如果数据集为空，MySQL将返回默认值

我正在尝试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

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如果我在数据库中有一个条目，这个查询就可以正常工作。但是，如果找不到条目，它将返回一个空数据集。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（用于“value”），而不是一个空的数据集

添加一个联合如何

(   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，如果您想执行一次合并，这样您就可以更容易地更改返回的部分（如果为null），您可以像这样从中进行选择

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

一种（丑陋的）方法：创建一个只有一行的表（与该表中的数据无关），然后使用：

SELECT coalesce(m.value, "NA") from oneline left join mytable m on m.chrom=1 and m.site=120;

以下是您得到的：

mysql> select * from oneline;
+------+
| id   |
+------+
|    1 |
+------+
1 row in set (0,00 sec)

mysql> select * from mytable;
+-------+-------+------+
| value | chrom | site |
+-------+-------+------+
| test  |     1 |  120 |
+-------+-------+------+
1 row in set (0,00 sec)

mysql> SELECT coalesce(m.value, "NA") from oneline left join mytable m on m.chrom=1 and m.site=120;
+-------------------------+
| coalesce(m.value, "NA") |
+-------------------------+
| test                    |
+-------------------------+
1 row in set (0,00 sec)

mysql> SELECT coalesce(m.value, "NA") from oneline left join mytable m on m.chrom=1 and m.site=119;
+-------------------------+
| coalesce(m.value, "NA") |
+-------------------------+
| NA                      |
+-------------------------+
1 row in set (0,00 sec)

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你能发布一些相关数据吗？