如果没有返回行,则返回具有空值的行

Question

如果返回的一个值没有返回任何行,那么有很多关于如何返回NULL的问题.但是,当没有返回结果时,我没有发现任何关于多值查询返回NULL的信息.

主要查询:

SELECT UserOpinion.*, x, y, z... FROM subquery, (x) AS x, (y) AS y...
I trust you get the gist.

我的查询是一个子查询:

(SELECT TOP 1                       
        u.BADId,                        
        Date,                       
        State,                      
        Note,                       
        Comment,                        
        Alias,                      
        Title,                      
        UserId,
        o.Id AS OpinionId                   
    FROM                        
        [Opinion] o                 
    RIGHT JOIN                      
        [User] u                        
        ON                          
        o.UserId = u.Id                     
        WHERE                       
        (Date IS NULL OR (Date = (SELECT MAX(Date)                              
                                FROM [Opinion]                              
                                WHERE UserId = o.UserId )))
        AND                             
            u.BADId = 'myvalue') AS UserOpinion; 

例如,我尝试过以下方法:

(SELECT TOP 1 * FROM (SELECT TOP 1                      
        u.BADId,                        
        Date,                       
        State,                      
        Note,                       
        Comment,                        
        Alias,                      
        Title,                      
        UserId,
        o.Id AS OpinionId                   
    FROM                        
        [Opinion] o                 
    RIGHT JOIN                      
        [User] u                        
        ON                          
        o.UserId = u.Id                     
        WHERE                       
        (Date IS NULL OR (Date = (SELECT MAX(Date)                              
                                FROM [Opinion]                              
                                WHERE UserId = o.UserId )))
        AND                             
            u.BADId = 'myvalue' 
        UNION SELECT NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL) t) AS UserOpinion; 

无论' myvalue '是否存在,都会返回NULL .我想要的是,如果条目存在,它返回查询的值,或者如果不存在则返回每个列的null.

我也尝试过以下方法:

(COALESCE(SELECT TOP 1                      
    u.BADId,                        
    Date,                       
    State,                      
    Note,                       
    Comment,                        
    Alias,                      
    Title,                      
    UserId,
    o.Id AS OpinionId                   
FROM                        
    [Opinion] o                 
RIGHT JOIN                      
    [User] u                        
    ON                          
    o.UserId = u.Id                     
    WHERE                       
    (Date IS NULL OR (Date = (SELECT MAX(Date)                              
                            FROM [Opinion]                              
                            WHERE UserId = o.UserId )))
    AND                             
        u.BADId = 'myvalue'), NULL) AS UserOpinion; 

这里的主要问题是如果' myvalue '没有匹配,整个查询都不会返回任何内容.其他查询不需要myvalue,并且进行多个独立查询不是性能约束的选项.

我远非SQL专家,任何正确方向的建议将不胜感激.

类似问题但不适用于此:

• 1,2,3等...

Answer 1

你可以这样做left outer join:

with s as (<your subquery here>)
select s.*
from (select 1 as x) x left join
     s
     on x.x = 1;

这将返回表中的值.如果您不匹配,则查询返回一行,其中所有列都是NULL.

注意:CTE不是必需的.我只是强调了查询的逻辑.您可以将子查询放在何处s.