Postgres 选择连接的列名答案

【问题标题】：Postgres select concatenated column namePostgres 选择连接的列名
【发布时间】：2017-04-12 14:13:32
【问题描述】：

举个例子，一个表month有五列：

月.周1
month.week2
month.week3
month.week4
month.week5

col的数量由函数决定

EXTRACT(WEEK FROM NOW()) - EXTRACT(WEEK FROM DATE_TRUNC('month', NOW())) + 1

如何选择列colX？这就是我目前所拥有的

SELECT month.week || (
    EXTRACT(WEEK FROM NOW()) 
    - EXTRACT(WEEK FROM DATE_TRUNC('month', NOW())) + 1
)::text 
FROM month

但这给了我错误

错误：month.week 列不存在 SQL 状态：42703

【问题讨论】：

标签： postgresql

【解决方案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

或

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

【讨论】：

或者只是CASE (week expression) WHEN 1 THEN ... WHEN 2 ...以避免重复（非常长的）周表达式。

【解决方案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)

【讨论】：