Oracle Interview Questions

Oracle is one of the popular database provided by Oracle Corporation, which works on relational management concepts and hence it is referred as Oracle RDBMS as well.

Oracle follows a number of format for every release.

For Example, release 10.1.0.1.1 can be referred as below mentioned:

10: Major DB Release Number

1: DB Maintenance Release Number

0: Application Server Release Number

1: Component Specific Release Number

1: Platform Specific Release Number

Both VARCHAR & VARCHAR2 are Oracle data types that are used to store character strings of variable length.

VARCHAR can store characters up to 2000 bytes while VARCHAR2 can store up to 4000 bytes.

VARCHAR will hold the space for characters defined during declaration even if all of them are not used whereas VARCHAR2 will release the unused space.

The finer differences between the two include:

• TRUNCATE is a DDL operation while DELETE is a DML operation.
• TRUNCATE drops the structure of a database and hence cannot be rolled back while DELETE command can be rolled back.
• The TRUNCATE command will free the object storage space while the DELETE command does not.

RAW datatype is used to store variable-length binary data or byte strings.

The difference between RAW & VARCHAR2 datatype is that PL/SQL does not recognize this data type and hence, cannot do any conversions when RAW data is transferred to different systems. This data type can only be queried or inserted in a table.

Syntax: RAW (precision)

oins are used to extract data from multiple tables using some common column or condition.

There are various types of Joins as listed below:

• INNER JOIN
• OUTER JOIN
• CROSS JOINS or CARTESIAN PRODUCT
• EQUI JOIN
• ANTI JOIN
• SEMI JOIN

SUBSTR function returns the sub-part identified by numeric values from the provided string.

Example: [Select SUBSTR (‘India is my country’, 2, 4) from dual] will return “Indi”.

INSTR will return the position number of the sub-string within the string.

Example: [SELECT INSTR (‘India is my country’, ‘a’) from dual] will return 6.

We can use the below example query to fetch the duplicate records.

SELECT EMP_NAME, COUNT (EMP_NAME)

FROM EMP

GROUP BY EMP_NAME

HAVING COUNT (EMP_NAME) > 1;

Using ON DELETE CASCADE will automatically delete a record in the child table when the same is deleted from the parent table. This statement can be used with Foreign Keys.

We can add ON DELETE CASCADE option on an existing table using the below set of commands.

Syntax:

ALTER TABLE CHILD_T1 ADD CONSTRAINT CHILD_PARENT_FK REFERENCES

PARENT_T1 (COLUMN1) ON DELETE CASCADE;

NVL is a function, which helps the user to substitute a value if null is encountered for an expression.

It can be used as the below syntax.

[NVL (Value_In, Replace_With)]

Primary key is used to identify each table row uniquely, while a Unique Key prevents duplicate values in a table column.

Given below are few differences:

• The primary key can be only one on the table while unique keys can be multiple.
• The primary key cannot hold null value at all while Unique key allows multiple null values.
• The primary key is a clustered index while a unique key is a non-clustered index.

TRANSLATE command translates characters one by one in the provided string with the substitution character. REPLACE will replace a character or a set of characters with a complete substitution string.

Example:

TRANSLATE (‘Missisippi’,’is’,’15) => M155151pp1

REPLACE (‘Missisippi’,’is’,’15) => M15s15ippi

We can find the current Date & Time using SYSDATE in Oracle.

Syntax:

SELECT SYSDATE into CURRENT_DATE from dual;

COALESCE function is used to return the first non-null expression from the list of arguments provided in the expression. Minimum two arguments should be there in an expression.

Syntax:

COALESCE (expr 1, expr 2, expr 3…expr n)

The Query will be as follows:

SELECT TOP 1 RANK

FROM (SELECT TOP 5 RANK

FROM STUDENT_REPORT

ORDER BY RANK DESC) AS STUDENT

ORDER BY RANK ASC;

GROUP BY clause is used to identify and group the data by one or more columns in the query results. This clause is often used with aggregate functions like COUNT, MAX, MIN, SUM, AVG etc.

Syntax:

SELECT COLUMN_1, COLUMN_2

FROM TABLENAME

WHERE [condition]

GROUP BY COLUMN_1, COLUMN_2

The quickest way to fetch the data would be to use ROWID in the SQL Query.

Both DECODE & CASE statements will function like IF-THEN-ELSE statement and they are the alternatives for each other. These functions are used in Oracle to transform the data values.

Example:

DECODE Function

Select ORDERNUM,
DECODE (STATUS,O, ‘ORDERED’,P, ‘PACKED,’S’,’SHIPPED’,’A’,’ARRIVED’)
FROM ORDERS;

CASE Function

Select ORDERNUM
, CASE (WHEN STATUS =O then ‘ORDERED’
WHEN STATUS =P then PACKED
WHEN STATUS =S then ’SHIPPED’
ELSE ’ARRIVED’) END
FROM ORDERS;

Both the commands will display Order Numbers with respective Status as,

If,

Status O= Ordered
Status P= Packed
Status S= Shipped
Status A= Arrived

Integrity constraints are required to enforce business rules so as to maintain the integrity of the database and prevent the entry of invalid data into the tables. With the help of the below-mentioned constraints, relationships can be maintained between the tables.

Various integrity constraints available include Primary Key, Foreign Key, UNIQUE KEY, NOT NULL & CHECK.

MERGE statement is used to merge the data from two tables. It selects the data from the source table and inserts/updates it in the other table based on the condition provided in the MERGE query.

MERGE INTO TARGET_TABLE_1
USING SOURCE_TABLE_1
ON SEARCH_CONDITION
WHEN MATCHED THEN
INSERT (COL_1, COL_2…)
VALUES (VAL_1, VAL_2…)
WHERE
WHEN NOT MATCHED THEN
UPDATE SET COL_1=VAL_1, COL_2=VAL_2…
WHEN

Aggregate functions perform summary operations on a set of values to provide a single value. There are several aggregate functions that we use in our code to perform calculations.

Few of them are listed below:

• AVG
• MIN
• MAX
• COUNT
• SUM
• STDEV

Set operator facilitates the user to fetch the data from two or more than two tables at once if the columns and relative data types are same in the source tables.

• UNION operator returns all the rows from both the tables except the duplicate rows.
• UNION ALL returns all the rows from both the tables along with the duplicate rows.
• MINUS returns rows from the first table, which does not exist in the second table.
• INTERSECT returns only the common rows in both the tables.

We can use the TO_CHAR function to do the above conversion.

The syntax will be as follows:

SELECT to_char (to_date (30-01-2018?, ‘DD-MM-2021Y), ‘2021Y-MM-DD) FROM dual;]

Transaction occurs when a set of SQL statements are executed in one go. To control the execution of these statements, Oracle has introduced TCL i.e. Transaction Control Statements that use a set of statements.

The set of statements include:

COMMIT: Used to make a transaction permanent.

ROLLBACK: Used to roll back the state of DB to last the commit point.

SAVEPOINT: Helps to specify a transaction point to which rollback can be done later.

An object used to store the data or references of the data in a database is known as a Database object.

The database consists of various types of DB objects such as tables, views, indexes, constraints, stored procedures, triggers etc.

A nested table is a database collection object, which can be stored as a column in a table. While creating a normal table, an entire nested table can be referenced in a single column. Nested tables have only one column with no restriction of rows.

Example:

CREATE TABLE EMP (
EMP_ID NUMBER,
EMP_NAME TYPE_NAME)

Here we are creating a normal table as EMP and referring a nested table TYPE_NAME as a column.

BLOB stands for Binary Large Object, which is a datatype that is generally used to hold images, audio & video files or some binary executables.

This datatype has the capacity of holding data up to 4 GB.

Schema is a collection of database objects owned by a database user who can create or manipulate new objects within this schema.

The schema can contain any DB objects like table, view, indexes, clusters, stored procs, functions etc.

Whenever a new database is created, a database specific data dictionary gets created by the system. This dictionary maintains all the metadata related to the database and owned by the SYS user.

It has a set of read-only tables and views and it is physically stored in the SYSTEM tablespace.

A view is a user-defined database object that is used to store the results of a SQL query, which can be referenced later. Views do not store this data physically but as a virtual table, hence it can be referred as a logical table.

A table can hold data but not SQL Query results whereas View can save the query results, which can be used in another SQL Query as a whole.

The table can be updated or deleted while Views cannot be done so.