Internal tables are holds the data which is having the same structure and storing it in working memory in ABAP. The data is stored line by line in the memory. The main purpose of internal table is for storing and formatting data from a database table within a program. It is used to minimize the DB access time in report programs.
Internal table are dynamic data objects, since they can contain any number of lines of a particular type. The maximum memory that can be occupied by an internal table (including its internal administration) is 2 gigabytes.
Types of Internal Tables :
1. Standard Internal Tables :
Standard tables have an internal linear index. 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. 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. WE can fill a standard table by appending lines (ABAP APPEND statement), and read, modify and delete entries by specifying the index (INDEX option with the relevant ABAP command).
2. Sorted tables :
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. Entries are inserted according to the sort sequence defined through the table key.
3. Hashed 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. we cannot access a hashed table using its index.
<b>Standard Internal tables:</b> Ordered and Indexed (Maintains the Order of intertion and can be read using INDEX).
<b>Sorted Table:</b> Ordered and Indexed. The table will be ordered based on the SORT by KEY of the table, can have both Unique and non-uique key. Can be used to READ using BINARY SEARCH.
<b>Hashed Tables:</b> Non Ordered No Indexed but always Unique.
Read only using UNIQUE KEY, NO INDEX read possible.
Faster interms of read as we always read by KEY and we use HASHING to store the data.
Internal tables provide a means of taking data from a fixed structure and storing it in working memory in ABAP. The data is stored line by line in memory, and each line has the same structure. In ABAP, internal tables fulfill the function of arrays. Since they are dynamic data objects, they save the programmer the task of dynamic memory management in his or her programs. You should use internal tables whenever you want to process a dataset with a fixed structure within a program. A particularly important use for internal tables is for storing and formatting data from a database table within a program. They are also a good way of including very complicated data structures in an ABAP program.
Like all elements in the ABAP type concept, internal tables can exist both as data types and as data objects A data type is the abstract description of an internal table, either in a program or centrally in the ABAP Dictionary, that you use to create a concrete data object. The data type is also an attribute of an existing data object.