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To use collections in an application, you define one or more PL/SQL types, then define variables of those types.You can define collection types in a procedure, function, or package.Nested tables and varrays can also be attributes of object types. You can think of them as one-dimensional arrays with no declared number of elements.You can model multi-dimensional arrays by creating nested tables whose elements are also nested tables.howmany := continent_population(continent_population. LAST); END; / Associative arrays help you represent data sets of arbitrary size, with fast lookup for an individual element without knowing its position within the array and without having to loop through all the array elements.

Oracle stores the rows of a nested table in no particular order.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.To reference an element, you use standard subscripting syntax (see Figure 5-2).For example, Associative arrays are sets of key-value pairs, where each key is unique and is used to locate a corresponding value in the array. Assigning a value using a key for the first time adds that key to the associative array.

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