postgresql - postgres中的子查询错误

Question

我有下表，customers_categorization:

customer__id    visit_month    cust_type
2170809793      0              lagger
2170809793      10             retained
2170809793      11             retained
2170809793      12             lagger
2170809793      17             lagger
2170809793      23             retained
2170809793      24             lost
2174188545      0              lost

我正在尝试在 Postgresql 中编写一个 SQL 查询，我想在其中找到按月返回的客户比例:

这是我目前所拥有的:

select visit_month,
       count(select customer__id from customers_categorization where cust_type=’retained’)/count(customer__id) as retention_rate
    from customers_categorization
    group by 1;

我不断在“select”或“select”附近收到“语法错误”。我不确定为什么这不起作用。postgresql 是否不接受带有 select 语句的子查询？

任何帮助都会很棒!

Answer 1

使用 count() 和 filter 的语法:

select
    visit_month,
    count(customer__id) filter (where cust_type = 'retained') /
        count(customer__id) as retention_rate
from customers_categorization
group by 1;

或使用大小写:

select
    visit_month,
    count(case when cust_type = 'retained' then customer__id end) /
        count(customer__id) as retention_rate
from customers_categorization
group by 1;