基数函数

CARDINALITY 函数对集合包含的元素数目计数。可以将 CARDINALITY 函数与简单或嵌套集合配合使用。将集合中的任何重复作为个别元素计数。下列查询显示一个查询，它对 manager 表中的每一列返回 department 值和每个 direct_reports 集合中的元素数。

图: 查询

SELECT department, CARDINALITY(direct_reports) FROM manager;

图: 查询结果

department   marketing 5
          
          department   engineering 7
          
          department   publications 4
          
          department   accounting 3

还可以从谓词表达式中对集合的元素数进行求值，如下所示。

SELECT department, CARDINALITY(direct_reports) FROM manager
          WHERE CARDINALITY(direct_reports) < 6 
          GROUP BY department;

图: 查询结果

department   accounting 3
            
            department   marketing 5
            
            department   publications 4