Question

我在具有潜在NULL值的字段上使用ARRAY_AGG，然后我想要稍后检查该数组是否为NULL。例如：

WITH test AS
(
SELECT 'A' AS call_id, NULL AS outcome
UNION ALL
SELECT 'A' AS call_id, 'success' AS outcome
)

SELECT *
  FROM (
        SELECT call_id, ARRAY_AGG(outcome) AS outcome
          FROM test
          GROUP BY call_id
       ) AS sub
  WHERE outcome && ARRAY[NULL]

但是这个查询的结果是没有返回任何行。 ARRAY_AGG将创建一行call_id =＆＃39; A＆＃39;并且outcome数组等于{，＆＃39;成功＆＃39;}。如何检查此数组是否为NULL值？

Answer 1

除了编写函数之外，我无法想到将直接使用数组执行此操作。但是在聚合期间计算NULL值呢？

WITH test (call_id, outcome) AS
(
  values 
    ('A', null), 
    ('A', 'success'), 
    ('B', 'failure')
)
SELECT *
FROM ( 
  SELECT call_id, 
         array_agg(outcome) AS outcome, 
         count(case when outcome is null then 1 end) as null_count
  FROM test
  GROUP BY call_id
) AS sub
where null_count > 0;

您还可以使用having子句将其直接集成到select语句中：

SELECT call_id, 
       array_agg(outcome) AS outcome
FROM test
GROUP BY call_id
having count(case when outcome is null then 1 end) > 0

另一种选择是将NULL值替换为outcome列中不存在的值，然后检查该“魔法”值。

Answer 2

你可以尝试取消嵌套你的outcome数组并检查它是否存在空值。像这样：

WITH test AS
(
  SELECT 'A' AS call_id, null AS outcome
  UNION ALL
  SELECT 'A' AS call_id, 'success' AS outcome
)
SELECT *
FROM (
  SELECT call_id, ARRAY_AGG(outcome) AS outcome
  FROM test
  GROUP BY call_id
) AS sub
WHERE EXISTS (select * from unnest(outcome) where unnest is null)

ARRAY_AGG中的NULLS和检查数组中的空值（Postgres 9.5.1）

2 个答案: