Internal table is a temporary table that is created, used during the program execution, and deleted before the program terminated. Internal tables used to store the dynamic data sets from a fixed structure in the main/working memory in ABAP. The lifetime of the table is until end of the program execution. Internal tables fulfill the function of arrays in ABAP.

The Internal table type are -
Standard Internal Tables -
This table type is particularly appropriate if we want to address individual table entries using the index. This is the quickest way to access table entries. To fill a standard table, append lines using the (APPEND) statement. Standard tables have a linear index. We can access them using either the index or the key. If we use the key, the response time is in linear relationship to the number of table entries. The key of a standard table is always non-unique, and we may not include any specification for the uniqueness in the table definition.

Sorted Internal Tables -
Sorted tables are always saved sorted by key. They also have a linear key. Like standard tables, we can access them using either the table index or the key. The key of a sorted table can be either unique, or non-unique, and we should specify either UNIQUE or NON-UNIQUE in the table definition. Standard tables and sorted tables both belong to the generic group index tables. This table type is particularly suitable if we want the table to be sorted while we are still adding entries to it.

Hashed Internal Tables -
Hashes tables have no internal linear index. We can only access hashed tables by specifying the key. The response time is constant, regardless of the number of table entries, since the search uses a hash algorithm. The key of a hashed table must be unique, and we must specify UNIQUE in the table definition. This table type is particularly suitable if we want mainly to use key access for table entries. We cannot access hashed tables using the index.

Index Tables -
Index table is only used to specify the type of generic parameters in a FORM or FUNCTION. That means we can't create a table of type INDEX. Internal tables are not DB tables. Standard and Sorted tables in combined are basically called as Index tables and there nothing else.

Both are temporary memory areas and are used to store the data at run-time.
Internal tables and work areas represents the instances of database tables.

Append statement used to add a record at the end of internal table in work area.
Insert statement used to add a record at specified location of internal table in work area.

8KB memory is allocated to internal table and work areas initially and increases dynamically when required.

There are three types of internal tables and those are -

1. Line
2. Key
3. Table

No, records will be displayed.

Sum: We can only use this statement within a LOOP. If we use SUM in an AT - ENDAT block, the system calculates totals for the numeric fields of all lines in the current line group and writes them to the corresponding fields in the work area. If you use the SUM statement outside an AT - ENDAT block, the system calculates totals for the numeric fields of all lines of the internal table in each loop pass and writes them to the corresponding fields of the work area.
COLLECT: It allows us to create unique or summarized datasets. The system first tries to find a table entry corresponding to the table key. The key values are taken either from the header line of the internal table itab, or from the explicitly specified work area wa.

• It is a standard data type object, that exists only during the runtime of the program. They are used to perform table calculations on subsets of database tables and for re-organizing the contents of database tables according to users need.
• Using SY-DBCNT.
• The number of memory allocations the system needs to allocate for the next record population.

Key, Line, and Table are the three different data types of internal tables.

The append statement is used to add a record at the end of the internal table.
The Insert statement used to add a record at a specific location.

Data clusters provides a way to group every type of complex internal data object from an application in ABAP/4. They are also placed in the memory of ABAP/4 for a limited time or for a longer time. The databases of this type have the name accordingly, they are named cluster databases and their construction as predefined. The placing of this type of data clusters is a feature of ABAP/4. With the support of SQL statements, it is possible to enter cluster databases and the data clusters that have been placed in ABAP/4 can be decoded only by ABAP/4.

We should configure the occurs value to make optimizations. Below are some notes to keep in mind before configuration -

• The default size declared should maintain in the roll area for the faster program access.
• The whole data area of the application should be less than 64 kilobytes.
• The data inserted that is bigger than the default size is placed in the roll file and the program getting accessed slow. So the data should be lesser size than default size for insertion.

SELECT * FROM SPFLI INTO TABLE ITAB PACKAGE SIZE N (N being the variable). We should give a name to the WILDCARD (*) characters that are meant to be compared with numeric and character strings.

The MODIFY LINE statement is meant for changing the lines of a full list from inside the program.
READ LINE statement is used for reading data on the lines from the current levels. READ LINE is the same as READ CURRENT LINE and they are both related to HIDE.