[英]Count NULL values by column in SQL
Suppose I have the following table:假设我有下表:
table
| a | b | c |
|:-----|:----|:-----|
| 1 | a | NULL |
| NULL | b | NULL |
| 3 | c | NULL |
| 4 | d | 23 |
| NULL | e | 231 |
How can I count the number of NULL values by each column?如何计算每列的 NULL 值数量?
My final result would be:我的最终结果是:
| column_name | n_nulls |
|:---------------|:----------|
| a | 2 |
| b | 0 |
| c | 3 |
You can use union all
:您可以使用union all
:
select 'a', count(*) - count(a) as n_nulls from t
union all
select 'b', count(*) - count(b) as n_nulls from t
union all
select 'c', count(*) - count(c) as n_nulls from t;
Redshift is a column-store database, so there probably is not a more efficient method. Redshift 是列存储数据库,因此可能没有更有效的方法。
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