collection 方法是一个内置的 PL/SQL 子程序，可以返回 collection 信息，或是在 collection 上执行操作，很方便。你可以通过点记
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collection 方法是一个内置的 PL/SQL 子程序，可以返回 collection 信息，或是在 collection 上执行操作，很方便。
你可以通过点记号来调用 collection 方法。语法如下图所示：
图1 Collection Method 调用
不能在 SQL 语句调用 collection 方法。
当 collection 为空时，你只能使用 EXISTS 方法，使用其他方法都会抛出 COLLECTION_IS_NULL 异常。
EXISTS 方法
若 collection 中第 n 个元素存在，则 EXISTS(n) 返回 TRUE；否则，返回 FALSE。EXISTS 方法结合 DELETE 方法，会把 collection 变成稀疏 nested tables（sparse nested tables）。通过 EXISTS 方法，避免引用一个不存在的元素，从而产生异常。当传递一个超出范围的标值时，EXISTS 方法返回 FALSE，而不是产生 SUBSCRIPT_OUTSIDE_LIMIT 异常。
示例1：演示检查元素是否存在
; n NumList := NumList(1,3,5,7); DBMS_OUTPUT.PUT_LINE( DBMS_OUTPUT.PUT_LINE('OK, element #99 does not exist at all.'); END IF;END;/
COUNT 方法
COUNT 返回 collection 中元素的当前数量。当你不知道 collection 中有多少元素时，很有用。例如，当你把获取的表的一列，放到一个 nested table 时，元素的数量取决于结果集的大小。
对于 varray，COUNT 总是等于 LAST。通过 EXTEND 和 TRIM 方法，你可以增加或减少 varray 的大小，，因此，COUNT 值是变化的，取决于 LIMIT 方法的值。
对于 nested tables，COUNT 方法通常等于 LAST 方法。然而，若你从 nested table 删除元素，则 COUNT 小于 LAST。当你整理元素时，COUNT 会忽略已删除的元素。使用不带参数的 DELETE 方法会设置 COUNT 为 0。
备注：FIRST 方法和 LAST 方法返回最大和最小的索引数。后面说明。
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