ORACLE PL/SQL INTERVIEW QUESTIONS

PL/SQL:Basiscs of PL/SQL

1. What is PL/SQL?

PL/SQL is a procedural language that has both interactive SQL and procedural programming language constructs such as iteration, conditional branching.

2. What is the basic structure of PL/SQL?

PL/SQL uses block structure as its basic structure. Anonymous blocks or nested blocks can be used in PL/SQL.A set of related declarations and procedural statements is called block.

3. What are the components of a PL/SQL Block?

Declarative part,
Executable part and Exception part.

4. What is difference between Anonymous block and Stored PL/SQL block?

· If you want to run Anonymous block, Each and Every time is complied and executed.
· But stored PL/SQL block is stored in compiled form in the database. Then we can execute without recompiling, using EXECUTE command.

5. What are the data types available in PL/SQL?

Some scalar data types such as NUMBER, VARCHAR2, DATE, CHAR, LONG, BOOLEAN.
Some composite data types such as RECORD & TABLE.

6. What are % TYPE and % ROWTYPE? What are the advantages of using these over data types?

% TYPE provides the data type of a variable or a database column to that variable.
% ROWTYPE provides the record type that represents a entire row of a table or view or columns selected in the cursor.
The advantages are:
i. Need not know about variable's data type
ii. If the database definition of a column in a table changes, the data type of a variable changes accordingly.

7. What is difference between % ROWTYPE and TYPE RECORD?

% ROWTYPE is to be used whenever query returns a entire row of a table or view.
TYPE rec RECORD is to be used whenever query returns columns of different table or views and variables.

E.g. TYPE r_emp is RECORD (eno emp.empno% type, ename emp.ename %type);
e_rec emp% ROWTYPE
cursor c1 is select empno,deptno from emp; e_rec c1 %ROWTYPE.
8. What is PL/SQL table?

Objects of type TABLE are called "PL/SQL tables", which are modeled as (but not the same as) database tables, PL/SQL tables use a primary PL/SQL tables can have one column and a primary key.

9. What is a cursor? Why Cursor is required?

Cursor is a named private SQL area from where information can be accessed. Cursors are required to process rows individually for queries returning multiple rows.

10. Explain the two types of Cursors?

There are two types of cursors, Implicit Cursor and Explicit Cursor.
PL/SQL uses Implicit Cursors for queries. User defined cursors are called Explicit Cursors. They can be declared and used.

11. What are the PL/SQL Statements used in cursor processing?

DECLARE CURSOR cursor name,
OPEN cursor name,
FETCH cursor name INTO or Record types,
CLOSE cursor name.

12. What are the cursor attributes used in PL/SQL ?

%ISOPEN - to check whether cursor is open or not
% ROWCOUNT - number of rows fetched/updated/deleted.
% FOUND - to check whether cursor has fetched any row. True if rows are fetched.
% NOT FOUND - to check whether cursor has fetched any row. True if no rows are fetched.
These attributes are preceded with SQL for Implicit Cursors and with Cursor name for Explicit Cursors.

13. What is a cursor for loop?

Cursor for loop implicitly declares %ROWTYPE as loop index, opens a cursor, fetches rows of values from active set into fields in the record and closes when all the records have been processed.

eg. FOR emp_rec IN C1 LOOP
salary_total := salary_total +emp_rec sal;
END LOOP;

14. What is a database trigger? Name some usages of database trigger?

Database trigger is stored PL/SQL program unit associated with a specific database table.

Usages are Audit data modifications, Log events transparently, Enforce complex business rules Derive column values automatically, Implement complex security authorizations. Maintain replicate tables.


15. Explain the usage of WHERE CURRENT OF clause in cursors?

WHERE CURRENT OF clause in an UPDATE, DELETE statement refers to the latest row fetched from a cursor.

16. What will happen after commit statement?
Cursor C1 is
Select empno, ename
from emp;
Begin
Open C1;
loop
Fetch C1 into eno, ename;
Exit When C1 %NOTFOUND;
----- commit;
end loop;
end;
The cursor having query as SELECT.... FOR UPDATE gets closed after COMMIT/ROLLBACK.
The cursor having query as SELECT.... does not get closed even after COMMIT/ROLLBACK.

17. How many types of database triggers can be specified on a table? What are they?

Insert Update Delete

Before Row o.k. o.k. o.k.

After Row o.k. o.k. o.k.

Before Statement o.k. o.k. o.k.

After Statement o.k. o.k. o.k.

If FOR EACH ROW clause is specified, then the trigger for each Row affected by the statement.
If WHEN clause is specified, the trigger fires according to the returned Boolean value.

18. Is it possible to use Transaction control Statements such a ROLLBACK or COMMIT in Database Trigger? Why?

It is not possible. As triggers are defined for each table, if you use COMMIT or ROLLBACK in a trigger, it affects logical transaction processing.

19. What are two virtual tables available during database trigger execution?

The table columns are referred as OLD.column_name and NEW.column_name.

For triggers related to INSERT only NEW.column_name values only available.
For triggers related to UPDATE only OLD.column_name NEW.column_name values only available. For triggers related to DELETE only OLD.column_name values only available.

20. What happens if a procedure that updates a column of table X is called in a database trigger of the same table?
Mutation of table occurs.