more Sybase Interview Questions and answers

What is recursive stored procedure?

you can write stored procedures that call themselves. Recursion can be defined as a method of problem solving wherein the solution is arrived at by repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps. Listing 1–31 presents an example that features a stored procedure that calculates the factorial of a number:

Differences between clustered and non-clustered



I'd like to talk about the difference between a clustered and a non-clustered
index. The two are very different and it's very important to understand the
difference between the two to in order to know when and how to use each.

I've pondered hard to find the best analogy that I could think of and I've come
up with ... the phone book. Yes, a phone book.

Imagine that each page in our phone book is equivalent to a Sybase 2K data
page. Every time we read a page from our phone book it is equivalent to one
disk I/O.

Since we are imagining, let's also imagine that our mythical ASE (that runs
against the phone book) has only enough data cache to buffer 200 phone pages.
When our data cache gets full we have to flush an old page out so we can read
in a new one.

Fasten your seat belts, because here we go...

Clustered Index

A phone book lists everyone by last name. We have an A section, we have a B
section and so forth. Within each section my phone book is clever enough to
list the starting and ending names for the given page.

The phone book is clustered by last name.

create clustered index on phone_book (last_name)

It's fast to perform the following queries on the phone book:

* Find the address of those whose last name is Cisar.
* Find the address of those whose last name is between Even and Fa

Searches that don't work well:

* Find the address of those whose phone number is 440-1300.
* Find the address of those whose prefix is 440

In order to determine the answer to the two above we'd have to search the
entire phone book. We can call that a table scan.

Non-Clustered Index

To help us solve the problem above we can build a non-clustered index.

create nonclustered index on phone_book (phone_number)

Our non-clustered index will be built and maintained by our Mythical ASE as

1. Create a data structure that will house a phone_number and information
where the phone_number exists in the phone book: page number and the row
within the page.

The phone numbers will be kept in ascending order.

2. Scan the entire phone book and add an entry to our data structure above for
each phone number found.
3. For each phone number found, note along side it the page number that it was
located and which row it was in.

any time we insert, update or delete new numbers, our M-ASE will maintain this
secondary data structure. It's such a nice Server.

Now when we ask the question:

Find the address of those whose phone number is 440-1300

we don't look at the phone book directly but go to our new data structure and
it tells us which page and row within the page the above phone number can be
found. Neat eh?

Draw backs? Well, yes. Because we probably still can't answer the question:

Find the address of those whose prefix is 440

This is because of the data structure being used to implement non-clustered
indexes. The structure is a list of ordered values (phone numbers) which point
to the actual data in the phone book. This indirectness can lead to trouble
when a range or a match query is issued.

The structure may look like this:

|Phone Number | Page Number/Row |
| 440-0000 | 300/23 |
| 440-0001 | 973/45 |
| 440-0002 | 23/2 |
| ... | |
| 440-0030 | 973/45 |
| 440-0031 | 553/23 |
| ... | |

As one can see, certain phone numbers may map to the same page. This makes
sense, but we need to consider one of our constraints: our Server only has room
for 200 phone pages.

What may happen is that we re-read the same phone page many times. This isn't a
problem if the phone page is in memory. We have limited memory, however, and we
may have to flush our memory to make room for other phone pages. So the
re-reading may actually be a disk I/O.

The Server needs to decide when it's best to do a table scan versus using the
non-clustered index to satisfy mini-range type of queries. The way it decides
this is by applying a heuristic based on the information maintained when an
update statistics is performed.

In summary, non-clustered indexes work really well when used for highly
selective queries and they may work for short, range type of queries.

Suggested Uses

Having suffered many table corruption situations (with 150 ASEs who wouldn't? :
-)), I'd say always have a clustered index. With a clustered index you can fish
data out around the bad spots on the table thus having minimal data loss.

When you cluster, build the cluster to satisfy the largest percentage of range
type queries. Don't put the clustered index on your primary key because
typically primary keys are increasing linearly. What happens is that you end up
inserting all new rows at the end of the table thus creating a hot spot on the
last data page.

For detail rows, create the clustered index on the commonly accessed foreign
key. This will aid joins from the master to it.

Use nonclustered index to aid queries where your selection is very selective.
For example, primary keys. :-)

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What is the difference between group by and order by

Group by controls the presentation of the rows, order by controls the presentation of the columns for the results of the SELECT statement. The GROUP BY keyword is used when we are selecting multiple columns from a table (or tables) and at least one arithmetic operator appears in the SELECT statement.

What is cascade delete

Normally, if you try and delete a record from a table that is constrained by a foreign key, you?ll get an error message. This procedure checks for any foreign keys for the table, deletes any child records, then deletes the intended record.
It references the system tables sysforeignkeys, sysobjects and syscolumns. Sysforeignkeys does what it says on the tin ? it?s a list of all foreign keys in the database. It doesn?t contain actual table and field names, instead it contains links to the sysobjects (tables, stored procedures, views etc) and syscolumns (fields).

The procedure works like this ? if we want to delete a record from table X, we look in the sysforeignkeys table for all references where table X is the parent table. It may be involved in several such FK?s. All we do is recursively go through these FK?s, deleting the child table records that are linked to the record we want to delete.

For example,

- delete all records from table X where field1 equals '234'

DELETE FROM X WHERE field1 = '234'
- table Y is linked to X through the Y_ID field, so

- table Z is linked to Y through the Z_ref field

(SELECT Y_ID FROM X WHERE field1 = '234'))
As you can see from the above example, if one of the child tables is also involved in a FK constraint, we also need to delete the relating rows in it?s child tables.

How would you join table A (containing 500 rows) and table B
(containing 1000 rows)?

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