Question

为了举例，表unique_ptr中有五列：

month.week1
month.week2
month.week3
month.week4
month.week5

month的数量由函数

决定

col

如何选择列EXTRACT(WEEK FROM NOW()) - EXTRACT(WEEK FROM DATE_TRUNC('month', NOW())) + 1？这就是我到目前为止所拥有的

colX

但这给了我错误

错误：列month.week不存在 SQL状态：42703

Answer 1

用例陈述

SELECT
    CASE WHEN (week expresion) = 1 THEN month.week1
         WHEN (week expresion) = 2 THEN month.week2
         WHEN (week expresion) = 3 THEN month.week3
         WHEN (week expresion) = 4 THEN month.week4
         ELSE month.week5
    END as WeekValue
FROM month

OR

SELECT
    CASE (week expresion) 
         WHEN 1 THEN month.week1
         WHEN 2 THEN month.week2
         WHEN 3 THEN month.week3
         WHEN 4 THEN month.week4
         ELSE month.week5
    END as WeekValue
FROM month

Answer 2

动态sql示例：

t=# create table so58(i int,w1 text);
CREATE TABLE
t=# create or replace function so59(_n int) returns table (i int,w text) as $$begin
return query execute format('select i,w%s from so58',_n);
end;
$$ language plpgsql;
CREATE FUNCTION
t=# select * from so59(1);
 i | w
---+---
(0 rows)

Postgres选择连接列名称

2 个答案: