Question

将表格的几列组合起来为另一个查询设置变量的最简单方法是什么？

我想避免在我的查询中不断重复if if then语句

SELECT 
CASE
WHEN team_id = away_team_id THEN away_score * 5
WHEN team_id = home_team_id THEN home_score * 5
END AS team_score_times_five, 
CASE
WHEN team_id = away_team_id THEN away_score - 5
WHEN team_id = home_team_id THEN home_score - 5
END AS team_score_minus_five
FROM t1

相反，我宁愿设置一个变量

CASE
WHEN team_id = away_team_id THEN away_score
WHEN team_id = home_team_id THEN home_score
END AS team_score

然后更干净地查询

SELECT team_score * 5 AS team_score_times_five, team_score - 5 AS team_score_minus_five FROM t1

Answer 1

只需使用FROM

后面的子查询

 SELECT team_score * 5 AS team_score_times_five, 
        team_score - 5 AS team_score_minus_five
 FROM (
   SELECT CASE
     WHEN team_id = away_team_id THEN away_score
     ELSE home_score END AS team_score
   FROM t1
 ) t

Answer 2

您也可以尝试common table expressions：

WITH common_table_expression AS (
    SELECT 
        CASE
          WHEN team_id = away_team_id THEN away_score
          WHEN team_id = home_team_id THEN home_score
          END AS team_score
    FROM t1
)
SELECT 
      team_score * 5 AS team_score_times_five
    , team_score - 5 AS team_score_minus_five
FROM common_table_expression

CTE允许您堆叠多个查询，这些查询将以更易读的方式嵌套，并且如果业务需求发生更改，将来更容易修改（具体取决于具体情况）。

This article有一个很棒的部分名为＆＃34;使用公用表表达式来获得极其可读的SQL＆＃34;这为你想要使用CTE的原因提供了额外的论据。

Postgres如何在select语句中避免重复

2 个答案: