this was intended to be a list of hints and tips that you might find useful when using sq

7/29/2019 This Was Intended to Be a List of Hints and Tips That You Might Find Useful When Using SQ

1/33

This was intended to be a list of hints and tips that you might find useful when using SQ,L

but a mere list of tips would be of little benefit without the knowledge to make use of

them, so we've expanded the list to include the explanations to increasing its usefulnessand to make it a proper tutorial.

One important point to remember is that Oracle caches the compiledform of SQL and is therefore able to re-use queries which are the same as previouslyexecuted queries. This saves the time and resources required to parse the statement and

determine the execution plan. How can you do this ?

Use Views

Views are a good way to ensure the same query is re-used as much as possible.

Remember that even just changing the case and spacing of the words could prevent a

query from being reused. A view is merely a pre-defined query, the text of which is

stored in the database. Therefore by using views you are using exactly the same queriesand eliminating the re-parsing overehead. As the load on the database increases this re-

parsing overhead becomes more and more significant. Materialised views take the

concept one stage further by actually running the query and storing the results in a table

Use Stored ProceduresAnother way is to use stored procedures which are program units that contain both SQL

and logic statements and are stored in the database. Oracle allows the use of PL/SQL and

Java stored procedures. Stored procedures and views also have the advantage that thequeries in the views/stored procedures have to be tuned only once, not in every place

where they're used. Like views, stored procedures also eliminate the overhead of sending

the queries from the client to the server as the queries are already on the server.


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Use Bind Variables

The use of bind variables in queries makes them generic and therefore re-usable.

For example, instead of writing a query like :-

SELECT name,addr FROM custs WHERE id = 12345;Change it to:-SELECT name,addr FROM custs WHERE id = ;

The first query will only be re-used when you request the details for customer number12345, whereas the second query will be re-used for any other customer.

For Better, Faster, Smarter, Oracle Solutions

SQL Tutorial - Hints And Tips To OptimiseYour SQL (ctd)

4. Use Selective Indexes

Ensure that tables are accessed via selective indexes, unless the table is very small or very

large, in which case it may be better not to use the indexes.

If the table were very small it could be cached completely, or all the columns could beindexed which means only the index would have to be read to satisfy any query.

Also make sure that you're not disabling the use of an index by:-

using an operator on the column (eg. + 1); the use of hints, if you're running Oracle(only applies if you're using the cost

based optimiser);

using NULL and not equal checks. (eg. 12345 ; or

IS NULL)


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5. Use Full-Table Scans

If the table is very large, depending on how many blocks are read, using an index mayremove everything else from the buffer cache and degrade the performance of all other

queries. In which case a full-table scan is better - only the last few blocks read are kept in

the buffer cache.

6. Optimise Joins

All other things being equal, the driving table is the one listed LAST in the

FROM clause, when using the rule-based optimiser. Changing the order of the

columns in the join condition does not change which table is used as the drivingtable. Choose the driving table carefully to ensure the minimum number of rows

are returned.

When using the cost-based optimiser, ensure that all the tables in the join have

been analysed (ask your dba), if they haven't, this may well cause poor

performance. The most usual way to optimise queries when using the cost-based

optimiser is to use hints, which instruct the parser as to which indexes should orshould not be used, or which tables should be scanned in full. You can also

experiment with the order of the tables in the join.

Indexes - these can still be used even if the where clause contains a "like"

condition but not if there is a "not like" condition.

Outer joins - the correct syntax for outer joins using Oracle syntax is:tab1.col1(+)= tab2.col1

or tab1.col1 = tab2.col1(+)

The bracketed plus sign follows the column of the table which has/may have themissing row.

An alternative is to use the ANSI standard outer join format which has the

advantge that it enables you to perform a full outer join in one statement:


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tab1.col1 left outer join tab2.col1 (return all rows from tab1)

tab1.col1 right outer join tab2.col1 (return all rows from tab2)

tab1.col1 full outer join tab2.col1 (return all rows from tab1

and tab2)

The final tip for this short tutorial is:

7. Name The Columns In A Query

There are three good reasons why it is better to name the columns in a query rather than

to use "select * from ...".

1. Network traffic is reduced. This can have a significant impact on performance ifthe table has a large number of columns, or the table has a long or long raw

column or have in-line clob or blob columns (all of which can be up to 2

Gigabytes in length). These types of columns will take a long time to transfer over

the network and so they should not be fetched from the database unless they arespecifically required.

2. The code is easier to understand, which means you need fewer comments!

3. It could save the need for changes in the future. If you are using views, not onlymight columns be added to or removed from the view, but the order of the

columns could well change - in which case using "SELECT *" at best would

fetch the wrong data and at worst would fail with an Oracle error which might

take a long while to understand.

This tutorial just scratches the surface of the subject, for more help see ourAdvancedSQL Tutorial

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For Better, Faster, Smarter, SQL Tutorials

Advanced SQL Tutorial

This advanced SQL tutorial focuses on the design of more complex SQL statements andthe strategies available for implementing them, it concentrates on sub queries and joins

because they are often inter-changeable, and views because these are often used to hide

the

complexity of queries involving sub-queries and joins.

As this is an advanced tutorial there is some consideration of performance issues, but this

aspect is more thoroughly explored in our series on Oracle performance tuning.

You might also be interested in our beginner's SQL tutorial, ourPL/SQL tutorials and our

Oracle tutorials
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Part 1 of this advanced SQL tutorial (this article) introduces sub-queries and looks

at non-correlated sub-queries. Part 2looks at the use of sub-queries. Part 3looks at nested

sub queries, sub-queries in the from clause and sub-queries that return no rows.Part 4 ofthis tutorial covers correlated sub-queries.

Sub Queries

Sub queries are also known as nested queries and are used to answer multi-part questions.

Sub queries and joins are often interchangeable and in fact the Oracle optimiser may welltreat a query containing a sub-query exactly as if it were a join.

Let's use a trivial example of finding the names of everybody who works in the same

department as a person called Jones to illustrate this point. The SQL could be written

using a sub query as follows:

SELECT name FROM emp WHERE dept_no =(SELECT dept_no FROM emp WHERE name = 'JONES')

or as a join statement, like this:-

SELECT e1.name FROM emp e1,emp e2

WHERE e1.dept_no = e2.dept_no AND e2name = 'JONES'

With a trivial example like this there would probably be very little difference in terms ofperformance of the SQL for such a simple query, but with more complex queries there

could well be performance implications. For this reason it is always worth trying a few

variations of the SQL for a query and examining the execution plans before deciding on aparticular approach, unless they're very simple queries.
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Learn more about Oracle performance tuning here.

Non Correlated Sub-Queries

There are, in fact, two types of sub query: correlated and non-correlated. The example

shown above is a non-correlated sub query. The difference between them is that acorrelated sub query refers to a column from a table in the parent query, whereas a non-

correlated sub query doesn't. This means that a non-correlated sub query is executed just

once for the whole SQL statement, whereas correlated sub queries are executed once perrow in the parent query.

Continue thisAdvanced SQL tutorial and learn more about sub-queries or seepart 4for

more on correlated sub queries.

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For Better, Faster, Smarter SQL Tutorials

Advanced SQL Tutorial (ctd)

This is part 2 of our Advanced SQL Tutorial. Part1introduced sub-queries and looked atnon-correlated sub-queries. This part looks at the uses of sub queries. There is also some

consideration of performance issues, but this aspect is more thoroughly explored in ourseries

on Oracle performance tuning. See also our beginner's SQL tutorial, the PL/SQL

tutorials and the otherOracle tutorials

Uses of Sub Queries

The most common use of sub queries is in the WHERE clause of queries to define the

limiting condition for the rows returned (i.e. what value(s) the rows must have to be ofinterest), as in the previous example. However, they can also be used in other
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parts of the query. Specifically, sub queries can be used:

to define the limiting conditions forSELECT, UPDATE and DELETE

statements in the following clauses:-o WHEREo HAVINGo START WITH

Instead of a table name in

o INSERT statementso UPDATE statementso DELETE

statements

o the FROM clause ofSELECT statements

To define the set of rows to be created in the target table of a CREATE TABLE

AS orINSERT INTO sql statement.

To define the set of rows to be included by a view or a snapshot in a CREATEVIEW orCREATE SNAPSHOT statement.

To provide the new values for the specified columns in an UPDATE statement

The first example of sub query in SQL shown above, used a simple equality expression as

we were interested in only one row, but we can also use the sub query to provide a set of

rows.

For example, to find the names of all employees in the same departments as Smith and

Jones, we could use the following SQL statement :-

SELECT name FROM emp WHERE dept_no IN(SELECT dept_no FROM emp WHERE name = 'JONES' OR name ='SMITH')

In fact, the original example could also return more than one row from the sub query ifthere were two or more people that were called Jones working in different departments.

In the first example a run-time SQL error would be generated in that case, because the


10/33

first example, by using '=', specified that the sub query should produce no more than one

row (it is perfectly legitimate for a sub query to return no rows).

We can reverse the question to ask for the names of all the employees that are NOT in thesame department as Jones, To do this, the sense of the sub query just has to be reversed

by prefixing it with 'NOT' or '!'. Again depending on whether there might be more thanone Jones, we would either use 'IN' or '='

SELECT name FROM emp WHERE dept_no NOT IN( SELECT dept_no FROM emp WHERE name = 'JONES')

Or

SELECT name FROM emp WHERE dept_no !=( SELECT dept_no FROM emp WHERE name = 'JONES')

Click on the link to continue this advanced SQL tutorial and learn about nested subqueries.

---------------------------------------

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For Better, Faster, Smarter, SQL Tutorials


This is the final part of our advanced SQL tutorial focusing on the design of more

complex SQL statements and sub queries in particular.Part 1 introduced sub-queries and

looked at non-correlated sub-queries.Part 2 covered the the uses of sub-queries andpart3 covered

nested sub queries, sub-queries in the from clause and sub-queries that return no rows.

The final part of our tutorial examines the use of correlated sub-queries. Performanceissues are only briefly mentioned as we have a whole series of articles on that topic (see

Oracle Performance Tuning).
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You might also be interested in ourbeginner's SQL tutorial,

ourPL/SQL tutorials and ourOracle tutorials.

Sub Queries (ctd)

Correlated Sub-Queries

As we've seen already, there are two types of sub query: correlated and non-correlated.

We've already looked at non-correlated sub queries (see advanced SQL tutorial part 1).

All of the examples of sub queries up until now have been non-correlated sub queries.

Just like non-correlated sub queries, correlated sub queries are used to answer multi-partquestions, but they are most often used to check for existence or absence of matching

records in the parent table and the related table in the sub query.

A correlated sub query refers to a column from a table in the parent query. As mentionedin part 1 sub-queries (both correlated and non-correlated) and joins are usually

interchangeable. However the SQL may be significantly faster when a correlated sub-

query is used as correlated sub queries refer to a column from their parent queries, they

are executed once per row in the parent query whereas non-correlated sub queries areexecuted once for the whole statement.

For example, using the emp and dept tables from before, to find out which departments

have no employees assigned to them, we can write the SQL statement in 3 different ways

- as a non-correlated sub query, as an outer join, or as a correlated sub-query.

Example 1 - non-correlated sub query

SELECT dept.name FROM deptWHERE dept.id NOT IN

(SELECT dept_idFROM emp
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WHERE dept_id IS NOT NULL)

Example 2- outer join

SELECT dept.name FROM dept,empWHERE emp.dept_id (+) = dept.id

Example 3 - correlated sub query

SELECT dept.name FROM deptWHERE NOT EXISTS (SELECT dept_id

FROM empWHERE emp.dept_id = dept.id)

The second example is an outer join SQL statement which may produce differnt reults to

the other 2 queries as it returns both matching rows and the non-matching rows on oneside of the join. In this case the query would return the names of departments which have

no employees assigned to them plus the names of those departments that do have

employees assigned to them.

The first and the third SQL statements would produce exactly the same results, but thefirst would probably be slower than the third if the dept_id column in the emp table were

indexed (depending on the sizes of the tables).

The first SQL statement can not use any indexes - the where clause of the sub query is

just checking for NOT NULL rows - so a full table scan would be performed. Also the

sub query would be executed once for each row in the dept table.

On the other hand, the sub query in the third example can use the index and since only

the dept_id is returned by the sub query, there is no need for any subsequent table access.

For these reasons, the third query would normally perform better than the first.

As you can see there are nearly always several ways in which the SQL for a query maybe written, and it is therefore best to try alternative SQL statements particularly for

complex queries before deciding on the preferred one.

This advanced SQL tutorial only touched on performance tuning which is a whole subject

in itself. For more help on Oracle performance tuning and other aspects of Oracle see theoracle tips and tricks section and see ourOracle resources section for book reviews and

links to other excellent Oracle resources.

---------------------------------------
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Looking for Oracle training?

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throughout the UKcontact us with your requirements or click on the link for theschedules, prices and course outlines for ourOracle SQL and PL/SQL training courses

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Introduction to Oracle Introduction to SQL Advanced SQL Oracle 8 Oracle 9i Oracle 10g Oracle PerformanceTuning PL/SQL More free guides and tutorialsOracle UNIX Linux

For Better, Faster, Smarter SQL Tutorials


This is part 3 of our advanced SQL tutorial focusing on the design of more complex SQL

statements and sub queries in particular.Part1 introduced sub-queries and looked at non-correlated sub-queries. Part 2 looked at the uses of sub-queries. This part will look at

nested sub queries, sub-queries in the from clause and sub-queries that return no

rows andpart 4covers correlated sub-queries.

Performance issues are not thoroughly explored even though this is an advanced sqltutorial as that subject is covered by another series of articles -Oracle Performance

Tuning.

Sub Queries (ctd)
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Nested Sub-Queries

The SQL syntax allows queries to be nested, meaning that a sub query itself can contain a

sub query, enabling very complex queries to be built as there is no syntacttical limit to thelevel of besting. However, very complex queries should be avoided as they are difficult to

understand and to maintain and may not perform that well either.

For example, the SQL statement to find the departments that have employees with a

salary higher than the average employee salary could be written as:

SELECT name FROM deptWHERE id IN

(SELECT dept_id FROM empWHERE sal >

(SELECT avg(sal)FROM emp)

)

Any of the other comparison operators instead of '=' or 'IN' such as '>', or '


17/33

Sub Queries That Return No Rows

Up until now the queries shown have all been expected to produce a result, but when

creating tables, it can be very useful to write the SQL to use a sub query which will notreturn any rows - when just the table structure is required and not any of the data.

In the following example we create a copy of the policy table with no rows:CREATE TABLE new_policy ASSELECT * from policy WHERE 1=0;

The sub query returns no data but does return the column names and data types to the

'create table' statement.

Click on the link to continue this advanced SQL tutorial and learn about correlated sub-

queries.


---------------------------------------

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Introduction to Oracle Introduction to SQL Advanced SQL Oracle 8 Oracle 9i Oracle 10g Oracle PerformanceTuning PL/SQL More free guides and tutorialsOracle UNIX Linux

For Better, Faster, Smarter, Oracle Training and

Consultancy

A Short Oracle Tutorial For Beginners

Introduction

This is just a brief Oracle tutorial for beginners, to provide a short history of databases

and Oracle's role in them, explain relational theory and give you an idea on howrelational databases work with a few examples. There is also a very brief discussion of

object-oriented design as it applies to databases.
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19/33

Time and space don't permit an in-depth

discussion of all the features available in Oracle, but if you would like to learn more just

contact usand ask for our free Oracle tutorial mini-course.

History of Databases - From Trees To Objects

The storage and management of data is probably the biggest headache for all businesses.

It has been so for a long while and is likely to continue for a long while too. As

companies try to store more and more details about their customers and their buyinghabits and as regulatory requirements for storing more data for longer, so companies will

need to store and manage more and more data. The only way this can be done at a

reasonable cost is by the use of computers.

In the late 1960s/early 1970s, specialised data management software appeared - the firstdatabase management systems (DBMS). These early DBMS were either hierarchical

(tree) or network (CODASYL) databases. These early systems were very complex andinflexible and so it adding new applications or reorganising the data was very difficultand time-consuming.

In 1970 the relational data model was defined by E.F. Codd (see "A Relational Model of

Data for Large Shared Data Banks" Comm. ACM. 13 (June 6, 1970), 377-387). This

delivered a solution to the problems of tree and network databases due to the concept ofnormalisation which involves the separation of the logical and physical representation of

data.

In 1974 IBM started a project called System/R to prove the theory of relational databases.

This led to the development of a query language called SEQUEL (Structured EnglishQuery Language) later renamed to Structured Query Language (SQL) for legal reasons

and now the query language of all databases.

In 1978 a prototype System/R implementation was evaluated at a number of IBM

customer sites. By 1979 the project finished with the conclusion that relational databaseswere a feasible commercial product.
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20/33

IBM's research into relational databases had also come to the attention of a group of

engineers in California who were so convinced of the potential that they formed a

company called Relational Software, Inc. in 1977 to build such a database. Their productwas called Oracle and the first version for VAX/VMS was released in 1979, thereby

becoming the first commercial rdbms, beating IBM to market by 2 years.

In the 1980s the company was renamed to Oracle Corporation. Throughout the 1980s,

new features were added and performance improved as the price of hardware came downand Oracle became the largest independent rdbms vendors. By 1985 they boasted of

having more than 1000 installations.

As relational databases became accepted, companies wanted to expand their use to storeimages, spreadsheets, etc. which can't be described in 2-dimensional terms. This led to

the Oracle database becoming an object-relational hybrid in version 8.0, i.e. a relational

database with object extensions, enabling you to have the best of both worlds.

This Oracle tutorial continues in part 2 with an explanation of relational databsesincluding a few examples.

Oracle tutorial part 2

---------------------------------------

Looking for more Oracle tutorials? Suggestions for further reading, book reviews andlinks to other resources for information about Oracle are available from the resources

section. For links to other articles and tutorials on SQL, PL/SQL and Oracle go toOracle

Tips & Tricks.

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As mentioned before, a relational database is based on the separation and independence

of the the logical and physical representations of the data. This provides enormous

flexibility and means you can store the data physically in any way without affecting howthe data is presented to the end user. The separation

of physical and logical layers means that you can change either layer without affecting

the other.

A relational database can be regarded as a set of 2-dimensional tables which are known

as "relations" in relational database theory. Each table has rows ("tuples") and columns("domains"). The relationships between the tables is defined by one table having a

column with the same meaning(but not necessarily value) as a column in another table.

For example consider a database with just 2 tables :

emp(id number,name varchar2(30)

,job_title varchar2(20),dept_id number)

holding employee information and

dept(id number,name varchar2(30))

holding department information.

There is an implied relationship between these tables because emp has a column called

dept_id which is the same as the id column in dept. In Oracle this is usually implementedby what's called a foreign-key relationship which prevents values being stored that arenot present in the referenced table.

Relational databases obtain their flexibility from being based on set theory (also known

as relational calculus) which enables sets or relations to be combined in various ways,

including:


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join/intersection

union (i.e. the sum of 2 sets);

exclusive "OR" (i.e. the difference between 2 sets)

and outer-join which is a combination of intersecting and exclusive or ing.

The intersection orjoinbetween 2 sets (in this case, tables) produces only thoseelements that exist in both sets.

Therefore, if we join Emp and Dept on department id, we will be left with only thoseemployees who work for a department that is in the dept table and only those departments

which have employees who are in the emp table.

The unionproduces the sum of the tables - meaning all records in Emp and all records in

Dept. and this may be with or without duplicates.

Let's use the following data to provide specific examples:

Emp

Id Name Dept Id

1 Bill Smith 3

2 Mike Lewis 2

3 Ray Charles 3

4 Andy Mallory 4

5 Mandy Randall 6

6 Allison White 1

Dept

Id Name

1 HR

2 IT

3 Marketing

4 Sales

5 Finance


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Thejoin of Emp and Dept. on the department id would produce the following result:

Emp.Id Emp.Name Dept.Id Dept.Name

1 Bill Smith 3 Marketing

2 Mike Lewis 2 IT

3 Ray Charles 3 Marketing

4 Andy Mallory 4 Sales

6 Allison White 1 HR

The union of Emp and Dept. would produce the following results

Id Name

1 Bill Smith

2 Mike Lewis

3 Ray Charles

4 Andy Mallory

5 Mandy Randall

1 HR

2 IT

3 Marketing

4 Sales

5 Finance

The union operator is only allowed when the number and data types of the columns in the2 sets are the same. It is not normally be used to combine sub sections from one or more

tables rather than entire tables.

There are other operators and variations but there isn't the space or the time to providefull details in this short Oracle tutorial.

The later versions of Oracle (Oracle 8 onwards) support both relational and object-

oriented features. The relational features are more prominent at the moment, but this is

beginning to change. In this context an object has both attributes and methods (programs


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stored with the object that performs a certain action or task) and in a true object-oriented

database would belong to a class and would allow multilevel inheritance.

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The resources section has recommendations for further reading to enhance your

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New subscribers can access Oracle 9i training material here and advice on general Oracle

performance tuning here

2. Oracle Questions

From: V.Mahesh - please send me details about constraints in oracle ( table level and

column level )

There are 5 different types of constraints available in Oracle NOT NULL, CHECK,

UNIQUE, PRIMARY KEY and FOREIGN KEY. All but NOT NULL can be definedat either the table level or the column level.

NOT NULL constraints prevent column values being left undefined and so can only be

defined at the column level.

Example:CREATE TABLE emp(
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id NUMBER NOT NULL,dept_id NUMBER,salary NUMBER)

The other constraints must be defined at the table level if they apply to more than one

column.

CHECK constraints can be applied to one or more columns and are used to restrict the

range of allowable values for those columns. For example suppose we define anemployee table EMP and want to ensure that the id is always positive, we could define it

as follows:

Learn how to develop your Oracle systems better, faster and smarter with high

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more info orclick here for details of our scheduled Oracle courses.

CREATE TABLE emp(id NUMBER CONSTRAINT emp_id_non_zero CHECK (id > 0),dept_id NUMBER,salary NUMBER)

UNIQUE and PRIMARY KEY constraints are very similar except that a table can have

only one primary key (but many unique columns are allowed) and all columns in the

primary key must also be NOT NULL.

Example:CREATE TABLE sales(invoice_no NUMBER UNIQUE,

..cust_id NUMBER)

CREATE TABLE sales(invoice_no NUMBER PRIMARY KEY,..cust_id NUMBER)

FOREIGN KEY constraints are used to define the relationship between one table and

another and to prevent orphaned records, thereby ensuring for example that all employees

belong to an existing department.

Example:CREATE TABLE emp(emp_id NUMBER ,dept_id NUMBER,salary NUMBER,CONSTRAINT fk_emp_dept FOREIGN KEY (dept_id) REFERENCESdept(id));
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From: Nath, Vivek - I have been doing large inserts in my database and on couple of

tables and the system has been degrading by the minute.Can i know some tips and tricks

how to manage this and improve the system performance???

There's no a lot of information to go on there, but there are some general things that can

be done. If a lot of large inserts have been made, it is possible that the index has becomeskewed or fragmented and may need rebuilding, but a better solution would be to drop

the indexes all together whilst the loading is taking place and re-create them once theload has finished. The same applies to constraints if they are enforced by indexes. You

might also want to check the size of the extents that are created to make sure they are the

optimal size for the type of application and the operating system you're using. One finalcheck would be to look at the queries that acccess the particular tables - are they using

full-table scans when an index read followed by table access by row id would be better or

vice versa.

For more Oracle performance tuning tips see:

http://www.smart-soft.co.uk/Oracle/oracle-tips-and-tricks.htm

---------

From: shiva shankar - What is the exact definition of normalisation.How many types are

there please explain them in detail

Normalization is the process of decomposing objects into their constituent parts,removing repeating groups and any items not functionaly dependent on the key. The

process was defined by Dr. E.F. Codd in 1970 when he defined the relational (database)

model.

There are 6 levels of normalization called normal forms (abbreviated NF): 1st normalform (1NF); 2nd normal form 2NF; 3rd normal form (3NF); Boyce-Codd normal form

(BCNF); 4th normal form (4NF); and 5th normal form (5NF). Each is a refinement of the

previous normal form, although it is very rare to progress beyond 3NF.

It should be stressed that normalization is an analysis tool, not a programming tool, so in

practice most databases will be denormalized to a certain extent to improve performance.

The trade off is redundant storage of information.

---------

From: "Manish" - how to see all the indexes created on a particular table? how to viewall the indexes in a tablespace?

To see all the indexes created on a particular table you need to query all_indexes

specifying the name of the table of interest as follows:

SELECT index_name FROM all_indexes WHERE table_name = 'EMP';
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To see all the indexes in a particular tablespace you just have to query all_indexes

specifying the name of the tablespace of interest as follows:

SELECT index_name FROM all_indexes WHERE tablespace_name ='USERS';

---------


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From: Bill Rowe - I read your articles on optimizing views and wondered what the

differences between cursors in packages and views might be. I've been reading

Feuerstein's O'Reilly books on PL/SQL and wonder if a cursor would work as well as aview, if the cursor were in a package specification.

This is an interesting question. On one level, views and cursors are completely different:cursors are defined in PL/SQL and views are defined in the database, but a more

fundamental level they are very similar - they are both pre-compiled queries so theycould be used in similar ways. However using views would provide more flexibility, they

can be combined into other queries which you could not do with cursors, they can also be

used for database updates which again you could not do with a cursor. The answer,therfore, is that a cursor would work as well as a view if you only need to use it for

querying the database and you don't want to combine the view with other queries.

---------

From: Hrishikesh Srivatsa - I have a question regarding a SQL query. There is anemployee table emp which has 3 fields namely EmpNo (Employee number) EmpName

(Employee name) MgrEmpNo (Employee's manager No). The CEO's Employee Manager

No can be assumed to be null or a special value.

A query has to be written to display the organization's heirarchy.

The answer to this is to use the hierarchical query clause of the select statement - i.e. usethe connect by prior keyword to indicate the relationship between the parent and

child columns and the start with keywords to indicate the root of the tree. In this

example, the query would be :

SELECT EmpName FROM empCONNECT BY PRIOR EmpNo = MgrEmpNoSTART WITH MgrEmpNo IS NULL;


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this was intended to be a list of hints and tips that you might find useful when using sq

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