Tuesday, 25 June 2019

Top 20 Oracle PL/SQL Interview Questions

The following technical interview questions and answers are as per my experiences while working in Oracle PL/SQL projects. These may be useful for the freshers and experienced developers whenever facing PL/SQL interview.


Ques: 1. What is a mutating table error? How can you get this error?

Ans: This error can come with triggers, whenever a trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables, so the database is selecting from one while updating the other. This is all because of the table is in middle of a transaction and referencing the same table again in the middle of the updating action causes the trigger to mutate.


Ques: 2. What is the key difference between SQL and PL/SQL?

Ans: SQL and PL/SQL are used to access data within Oracle databases. SQL is a limited language that allows you to directly interact with the database. You can write queries, manipulate objects and data of oracle database with SQL language. SQL doesn't include the programming concepts.
By using PLSQL, you can extract and manipulate the objects and data of oracle database. And can do all the things that normal programming languages can have, such as looping and controlled executions.


Ques: 3. What is the difference between Form triggers and Database level triggers?

Ans: Form level triggers are use in forms and fire on any level like item level, row level or on block level on requirement of application. And database triggers are written in database directly and fire on behalf of any transaction like Insert, Update and delete on table automatically.
The key difference in form level triggers and database trigger is that form level trigger fire on user or application requirement and database triggers will fire automatically.


Ques: 4. What is Pseudo column?

Ans: Pseudo columns are database columns which are used for different purposes in oracle database.
ROWNUM, ROWID, SYSDATE, UID, USER, ORA_ROWSCN, SYSTIMESTAMP are pseudo columns in oracle database.


Ques: 5. What is exception handling in oracle? How can they be handled?

Ans: When an error occurs, exception is raised normally, and execution is stopped. Control transfers to exception handling part. Exception is an error situation which arises during program execution. Exception handlers are routines written to handle the exception. The exceptions can be internally defined User-defined exception. In oracle, exception can be handled by using these exception statements:
EXCEPTION WHEN
DUP_VAL_ON_INDEX
NOT_LOGGED_ON
TOO_MANY_ROWS
VALUE_ERROR
NO_DATA_FOUND


Ques: 6. What is the key difference between OPEN-FETCH-CLOSE and FOR LOOP in CURSOR?

Ans: A FOR LOOP in cursor implicitly declares its loop index as a %ROWTYPE record, opens a cursor, repeatedly fetches rows of values from the result set into fields in the record, and closes the cursor when all rows have been processed.
While using the OPEN-FETCH-CLOSE, we have to explicitly open the query and closing the query.


Ques: 7. What is Dynamic SQL? How can we use it in Oracle?

Ans: Dynamic SQL is used by PL/SQL to execute Data Definition Language (DDL) statements, Data Control (DCL) statements, or Transaction Control statements within PL/SQL blocks. These statements can, probably will change from execution to execution means change at runtime. These statements are not stored within the source code but are stored as character variables in the program.
    The SQL statements are created dynamically at runtime by using variables. This is used either using native dynamic SQL or through the DBMS_SQL package. Dynamic SQL supports all SQL data types.


Ques: 8. What is the key difference Between Row Level Trigger and Statement Level Trigger?

Ans: Row level trigger executes once for each row after or before in the DML event.  It can be defined by using FOR EACH ROW.
Statement Level trigger executes once after or before the DML event, it doesn’t matter many rows are affected by the DML event.  


Ques: 9. What are the various steps included in the compilation process of a PL/SQL block?

Ans: In the compilation process of a PL/SQL block, the syntax checking, binding, and p-code generation are involved. Syntax checking involves checking PL/SQL code for compilation errors. Syntax errors have been corrected, a storage address is assigned to the variables that are used to hold data for Oracle. This process is called binding. After binding, P-code is generated for the PL/SQL block. P-code is a list of instructions to the PL/SQL engine. For named blocks, p-code is stored in the database, and it is used the next time the program is executed.


Ques: 10. What packages have Oracle provided to PLSQL developers?

Ans: Oracle provides the DBMS_ series of packages to PLSQL developers to smooth the programming. The below packages, the developer should be aware of:
DBMS_DDL
UTL_FILE
DBMS_OUTPUT
DBMS_JOB
DBMS_SQL
DBMS_PIPE
DBMS_TRANSACTION
DBMS_LOCK
DBMS_ALERT
DBMS_UTILITY


Ques: 11. How can you protect your PL/SQL source code?

Ans: Oracle provides a binary wrapper utility that can be used to scramble PL/SQL source code. This utility was introduced in Oracle 7.
This utility use human-readable PL/SQL source code as input and writes out portable binary object code. It can larger than the original in size. The binary code can be distributed without fear of exposing your used algorithms and methods. Oracle will still understand and know how to execute the code. Be careful, there is no "decode" command available. So, always keep your source code saved.


Ques: 12. What are SQLCODE and SQLERRM in PLSQL? What is their importance in programming in PLSQL?

Ans: In PLSQL, SQLCODE returns the value of the error number for the last encountered error. Whereas the SQLERRM returns the actual error message for the last encountered error. They are used in exception handling in PLSQL. These are very useful for the WHEN OTHERS exception.


Ques: 13. What do you understand by cursors in PLSQL?

Ans: Cursor is a pointer variable in a memory and use for data manipulation operations in PLSQL. Basically, cursor is a private SQL memory area. It is also used to improve the performance of the PLSQL block.

Two types of cursors are:

a). Implicit cursor: Implicit cursors use oracles to manipulate the data manipulation operations internally and developers have no control on this type of cursor. We use sql%notfound and sql%rowcount cursor attributes in implicit cursor.

b). Explicit cursor: Explicit cursors are created by the developers and can control it by using the Fetch, Open and close keywords.

There are five types of cursors attributes in PLSQL:

1). %isopen: used to verify whether this cursor is open or not.
2). %found: If cursor fetch the data then %found return true.
3). %notfound: If cursor fetches not data then %notfound return true.
4). %rowcount: It return no. of rows that are in cursor.
5). %bulk_rowcount: It is same as %rowcount but it is used in bulk.


Ques: 14. What is the Index-By-Tables in PLSQL?

Ans: Index-By-Tables is also known as Associative Arrays. These are the sets of key-value pairs, where each key is unique and is used to locate a corresponding value in the array. The key can be an integer or a string.

Syntax : TYPE tab_type_name IS TABLE OF element type
               INDEX  BY  BINARY_INTEGER;

Ex:  TYPE DeptTabTyp IS TABLE OF Dept%ROWTYPE
         INDEX BY BINARY_INTEGER;
         Dept_tab DeptTabTyp;


Ques: 15. What do you understand by bulk binding?

Ans: The bulk binding technique improves performance by minimizing the number of context switches between the PL/SQL and SQL engines. A DML operation statement can transfer all the elements of a collection in a single operation.

           For example, If the collection has 100 elements, It lets you to perform the 100 SELECT, INSERT, UPDATE, or DELETE statements using a single operation.              


Ques: 16. What do you under by AUTONOMOUS_TRANSACTION?

Ans: The pragma AUTONOMOUS_TRANSACTION instructs the PL/SQL compiler to mark a PLSQL block as autonomous i.e. independent. Autonomous transactions let the compiler stop the main transaction to do DML operations, commit or roll back those operations, then resume the main transaction.


Ques: 17. What are the two components of LOB datatype in PLSQL?

Ans: The two components of the LOB datatype in PLSQL are:

1). LOB locator: LOB Locator is a locator, which points to the location in the database where the actual value is stored. This value is stored along with the record in the table row and is like a pointer to the actual location of LOB value.

2). LOB value: It is referred to an actual image, file or value of the LOB datatype.


Ques: 18: What are cascading of triggers?

Ans: If we insert data in one table and that table have trigger on it then trigger fire. And in this trigger, there is another table that we are using for inserting the data in it and this table has also trigger on it then this trigger also fire. This is called cascading of triggers.


Ques: 19. What do you understand by Table Functions in PLSQL?

Ans: We can use the table function like the name of the database table. Table functions are functions that produce a collection of rows (either a nested table or a Varray) that can be queried like a physical database table or assigned to PL/SQL collection variable.


Ques 20: What is the use of NOCOPY in PLSQL?

Ans: In PLSQL programming, the NOCOPY is Compiler Hint. When the Parameters hold large data structures such as collections and records, all this time copying slows down the execution. To prevent this, we can specify NOCPY. This allows the PL/SQL Compiler to pass OUT and INOUT parameters by reference.






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