Why we need internal tables how it has its own significance for its purpose

Question

we can maintain tables in oracle also and some other features .but how internal tables differ from them in its functioning

Answer 1

u2022 In ABAP/4, you work mainly with tables. Tables are the essential data structures in the R/3 System. Long-life data is stored in relational database tables.
u2022 Besides database tables, you can create internal tables which exist only during the runtime of your program. ABAP/4 provides various operations for working with internal tables. You can, for example, search for, append, insert, or delete lines.
u2022 The number of lines in an internal table is not fixed. Depending on requirements, the system increases the size of internal tables at runtime.
u2022 You can use internal tables to perform table calculations on subsets of database tables. For example, you can read a certain part of a database table into an internal table (see Reading Data into an Internal Table).
From the internal table, you can then calculate totals or generate a ranked list.
In ABAP/4, you can distinguish between internal table data types, which define the structure of internal tables, and internal table data objects, which are the actual internal tables and can be filled with data. An internal table data type is an abstract definition of a data structure which can be used to declare data objects as internal tables.
u2022 Another use for internal tables is reorganizing the contents of database tables according to the needs of your program. For example, you can read data relevant for creating a telephone list from one or several large customer tables into an internal table. During the runtime of your program, you can then access this list directly without having to perform a time-consuming database query for each call.
Table type
The table type determines how ABAP will access individual table entries. Internal tables can be divided into three types:
Standard tables have an internal linear index. From a particular size upwards, the indexes of internal tables are administered as trees. In this case, the index administration overhead increases in logarithmic and not linear relation to the number of lines. The system can access records either by using the table index or the key. The response time for key access is proportional to the number of entries in the table. The key of a standard table is always non-unique. You cannot specify a unique key. This means that standard tables can always be filled very quickly, since the system does not have to check whether there are already existing entries.
Sorted tables are always saved sorted by the key. They also have an internal index. The system can access records either by using the table index or the key. The response time for key access is logarithmically proportional to the number of table entries, since the system uses a binary search. The key of a sorted table can be either unique or non-unique. When you define the table, you must specify whether the key is to be UNIQUE or NON-UNIQUE. Standard tables and sorted tables are known generically as index tables.
Hashed tables have no linear index. You can only access a hashed table using its key. The response time is independent of the number of table entries, and is constant, since the system access the table entries using a hash algorithm. The key of a hashed table must be unique. When you define the table, you must specify the key as UNIQUE.
I hope it helps.
Have a look at below link for details:
[Internal Tables|http://help.sap.com/saphelp_nw70/helpdata/en/fc/eb35de358411d1829f0000e829fbfe/frameset.htm]
Best Regards,
Vibha
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