PLSQL Important Question and Answers

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.

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.

What are the components of a PL/SQL block?

A set of related declarations and procedural statements is called block.

What are the components of a PL/SQL Block?

Declarative part, Executable part and Exception part.

What are the datatypes a available in PL/SQL?

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

What are % TYPE and % ROWTYPE? What are the advantages of using these over datatypes?

% 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.

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.

What is PL/SQL table?

Objects of type TABLE are called "PL/SQL tables", which are modelled 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.

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.

Explain the two types of Cursors?

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

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

DECLARE CURSOR cursor name, OPEN cursor name, FETCH cursor name INTO <variable list> or Record types, CLOSE cursor name.

What are the cursor attributes used in PL/SQL?

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

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;

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.

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.

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 modificateions, Log events transparently, Enforce complex business rules Derive column values automatically, Implement complex security authorizations. Maintain replicate tables.

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 retruned boolean value.

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 of ROLLBACK in a trigger, it affects logical transaction processing.

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.

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.

Write the order of precedence for validation of a column in a table ?

I. done using Database triggers. ii. done using Integarity Constraints.

What is an Exception? What are types of Exception?

Exception is the error handling part of PL/SQL block. The types are Predefined and user_defined. Some of Predefined execptions are. CURSOR_ALREADY_OPEN DUP_VAL_ON_INDEX NO_DATA_FOUND TOO_MANY_ROWS INVALID_CURSOR INVALID_NUMBER LOGON_DENIED NOT_LOGGED_ON PROGRAM-ERROR STORAGE_ERROR TIMEOUT_ON_RESOURCE VALUE_ERROR ZERO_DIVIDE OTHERS.

What is Pragma EXECPTION_INIT? Explain the usage?

The PRAGMA EXECPTION_INIT tells the complier to associate an exception with an oracle error. To get an error message of a specific oracle error. e.g. PRAGMA EXCEPTION_INIT (exception name, oracle error number)

What is Raise_application_error?

Raise_application_error is a procedure of package DBMS_STANDARD which allows issuing a user_defined error messages from stored sub-program or database trigger.

What are the return values of functions SQLCODE and SQLERRM?

SQLCODE returns the latest code of the error that has occured. SQLERRM returns the relevant error message of the SQLCODE.

Where the Pre_defined_exceptions are stored?

In the standard package. Procedures, Functions & Packages;

What is a stored procedure?

A stored procedure is a sequence of statements that perform specific function.

What is difference between a PROCEDURE & FUNCTION?

A FUNCTION is alway returns a value using the return statement.  A PROCEDURE may return one or more values through parameters or may not return at all.

What are advantages of Stored Procedures?

Extensibility, Modularity, Reusability, Maintainability and one time compilation.

What are the modes of parameters that can be passed to a procedure?

IN,OUT,IN-OUT parameters.

What are the two parts of a procedure?

Procedure Specification and Procedure Body.

Give the structure of the procedure?

PROCEDURE name (parameter list.....) is local variable declarations BEGIN Executable statements. Exception. exception handlers end;

Give the structure of the function?

FUNCTION name (argument list .....) Return datatype is local variable declarations Begin executable statements Exception execution handlers End;

Explain how procedures and functions are called in a PL/SQL block ?

Function is called as part of an expression. sal := calculate_sal ('a822'); procedure is called as a PL/SQL statement calculate_bonus ('A822');

What is Overloading of procedures?

The Same procedure name is repeated with parameters of different datatypes and parameters in different positions, varying number of parameters is called overloading of procedures. e.g. DBMS_OUTPUT put_line

What is a package? What are the advantages of packages?

Package is a database object that groups logically related procedures. The advantages of packages are Modularity, Easier Applicaton Design, and Information. Hiding,. Reusability and Better Performance.

What are two parts of package?

The two parts of package are PACKAGE SPECIFICATION & PACKAGE BODY. Package Specification contains declarations that are global to the packages and local to the schema. Package Body contains actual procedures and local declaration of the procedures and cursor declarations.

What is difference between a Cursor declared in a procedure and Cursor declared in a package specification?

A cursor declared in a package specification is global and can be accessed by other procedures or procedures in a package. A cursor declared in a procedure is local to the procedure that can not be accessed by other procedures.

How packaged procedures and functions are called from the following ? a. Stored procedure or anonymous block b. an application program such a PRC *C, PRO* COBOL c. SQL *PLUS

a. PACKAGE NAME.PROCEDURE NAME (parameters); variable := PACKAGE NAME.FUNCTION NAME (arguments); EXEC SQL EXECUTE b.  BEGIN PACKAGE NAME.PROCEDURE NAME (parameters) variable := PACKAGE NAME.FUNCTION NAME (arguments); END; END EXEC; c. EXECUTE PACKAGE NAME.PROCEDURE if the procedures does not have any out/in-out parameters. A function can not be called.

Name the tables where characteristics of Package, procedure and functions are stored?

User_objects, User_Source and User_error.