q. What Oracle lock modes do you know? A. Oracle has two lock modes: shared or exclusive. Shared locks are set on database resources so that many transactions can access the resource. Exclusive locks are set on resources that ensure one transaction has exclusive access to the database resource

Q. What is query optimization?A. Query optimization is the part of the query process in which the database system compares different query strategies and chooses the one with the least expected cost

Q. What is transaction?A. A transaction is a collection of applications code and database manipulation code bound into an indivisible unit of execution.it consists from:BEGIN-TRANSACTION Name Code END TRANSACTION Name

Q. What databases do you know? Informix DB2 SQL Oracle

Q. Explain SQL SELECT example: select j.FILE_NUM from DB_name.job j, DB_name.address awhere j.JOB_TYPE ='C' AND j.COMPANY_NAME = 'TEST6' AND j.OFFICE_ID = '101' AND j.ACTIVE_IND = 'Y' AND a.ADDRESS_STATUS_ID = 'H' AND a.OFFICE_ID = '101' AND a.FILE_NUM = j.FILE_NUM order by j.FILE_NUM;

Answer: j and a aliases for table names. this is outer joint select statament from two tables.

Q. Describe some Conversion Functions that you know

A. TO_CHAR converts a number / date to a string. TO_DATE converts a string (representing a date) to a date.

TO_NUMBER converts a character string containing digits to a numeric data type, it accepts one parameter which is a column value or a string literal

Q. In what sequence SQL statement are processed?A. The clauses of the subselect are processed in the following sequence (DB2):1. FROM clause2. WHERE clause

3. GROUP BY clause4. HAVING clause5. SELECT clause6. ORDER BY clause7. FETCH FIRST clause

Q. What is a pseudo column. Give some examples? It is a column that is not an actual column in the table. Eaxmple USER, UID, SYSDATE, ROWNUM, ROWID, NULL, AND LEVEL.

Q. Suppose a customer table is having different columns like customer no, payments.What will be the query to select top three max payments?

SELECT customer_no, payments from customer C1 WHERE 3<=(SELECT COUNT(*) from customer C2 WHERE C1.payment <= C2.payment)

Q. Find out nth highest salary from emp table

SELECT DISTINCT (a.sal) FROM EMP A WHERE &N = (SELECT COUNT (DISTINCT (b.sal)) FROM EMP B WHERE a.sal<=b.sal);

Q. What are the difference between DDL, DML and DCL commands?

DDL is Data Definition Language statements. Some examples: CREATE - to create objects in the database ALTER - alters the structure of the database DROP - delete objects from the database TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed COMMENT - add comments to the data dictionary GRANT - gives user's access privileges to database REVOKE - withdraw access privileges given with the GRANT command DML is Data Manipulation Language statements. Some examples: SELECT - retrieve data from the a database INSERT - insert data into a table UPDATE - updates existing data within a table DELETE - deletes all records from a table, the space for the records remain CALL - call a PL/SQL or Java subprogram

EXPLAIN PLAN - explain access path to data LOCK TABLE - control concurrency DCL is Data Control Language statements. Some examples: COMMIT - save work done SAVEPOINT - identify a point in a transaction to which you can later roll back ROLLBACK - restore database to original since the last COMMIT SET TRANSACTION - Change transaction options like what rollback segment to use

Can we drop a column from a table?

yes. ALTER TABLE table_name DROP COLUMN column_name;

Q. Describe some Group Functions that you know

A. 1) The COUNT function tells you how many rows were in the result set. SELECT COUNT(*) FROM TESTING.QA

2) The AVG function tells you the average value of a numeric column. SELECT MAX(SALARY) FROM TESTING.QA

3) The MAX and MIN functions tell you the maximum and minimum value of a numeric column.

SELECT MIN(SALARY) FROM TESTING.QA

4) The SUM function tells you the sum value of a numeric column. SELECT SUM(SALARY) FROM TESTING.QA

Group functions: Group functions operate on sets of rows to give one result per group

Count function : COUNT(*) returns the number of rows in a table.

SELECT COUNT(*)FROM employeesWHERE department_id = 50;

Having clause If you restrict rows based on the result of a group function, you must have a GROUP BY clause as well as the HAVING clause.

SELECT department_id, MAX(salary)FROM employeesGROUP BY department_idHAVING MAX(salary)>10000 ;

Query Syntax

Write a query to display the name, who is getting the max salary from EMP table

SELECT ename FROM EMPWHERE sal=(SELECT MAX(sal) FROM EMP)

Select ename, max( sal) from emp ( Wrong)

Write a query to display the employee name and respective manager name in the Emp table

SELECT a.ename, b.ename FROM EMP a , EMP bWHERE a.MGR= b.EMPNO

Write a query to display the employee name and respective manager name in the Emp table including the manager is null

SELECT a.ename ||' is working under '||b.enameFROM EMP a,EMP bWHERE a.mgr=b.empnoUNIONSELECT 'There is no manager to '||ename FROM EMP WHERE mgr IS NULL

Write a query to sum all the positive and negative values in the column Table name tab1

COL112-1-2

1)To get 2,3,N maximum salaries with complete row

select * from emp where sal=(select min(sal) from(select * from (select * from emp order by sal desc) where rownum<=2))

(Replace 2 with the desired Number)

2) To get 2 Max Salary with complete row

select * from emp  where sal=(select max(sal) mx from emp where  (Sal) <(select max(sal) from emp))

4)To Get Top N rowsselect * from (select * from emp order by sal desc) where rownum<=5))(replace 5 with the desired Number)

5)To Get Nth Rowselect * from emp where 7=(Select count(Rowid) from emp x whereemp.rowid>=x.rowid)(Replace 7 with the Desired Number)

6)To Get Employees whose salary is greater the the avg salary of deptselect empno,emp.deptno,sal,a.av  from emp, (select Deptno,avg(sal) av from emp group by deptno) awhere a.deptno=emp.deptno and emp.sal>a.avOther waySELECT * FROM EMP WHERE SAL>(sELECT AVG(SAL) FROM EMP X  WHEREX.DEPTNO=EMP.DEPTNO) ORDER BY EMPNO

7)Deletion of Duplicate RecordsDelete from emp where rowid in(select min(rowid) from emp x wherex.rowid=emp.rowid)--Select Case Exampleselect ename,sal, case when sal between 500 and 1000 then 'D'when sal between 1001 and 1500  then 'C'when sal between 1501 and 2500  then 'B'else 'A' end Rank from emp ,salgrade where

emp.sal between losal and hisal

--Rows from emp and Salgrade tablesselect ename,sal,grade from emp,salgrade whereemp.sal between losal and hisal

--Use of Rank to Retervie top rowsselect e.*,rank() over(order by sal desc)rank fromemp e where rownum<=5

Q. Describe TO_DATE function.A. The TO_DATE function returns a timestamp from a character string that has been interpreted using a character template.TO_DATE is a synonym for TIMESTAMP_FORMAT.

Write a syntax for To_Date function

To_date('2003/07/09', 'yyyy/mm/dd') would return a date value of July 9, 2003. To_date('070903', 'MMDDYY') would return a date value of July 9, 2003. To_date('20020315', 'yyyymmdd') would return a date value of Mar 15, 2002.

SELECT TO_DATE('January 15','MONTH DD') "Sample" FROM DUAL