我试图使用以下方法查询数据库：

SELECT coalesce(value, "NA") as value 
FROM mytable FORCE INDEX (chrs) FORCE INDEX (sites) 
WHERE chrom = 1 AND site = 120

如果我在数据库中有一个条目,这个查询工作正常.但是,如果未找到该条目,则返回空数据集.我想更改查询,以便如果在数据库表中找不到它,它仍将返回某种值(对于“值”)而不是空数据集

解决方法:

你怎么添加一个UNION？

(   SELECT coalesce(value, "NA") as value 
    FROM mytable FORCE INDEX (chrs) FORCE INDEX (sites) 
    WHERE chrom = 1 AND site = 120
)
UNION ALL
(   SELECT 'NA' as value
    FROM mytable
    WHERE NOT EXISTS 
    (   SELECT coalesce(value, "NA") as value 
        FROM mytable FORCE INDEX (chrs) FORCE INDEX (sites) 
        WHERE chrom = 1 AND site = 120
    )
)

或者如果你想做一次合并,这样你就可以更容易地更改返回的部分,你可以这样选择它.

SELECT COALESCE(value, "NA") as value
FROM
(   
    (   SELECT value
        FROM mytable FORCE INDEX (chrs) FORCE INDEX (sites) 
        WHERE chrom = 1 AND site = 120
    )
    UNION ALL
    (   SELECT NULL as value
        FROM mytable
        WHERE NOT EXISTS 
        (   SELECT value as value 
            FROM mytable FORCE INDEX (chrs) FORCE INDEX (sites) 
            WHERE chrom = 1 AND site = 120
        )
    )
) t

DEMO