data dictionary--keys

[Former Member]

what is difference between unique and non-unique key?plz explain with e.g.

[amit_khare]

Refer the threads -

Regards,

Amit

[Former Member]

hi,

The data type of an internal table is fully specified by its:

Line type

The line type defines the attributes of the individual fields. You can specify any ABAP data type.

Key definition

The key fields and their sequence determine the criteria by which the system identifies table lines.

Key type

You can define the key as either unique or non-unique. The uniqueness of the key must be compatible

with the access type you have chosen for the table. If the key is unique, there can be no duplicate

entries in the table.

Access type

Unlike database tables, the system assigns line numbers to certain kinds of internal tables. This means

that you can use the index to access lines as well as the key. We sometimes use the term "table type"

to refer to this.

A unique key is only possible with sorted and hashed tables

With a standard table, inserting an entry has the same effect as appending. With sorted tables with a nonunique

key, the entry is inserted before the first (if any) entry with the same key.

To read individual data records using the first variant, all fields of <wa1> that are key fields of <itab> must

be filled. <wa1> and <wa2> can be identical. If you use the WITH TABLE KEY addition in the second

variant, you must also specify the key fully. Otherwise, the system searches according to the sequence of

fields that you have specified, using a binary search where possible.

t_cust_list TYPE TABLE OF t_cust

WITH NON-UNIQUE KEY id.

If you want to access your data using the index and do not need your table to be kept in sorted order or to

have a unique key, that is, when the sequence of the entries is the most important thing, not sorting by

key or having unique entries, you should use standard tables.

flight_list TYPE SORTED

TABLE OF sflight

WITH UNIQUE KEY carrid connid fldate.

When you choose to use a sorted table, it will normally be because you want to define a unique key. The

mere fact that the table is kept in sorted order is not that significant, since you can sort any kind of internal

table. However, with sorted tables (unlike hashed tables), new data records are inserted in the correct sort

order. If you have a table with few entries but lots of accesses that change the contents, a sorted table

may be more efficient than a hashed table in terms of runtime.

Since the internal table represents a database table in this case, you should ensure that its records have

unique keys.

t_city_list TYPE HASHED

TABLE OF t_city

WITH UNIQUE KEY city country.

The hash algorithm calculates the address of an entry based on the key. This means that, with larger

tables, the access time is reduced significantly in comparison with a binary search. In a loop, however, the

hashed table has to search the entire table (full table scan). Since the table entries are stored unsorted, it

would be better to use a sorted table if you needed to run a loop through a left-justified portion of the key. It

can also be worth using a hashed table but sorting it.

A typical use for hashed tables is to buffer detailed information that you need repeatedly and can identify

using a unique key.

Hope this is helpful, Do reward.

Edited by: Runal Singh on Feb 6, 2008 5:46 PM