Why collections in oracle

Therefore, to pass an associative array variable as a parameter to a standalone subprogram, you must declare the type of that variable in a package specification. Doing so makes the type available to both the invoked subprogram which declares a formal parameter of that type and the invoking subprogram or anonymous block which declares and passes the variable of that type.

See Example An associative array is intended for temporary data storage. To make an associative array persistent for the life of a database session, declare it in a package specification and populate it in the package body.

A varray variable-size array is an array whose number of elements can vary from zero empty to the declared maximum size. The lower bound of index is 1; the upper bound is the current number of elements. The upper bound changes as you add or delete elements, but it cannot exceed the maximum size. When you store and retrieve a varray from the database, its indexes and element order remain stable. Figure shows a varray variable named Grades , which has maximum size 10 and contains seven elements.

Grades n references the n th element of Grades. The upper bound of Grades is 7, and it cannot exceed The database stores a varray variable as a single object. If a varray variable is less than 4 KB, it resides inside the table of which it is a column; otherwise, it resides outside the table but in the same tablespace. An uninitialized varray variable is a null collection. You must initialize it, either by making it empty or by assigning a non- NULL value to it. Appropriate Uses for Varrays. Table for a summary of varray characteristics.

This example defines a local VARRAY type, declares a variable of that type initializing it with a constructor , and defines a procedure that prints the varray. The example invokes the procedure three times: After initializing the variable, after changing the values of two elements individually, and after using a constructor to the change the values of all elements.

For an example of a procedure that prints a varray that might be null or empty, see Example Because you must store or retrieve all elements at the same time, a varray might be impractical for large numbers of elements. In the database, a nested table is a column type that stores an unspecified number of rows in no particular order.

Using these indexes, you can access the individual rows of the nested table variable. The indexes and row order of a nested table might not remain stable as you store and retrieve the nested table from the database. The amount of memory that a nested table variable occupies can increase or decrease dynamically, as you add or delete elements.

An uninitialized nested table variable is a null collection. Appropriate Uses for Nested Tables. Table for a summary of nested table characteristics. This example defines a local nested table type, declares a variable of that type initializing it with a constructor , and defines a procedure that prints the nested table. The example invokes the procedure three times: After initializing the variable, after changing the value of one element, and after using a constructor to the change the values of all elements.

After the second constructor invocation, the nested table has only two elements. Referencing element 3 would raise error ORA Conceptually, a nested table is like a one-dimensional array with an arbitrary number of elements. However, a nested table differs from an array in these important ways:. An array has a declared number of elements, but a nested table does not. The size of a nested table can increase dynamically.

An array is always dense. A nested array is dense initially, but it can become sparse, because you can delete elements from it. Figure shows the important differences between a nested table and an array. Nested table data is stored in a separate store table, a system-generated database table. When you access a nested table, the database joins the nested table with its store table.

This makes nested tables suitable for queries and updates that affect only some elements of the collection. You would create a separate lookup table, with multiple entries for each row of the main table, and access it through join queries. A collection constructor constructor is a system-defined function with the same name as a collection type, which returns a collection of that type. This topic applies only to varrays and nested tables. Associative arrays do not have constructors.

In this topic, collection means varray or nested table. If the parameter list is empty, the constructor returns an empty collection. Otherwise, the constructor returns a collection that contains the specified values. You can assign the returned collection to a collection variable of the same type in the variable declaration and in the executable part of a block. This example invokes a constructor twice: to initialize the varray variable team to empty in its declaration, and to give it new values in the executable part of the block.

For an example of a procedure that prints a varray that might be null, see Example Data Type Compatibility. You can assign a collection to a collection variable only if they have the same data type. Having the same element type is not enough.

Collection variables group1 and group2 have the same data type, triplet , but collection variable group3 has the data type trio. The assignment of group1 to group2 succeeds, but the assignment of group1 to group3 fails.

To a varray or nested table variable, you can assign the value NULL or a null collection of the same data type. Either assignment makes the variable null. The elements of the two nested tables must have comparable data types. The SQL SET function takes a nested table argument and returns a nested table of the same data type whose elements are distinct the function eliminates duplicate elements. Although a collection has only one dimension, you can model a multidimensional collection with a collection whose elements are collections.

In this example, nva is a two-dimensional varray—a varray of varrays of integers. In this example, ntb1 is a nested table of nested tables of strings, and ntb2 is a nested table of varrays of integers.

In this example, aa1 is an associative array of associative arrays, and ntb2 is a nested table of varrays of strings. To determine if one collection variable is less than another for example , you must define what less than means in that context and write a function that returns TRUE or FALSE. You cannot compare associative array variables to the value NULL or to each other.

Except for Comparing Nested Tables for Equality and Inequality , you cannot natively compare two collection variables with relational operators. This restriction also applies to implicit comparisons.

Comparing Nested Tables for Equality and Inequality. Table This example compares a varray variable and a nested table variable to NULL correctly. Two nested table variables are equal if and only if they have the same set of elements in any order.

This example compares nested table variables for equality and inequality with relational operators. You can compare nested table variables, and test some of their properties, with SQL multiset conditions. Collection methods make collections easier to use and your applications easier to maintain.

Table summarizes the collection methods. Returns TRUE if and only if specified element of varray or nested table exists. For detailed syntax, see " Collection Method Invocation ". In a subprogram, a collection parameter assumes the properties of the argument bound to it. You can apply collection methods to such parameters. For varray parameters, the value of LIMIT is always derived from the parameter type definition, regardless of the parameter mode.

Therefore, the deleted elements are included in the internal size of the collection, and you can restore a deleted element by assigning a valid value to it.

This example declares a nested table variable, initializing it with six elements; deletes and then restores the second element; deletes a range of elements and then restores one of them; and then deletes all elements. The restored elements occupy the same memory as the corresponding deleted elements. This example populates an associative array indexed by string and deletes all elements, which frees the memory allocated to them. Next, the example replaces the deleted elements—that is, adds new elements that have the same indexes as the deleted elements.

The new replacement elements do not occupy the same memory as the corresponding deleted elements. Finally, the example deletes one element and then a range of elements.

TRIM is a procedure that deletes elements from the end of a varray or nested table. TRIM operates on the internal size of a collection. Therefore, TRIM can delete a deleted element. Therefore, trimmed elements are not included in the internal size of the collection, and you cannot restore a trimmed element by assigning a valid value to it.

This example declares a nested table variable, initializing it with six elements; trims the last element; deletes the fourth element; and then trims the last two elements—one of which is the deleted fourth element. The collection can be empty, but not null. To make a collection empty or add elements to a null collection, use a constructor. For more information, see " Collection Constructors ". This example declares a nested table variable, initializing it with three elements; appends two copies of the first element; deletes the fifth last element; and then appends one null element.

EXISTS is a function that tells you whether the specified element of a varray or nested table exists. This example initializes a nested table with four elements, deletes the second element, and prints either the value or status of elements 1 through 6.

For an associative array indexed by string, the first and last elements are those with the lowest and highest key values, respectively. FIRST and team. Because a varray is always dense, team i inside the loop always exists.

An Index By table is better called as the docs do an Associative Array. These are simple collections of single attributes with an index. Nested tables also have indexes but their indexes are just row counts. With an associative array the index can be meaningful, i. So they are useful for caching data values for later use. The index can be a number, or since 9iR2 a string which can be very useful.

For instance, here is an associative array of salaries which is indexed by the employee identifier. Other than caching reference tables or similar look-up values there aren't many use cases for associative arrays.

Variable arrays are just nested tables with a pre-defined limit on the number of elements. So perhaps the name is misleading: they are actually fixed arrays. There's little we can do with VArrays which we can't do with nested tables except constrain the number of elements and it's pretty rare that we would want to do that. They are declared like this:. However we must be certain the query will return at most the number of elements specified in the VArray's declaration.

There are no known use cases for variable arrays. Okay that's a bit harsh, but almost all of the time you will use nested tables instead. The one big advantage of VArrays over nested tables is that they guarantee the order of the elements. So if you must get elements out in the same order as you inserted them use a VArray. Find out more. Associative arrays, also known as index-by tables, let you look up elements using arbitrary numbers and strings for subscript values.

These are similar to hash tables in other programming languages. Nested tables hold an arbitrary number of elements. They use sequential numbers as subscripts. You can define equivalent SQL types, allowing nested tables to be stored in database tables and manipulated through SQL. Varrays short for variable-size arrays hold a fixed number of elements although you can change the number of elements at runtime. You can define equivalent SQL types, allowing varrays to be stored in database tables.

They can be stored and retrieved through SQL, but with less flexibility than nested tables. Both nested tables and associative arrays formerly known as index-by tables use similar subscript notation, but they have different characteristics when it comes to persistence and ease of parameter passing.

Nested tables can be stored in a database column, but associative arrays cannot. Nested tables can simplify SQL operations where you would normally join a single-column table with a larger table. Associative arrays are appropriate for relatively small lookup tables where the collection can be constructed in memory each time a procedure is called or a package is initialized. They are good for collecting information whose volume is unknown beforehand, because there is no fixed limit on their size.

Their index values are more flexible, because associative array subscripts can be negative, can be nonsequential, and can use string values instead of numbers. Each varray is stored as a single object, either inside the table of which it is a column if the varray is less than 4KB or outside the table but still in the same tablespace if the varray is greater than 4KB. You must update or retrieve all elements of the varray at the same time, which is most appropriate when performing some operation on all the elements at once.

But you might find it impractical to store and retrieve large numbers of elements this way. You would usually create a separate lookup table, with multiple entries for each row of the main table, and access it through join queries.

Nested tables can be sparse: you can delete arbitrary elements, rather than just removing an item from the end. Nested table data is stored in a separate store table, a system-generated database table associated with the nested table. The database joins the tables for you when you access the nested table. This makes nested tables suitable for queries and updates that only affect some elements of the collection.

You cannot rely on the order and subscripts of a nested table remaining stable as the nested table is stored in and retrieved from the database, because the order and subscripts are not preserved in the database.

What is the difference between nested array and associative array? A nested table must be initialized as shown. The NOT keyword can be included to get the inverse. If the collection is not initialized the function will return NULL.

An initialised and empty collection will return true. The OF keyword is optional, but makes the code more readable. Having duplicate values in the subset results in false, if those duplicates are not present in the main set.

In the following example we return only those subsets that have a cardinality of 2. The SET function returns a collection containing the distinct values from a collection.

In addition to regular data types, collections can be based on record types, allowing the creation of two-dimensional collections.