如何在选择查询中使用案例别名答案

【问题标题】：How can i use case alias in a select query如何在选择查询中使用案例别名
【发布时间】：2021-04-30 01:59:52
【问题描述】：

在 SQL 中，我如何在选择查询中使用案例别名，有问题的是一个计数，例如，您将在下面看到选择别名设置为 as，但我不能在其余部分中使用别名选择查询。使用 MySQL。

下面有问题的行是： (win / total) * 100 as win_percent

    DECLARE @team TEXT;
    SET @team = "myTeam";
    SELECT 
        @team team,
        COUNT(CASE WHEN home = @team then 1 ELSE NULL END) as home,
        COUNT(CASE WHEN away = @team then 1 ELSE NULL END) as away,
        COUNT(CASE WHEN (away = @team or home = @team) then 1 ELSE NULL END) as total,
        COUNT(CASE WHEN winner = @team and (home = @team or away = @team)  then 1 ELSE NULL END) as win,    
        COUNT(CASE WHEN (home = @team or away = @team) and (winner <> @team and winner <> "0") then 1 ELSE NULL END) as lost,
        COUNT(CASE WHEN winner = "0" and (home = @team or away = @team) then 1 ELSE NULL END) as tie,
        (869 / 1928) * 100 as win_percent,
        (win / total) * 100 as win_percent
    from matches;

【问题讨论】：

不行。使用表达式而不是别名。或者将您的查询移动到子查询/CTE 中，并在外部查询中使用 win/total 列值。

标签： mysql sql database select mariadb

【解决方案1】：

您可以创建一个子查询并将最终的 win_percent 放在最外层的查询中。查询应如下所示：

SELECT a.*,
    (win / total) * 100 as win_percent
FROM (
SELECT 
        @team team,
        COUNT(CASE WHEN home = @team then 1 ELSE NULL END) as home,
        COUNT(CASE WHEN away = @team then 1 ELSE NULL END) as away,
        COUNT(CASE WHEN (away = @team or home = @team) then 1 ELSE NULL END) as total,
        COUNT(CASE WHEN winner = @team and (home = @team or away = @team)  then 1 ELSE NULL END) as win,    
        COUNT(CASE WHEN (home = @team or away = @team) and (winner <> @team and winner <> "0") then 1 ELSE NULL END) as lost,
        COUNT(CASE WHEN winner = "0" and (home = @team or away = @team) then 1 ELSE NULL END) as tie,
        (869 / 1928) * 100 as win_percent
    from matches
) AS a

【讨论】：

按预期工作谢谢，将在 3 分钟内设置为答案。

【解决方案2】：

我建议简化您的查询，使用方便的 MySQL 扩展，将布尔值视为数字，1 为真，0 为假：

SELECT @team team,
       SUM(home = @team) as home,
       SUM(away = @team) as away,
       SUM(@team IN (away, home))) as total,
       SUM(winner = @team and @team IN (away, home)) as win,    
       SUM(winner <> @team and winner <> 0 and @team IN (away, home)) as lost,    
       SUM(winner = '0' and @team IN (away, home)) as tie,
       (869 / 1928) * 100 as win_percent,
       AVG(CASE WHEN @team IN (away, home) THEN winner = @team END) as win_percent

【讨论】：