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
GBase 8s SQL 指南:教程
南大通用数据技术股份有限公司 - 95 -
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