Oracle SQL, PL/SQL FAQs

About Triggers:

1.      What is triggers?  What are the different types of triggers?

A Database Trigger is a stored procedure that is fired when a DML operation is performed on the table.In total there are 13 types of Triggers

Sytax for creating a trigger:

            CREATE OR REPLACE TRIGGER <TRIGGERNAME> before / after

                        [INSERT / UPDATE / DELTE ] ON <TABLE NAME>

                        {For each Statement / Row}

                        {When <condition…..>}

Types of Triggers:

Before

After

For each Row

For each Statement (default)

Instead of Trigger: This trigger is defined on a view rather than a table.

System Triggers: A new feature of Oracle8i, wherein the trigger is fired when the database startup / shutdown process.

Schema Triggers: These triggers are fired whenever a DDL statement is executed.  (Creation or Deletion of any DB Objects)

Order of  Trigger Firing:

·         Before Statement trigger (If present)

·         Each row affected by the statement

(a)    Execute row level trigger (If present)

(b)   Execute the statement itself

(c)    Execute the after row level trigger (If Present)

·         After statement trigger (If Present)

2.      What are the different types of joins available in Oracle?

Equi Join: When primary and foreign key relationship exists between the tables that are going to be joined.

Self Join : If comparision comes in a  single table

Cartesian Join: When tables are joined without giving any join condition.

Inner Join: The resultant set includes all the rows that satisfy the join condition.

Outer Join: The resultant set includes the rows which doesn’t satisfy the join condition.  The outer join operator Plus sign (+) will be included in the join condiiton.

Example: SELECT a. column1, a. column2, b.column3….. from a, b where

a.column1(+)=b.Column1

Here the rows from table a which doesn’t satisfy the join condition will also be fetched.

3.      What are Indexes?  What are the different types of Index? If a table consists of more than one Index how to enforce the statement to use the second Index?

An Index is a DB object, which is used to improve the performance of  the data retrieval.

CREATE INDEX <INDEX NAME> ON <TABLE name>.(<COLUMN name>)

Types of Indexes:

Bitmap Index (Used for Low cardinality column)

Btree Index  (Used for high cardinality column)

4.      What is Mutating Table?

Table under transition is called Mutating Table. 

5.      What is views?  What is Inline View??

Views are window to a table.  It contains no data, it is based on the actual table called the base table or a view.

Inline View means writing select statement in the Query itself instead of selecting a Column Name.

6. What is a Cursor?  When it is used?   What are different types of Cursors.

Cursor is a private SQL area created in SGA to do multi row operation in a PL/SQL programme

Explicit Cursor, Implicit Cursor.

Implicit Cursor: System (Oracle) automatically declares and uses for all DML SQL Statements.

Explicit Cursor: Cursor declared explicitly in the PL/SQL programme to do multi row operation

Syntax:

Declare

Cursor C1 is SELECT SAL, EMPNO FROM EMP

X number;

Y Varchar2(30);

Begin

            Open C1;

            Loop

            Fetch C1 INTO x, y;

            Exit when c1%NOTFOUND

            End Loop;

End;

7. What is  for Cursor?  When it is used?  Is it necessary to write an explicit

exit in case for Cursor?

A Cursor for loop can be used simplify the explicit cursor, no need to explicitly

Open, fetch and close.  No explicity EXIT statement is required.

8. What are Cursor attributes? What is use of FOR UPDATE in Cursor?

%Found

%NotFound

%RowCount

%IsOpen

     

FOR UPDATE  statement in Cursor is Used to Update a Column in the Selected table by using the CURRENT OF <cursor name>.

9. What is a Package?  What is the advantage of using Packages?

A Package is a PL/SQL Construct that allow related object to be stored together.  Package contains 2 parts, Package Specification and Package Body, each stored separately in the Data Dicitionary.

Once the Package is called all the related Procedure and functions of the package gets compiled and stored in the memory as P-code.

How do u call a Package.

<PackageName>.<Procedure / Function Name> (Related Parameters….)

10. Name some important Packages provided by Oracle?

DBMS_SQL, DBMS_JOBS, DBMS_DDL, DBMS_LOCK

11. What is Overloading?

Overloading is oops concept(Object Oriented Programming)

By Using the same name we can write any number of Procedure or functions in a package but either number of parameters in the procedure/function must be vary or parameter datatype must vary.

12. What is a Function?   Difference between Procedure and Function?

Function is a object that takes one or more arguments and returns only value. But in case of procedures we can return more than one parameters.

Function always returns a value, whereas procedure may or may not return a value.

13. What is the Package used in Oracle to do the File Operation?

UTL_FILE

14. What is Dynamic SQL?  How Dynamic SQL can be built?

The SQL statement which are built at run time are called the Dynamic SQL.  Dynamic SQL can be built by using DBMS_SQL package.

Procedure of Dynamic SQL

OPEN_CURSOR, PARSE, BIND_VARIABLE, DEFINE_COLUMN, EXECUTE, FETCH_ROWS, CLOSE_CURSOR.

Oracle8i onwards there is another built in to construct Dynamic SQL called EXECUTE_IMMEDIATE.

15. What is an exception?  What are the different types of Exception?  How do u

declare a user defined exception?

The error condition in PL/SQL  is termed as an exception.  Two types of

Exception:

Pre-Defined Exception: Example No_Data_Found, Storage_Error,

Zero_Error, Invlid_Cursor, Too_Many_Rows

User-Defined Exception: Anything

Syntax:

Declare

Xyz Exception;

Begin

            SELECT ENAME FROM EMP

            RAISE XYZ;

End;

16. what could happen if we use WHEN OTHERS before any predefined exceptions

According to the Oracle standards “ When Others “ exception must be the last exception. All the Predefined exceptions must be used before the “When others” exception.

            If “ When others” exception used before any pre-defined exceptions then  procedure/function shows the compilations errors

   

17. List out some features in 8i

Bitmap Indexes, Drop a Column, Bulk Insert and Bulk Update

Materialized views, Dynamic Sql(Execute Immediate etc)

     

18. List some 9iFeatures

External tables ( We query the data directly from a file like select * from “c:/abcd.csv” )

Multi Table Insert with single command, resumable process etc.

     

19. What are SQLCODE and SQLERRM and why are they important for PL/SQL 

           developers?

SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception

20. What is the use of Pragma_Init exception

By using this we can define our messages by handling the oracle messages

21. What are temporary tables? How many types?

Temporary tables are used to store the data temporarly. Mainly there are 2 types

They are transaction and Session types

Syntax: Create global temporary table <temp_tab> as select * from emp;

This temporarly table is used to store the data temparorly once you exit from session then that table will get erased

22. Some of the System Tables

a.       User_source table will stores the information of the user defined definitions

b.      All_Source and dba_source tables will stores the system defined schema objects definitions as well as user defined.

c.       All_Tab_Columns and ben_all_tab_columns are used to list  out the all the columns name and respected table names also.

23. Write a query to list out the employees with their respective manager levels?

select lpad('*', level * 2), empno, ename, mgr from emp

connect by prior empno = mgr start with empno = 7839

It results the hierarchy of the employees

 Note : For Answers Check the Next Page

* What is PL/SQL and what is it used for?

* Should one use PL/SQL or Java to code procedures and triggers?

* How can one see if somebody modified any code?

* How can one search PL/SQL code for a key?

* How can one keep a history of PL/SQL code changes?

* How can I protect my PL/SQL source code?

* Can one print to the screen from PL/SQL?

* Can one read/write files from PL/SQL?

* Can one call DDL statements from PL/SQL?

* Can one use dynamic SQL statements from PL/SQL?

* What is the difference between %TYPE and %ROWTYPE?

* How does one get the value of a sequence into a PL/SQL variable?

* Can one execute an operating system command from PL/SQL?

* How does one loop through tables in PL/SQL?

* How often should one COMMIT in a PL/SQL loop? / What is the best commit strategy?

* I can SELECT from SQL*Plus but not from PL/SQL. What is wrong?

* What is a mutating and constraining table?

* Can one pass an object/table as an argument to a remote procedure?

* Is it better to put code in triggers or procedures? What is the difference?

* Is there a PL/SQL Engine in SQL*Plus?

* Is there a limit on the size of a PL/SQL block?

* Where can one find more info about PL/SQL?

What is PL/SQL and what is it used for?

PL/SQL is Oracle's Procedural Language extension to SQL. PL/SQL's language syntax, structure and data types are similar to that of ADA. The PL/SQL language includes object oriented programming techniques such as encapsulation, function overloading, information hiding (all but inheritance). PL/SQL is commonly used to write data-centric programs to manipulate data in an Oracle database.

Should one use PL/SQL or Java to code procedures and triggers?

Internally the Oracle database supports two procedural languages, namely PL/SQL and Java. This leads to questions like "Which of the two is the best?" and "Will Oracle ever desupport PL/SQL in favour of Java?".

Many Oracle applications are based on PL/SQL and it would be difficult of Oracle to ever desupport PL/SQL. In fact, all indications are that PL/SQL still has a bright future ahead of it. Many enhancements are still being made to PL/SQL. For example, Oracle 9iDB supports native compilation of Pl/SQL code to binaries.

PL/SQL and Java appeal to different people in different job roles. The following table briefly describes the difference between these two language environments:

PL/SQL:

Data centric and tightly integrated into the database

Proprietary to Oracle and difficult to port to other database systems

Data manupilation is slightly faster in PL/SQL than in Java

Easier to use than Java (depending on your background)

Java:

Open standard, not proprietary to Oracle

Incurs some data conversion overhead between the Database and Java type systems

Java is more difficult to use (depending on your background)

How can one see if somebody modified any code?

Code for stored procedures, functions and packages is stored in the Oracle Data Dictionary. One can detect code changes by looking at the LAST_DDL_TIME column in the USER_OBJECTS dictionary view. Example:

        SELECT OBJECT_NAME,

               TO_CHAR(CREATED,       'DD-Mon-RR HH24:MI') CREATE_TIME,

               TO_CHAR(LAST_DDL_TIME, 'DD-Mon-RR HH24:MI') MOD_TIME,

               STATUS

        FROM   USER_OBJECTS

        WHERE  LAST_DDL_TIME > '&CHECK_FROM_DATE';

How can one search PL/SQL code for a key?

The following query is handy if you want to know where a certain table, field or expression is referenced in your PL/SQL source code.

        SELECT TYPE, NAME, LINE

        FROM   USER_SOURCE

        WHERE  UPPER(TEXT) LIKE '%&KEYWORD%';

* By useing DBA_DEPENDENCIES table you can find out. - Ezhil

How can one keep a history of PL/SQL code changes?

One can build a history of PL/SQL code changes by setting up an AFTER CREATE schema (or database) level trigger (available from Oracle 8.1.7). This way one can easily revert to previous code should someone make any catastrophic changes. Look at this example:

        CREATE TABLE SOURCE_HIST                     -- Create history table

          AS SELECT SYSDATE CHANGE_DATE, USER_SOURCE.*

             FROM   USER_SOURCE WHERE 1=2;

        CREATE OR REPLACE TRIGGER change_hist        -- Store code in hist table

               AFTER CREATE ON SCOTT.SCHEMA          -- Change SCOTT to your schema name

        DECLARE

        BEGIN

          if DICTIONARY_OBJ_TYPE in ('PROCEDURE', 'FUNCTION',

                          'PACKAGE', 'PACKAGE BODY', 'TYPE') then

             -- Store old code in SOURCE_HIST table

             INSERT INTO SOURCE_HIST

                SELECT sysdate, user_source.* FROM USER_SOURCE

                WHERE  TYPE = DICTIONARY_OBJ_TYPE

                  AND  NAME = DICTIONARY_OBJ_NAME;

          end if;

        EXCEPTION

          WHEN OTHERS THEN

               raise_application_error(-20000, SQLERRM);

        END;

        /

        show errors

How can I protect my PL/SQL source code?

PL/SQL V2.2, available with Oracle7.2, implements a binary wrapper for PL/SQL programs to protect the source code.

This is done via a standalone utility that transforms the PL/SQL source code into portable binary object code (somewhat larger than the original). This way you can distribute software without having to worry about exposing your

proprietary algorithms and methods. SQL*Plus and SQL*DBA will still understand and know how to execute such scripts. Just be careful, there is no "decode" command available.

The syntax is:

            wrap iname=myscript.sql oname=xxxx.plb

Can one print to the screen from PL/SQL?

One can use the DBMS_OUTPUT package to write information to an output buffer. This buffer can be displayed on the screen from SQL*Plus if you issue the SET SERVEROUTPUT ON; command. For example:

            set serveroutput on

            begin

               dbms_output.put_line('Look Ma, I can print from PL/SQL!!!');

            end;

            /

DBMS_OUTPUT is useful for debugging PL/SQL programs. However, if you print too much, the output buffer will overflow. In that case, set the buffer size to a larger value, eg.: set serveroutput on size 200000

If you forget to set serveroutput on type SET SERVEROUTPUT ON once you remember, and then EXEC NULL;. If you haven't cleared the DBMS_OUTPUT buffer with the disable or enable procedure, SQL*Plus will display the entire contents of the buffer when it executes this dummy PL/SQL block.

Can one read/write files from PL/SQL?

Included in Oracle 7.3 is an UTL_FILE package that can read and write operating system files. The directory you intend writing to has to be in your INIT.ORA file (see UTL_FILE_DIR=... parameter). Before Oracle 7.3 the only means of writing a file was to use DBMS_OUTPUT with the SQL*Plus SPOOL command.

Copy this example to get started:

            DECLARE

              fileHandler UTL_FILE.FILE_TYPE;

            BEGIN

              fileHandler := UTL_FILE.FOPEN('/tmp', 'myfile', 'w');

              UTL_FILE.PUTF(fileHandler, 'Look ma, I''m writing to a file!!!\n');

              UTL_FILE.FCLOSE(fileHandler);

            EXCEPTION

              WHEN utl_file.invalid_path THEN

                 raise_application_error(-20000, 'ERROR: Invalid path for file or path not in INIT.ORA.');

            END;

            /

Can one call DDL statements from PL/SQL?

One can call DDL statements like CREATE, DROP, TRUNCATE, etc. from PL/SQL by using the "EXECUTE IMMEDATE" statement. Users running Oracle versions below 8i can look at the DBMS_SQL package (see FAQ about Dynamic SQL).

            begin

               EXECUTE IMMEDIATE 'CREATE TABLE X(A DATE)';

            end;

NOTE: The DDL statement in quotes should not be terminated with a semicolon.

Can one use dynamic SQL statements from PL/SQL?

From PL/SQL V2.1 one can use the DBMS_SQL package to execute dynamic SQL statements. Eg:

            CREATE OR REPLACE PROCEDURE DYNSQL AS

              cur integer;

              rc  integer;

            BEGIN

              cur := DBMS_SQL.OPEN_CURSOR;

              DBMS_SQL.PARSE(cur, 'CREATE TABLE X (Y DATE)', DBMS_SQL.NATIVE);

              rc := DBMS_SQL.EXECUTE(cur);

              DBMS_SQL.CLOSE_CURSOR(cur);

            END;

            /

Another example:

            CREATE OR REPLACE PROCEDURE DEPARTMENTS(NO IN DEPT.DEPTNO%TYPE) AS

              v_cursor integer;

              v_dname  char(20);

              v_rows   integer;

            BEGIN

              v_cursor := DBMS_SQL.OPEN_CURSOR;

              DBMS_SQL.PARSE(v_cursor, 'select dname from dept where deptno > :x', DBMS_SQL.V7);

              DBMS_SQL.BIND_VARIABLE(v_cursor, ':x', no);

              DBMS_SQL.DEFINE_COLUMN_CHAR(v_cursor, 1, v_dname, 20);

              v_rows := DBMS_SQL.EXECUTE(v_cursor);

              loop

                if DBMS_SQL.FETCH_ROWS(v_cursor) = 0 then

                   exit;

                end if;

                DBMS_SQL.COLUMN_VALUE_CHAR(v_cursor, 1, v_dname);

                DBMS_OUTPUT.PUT_LINE('Deptartment name: '||v_dname);

              end loop;

              DBMS_SQL.CLOSE_CURSOR(v_cursor);

            EXCEPTION

              when others then

                   DBMS_SQL.CLOSE_CURSOR(v_cursor);

                   raise_application_error(-20000, 'Unknown Exception Raised: '||sqlcode||' '||sqlerrm);

            END;

            /

What is the difference between %TYPE and %ROWTYPE?

The %TYPE and %ROWTYPE constructs provide data independence, reduces maintenance costs, and allows programs to adapt as the database changes to meet new business needs.

%ROWTYPE is used to declare a record with the same types as found in the specified database table, view or cursor. Example:

            DECLARE

               v_EmpRecord  emp%ROWTYPE;

%TYPE is used to declare a field with the same type as that of a specified table's column. Example:

            DECLARE

               v_EmpNo  emp.empno%TYPE;

How does one get the value of a sequence into a PL/SQL variable?

As you might know, oracle prohibits this:

            i := sq_sequence.NEXTVAL;

(for some silly reason). But you can do this:

            select sq_sequence.NEXTVAL into :i from dual;

Thanks to Ronald van Woensel

Can one execute an operating system command from PL/SQL?

There is no direct way to execute operating system commands from PL/SQL in Oracle7. However, one can write an external program (using one of the precompiler languages, OCI or Perl with Oracle access modules) to act as a listener on a database pipe (SYS.DBMS_PIPE). Your PL/SQL program then put requests to run commands in the pipe, the listener picks it up and run the requests. Results are passed back on a different database pipe. For an Pro*C example, see chapter 8 of the Oracle Application Developers Guide.

In Oracle8 one can call external 3GL code in a dynamically linked library (DLL or shared object). One just write a library in C/ C++ to do whatever is required. Defining this C/C++ function to PL/SQL makes it executable. Look at this External Procedure example.

How does one loop through tables in PL/SQL?

Look at the following nested loop code example.

            DECLARE

               CURSOR dept_cur IS

               SELECT deptno

                 FROM dept

                ORDER BY deptno;

               -- Employee cursor all employees for a dept number

               CURSOR emp_cur (v_dept_no DEPT.DEPTNO%TYPE) IS

               SELECT ename

                 FROM emp

                WHERE deptno = v_dept_no;

            BEGIN

               FOR dept_rec IN dept_cur LOOP

                  dbms_output.put_line('Employees in Department '||TO_CHAR(dept_rec.deptno));

                  FOR emp_rec in emp_cur(dept_rec.deptno) LOOP

                     dbms_output.put_line('...Employee is '||emp_rec.ename);

                  END LOOP;

              END LOOP;

            END;

            /

How often should one COMMIT in a PL/SQL loop? / What is the best commit strategy?

Contrary to popular believe, one should COMMIT less frequently within a PL/SQL loop to prevent ORA-1555 (Snapshot too old) errors. The higher the frequency of commit, the sooner the extents in the rollback segments will be cleared for new transactions, causing ORA-1555 errors.

To fix this problem one can easily rewrite code like this:

            FOR records IN my_cursor LOOP

               ...do some stuff...

               COMMIT;

            END LOOP;

... to ...

            FOR records IN my_cursor LOOP

               ...do some stuff...

               i := i+1;

               IF mod(i, 10000) THEN    -- Commit every 10000 records

                  COMMIT;

               END IF;

            END LOOP;

If you still get ORA-1555 errors, contact your DBA to increase the rollback segments.

NOTE: Although fetching across COMMITs work with Oracle, is not supported by the ANSI standard.

I can SELECT from SQL*Plus but not from PL/SQL. What is wrong?

PL/SQL respect object privileges given directly to the user, but does not observe privileges given through roles. The consequence is that a SQL statement can work in SQL*Plus, but will give an error in PL/SQL. Choose one of the following solutions:

Grant direct access on the tables to your user. Do not use roles!

            GRANT select ON scott.emp TO my_user;

   

Define your procedures with invoker rights (Oracle 8i and higher);

Move all the tables to one user/schema.

What is a mutating and constraining table?

"Mutating" means "changing". A mutating table is a table that is currently being modified by an update, delete, or insert statement. When a trigger tries to reference a table that is in state of flux (being changed), it is considered "mutating" and raises an error since Oracle should not return data that has not yet reached its final state.

Another way this error can occur is if the trigger has statements to change the primary, foreign or unique key columns of the table off which it fires. If you must have triggers on tables that have referential constraints, the workaround is to enforce the referential integrity through triggers as well.

There are several restrictions in Oracle regarding triggers:

A row-level trigger cannot query or modify a mutating table. (Of course, NEW and OLD still can be accessed by the trigger) .

A statement-level trigger cannot query or modify a mutating table if the trigger is fired as the result of a CASCADE delete.

Etc.

Can one pass an object/table as an argument to a remote procedure?

The only way the same object type can be referenced between two databases is via a database link. Note that it is not enough to just use the same type definitions. Look at this example:

            -- Database A: receives a PL/SQL table from database B

            CREATE OR REPLACE PROCEDURE pcalled(TabX DBMS_SQL.VARCHAR2S) IS

            BEGIN

               -- do something with TabX from database B

               null;

            END;

            /

            -- Database B: sends a PL/SQL table to database A

            CREATE OR REPLACE PROCEDURE pcalling IS

               TabX DBMS_SQL.VARCHAR2S@DBLINK2;

            BEGIN

               pcalled@DBLINK2(TabX);

            END;

            /

Is it better to put code in triggers or procedures? What is the difference?

In earlier releases of Oracle it was better to put as much code as possible in procedures rather than triggers. At that stage procedures executed faster than triggers as triggers had to be re-compiled every time before executed (unless cached). In more recent releases both triggers and procedures are compiled when created (stored p-code) and one can add as much code as one likes in either procedures or triggers.

Is there a PL/SQL Engine in SQL*Plus?

No. Unlike Oracle Forms, SQL*Plus does not have a PL/SQL engine. Thus, all your PL/SQL is sent directly to the database engine for execution. This makes it much more efficient as SQL statements are not stripped off and sent to the database individually.

Is there a limit on the size of a PL/SQL block?

Yes, the max size is not an explicit byte limit, but related to the parse tree that is created when you compile the code. You can run the following select statement to query the size of an existing package or procedure:

            SQL> select * from dba_object_size where name = 'procedure_name';