mysql - 如果满足给定条件，则选择两列，否则选择另外两列

Question

我有以下 mysql 查询

mysql> SELECT COUNT(*) AS freq, column_name FROM table_name WHERE created_time >= CURDATE() GROUP BY column_name;
Empty set (0.41 sec)

在这种情况下，当结果为空时，我想为两列都返回 0，因此结果应该是这样的:

+--------+------------+
| freq   | column_name|
+--------+------------+
|      0 |       0    |
+--------+------------+

我已经尝试了以下方法，但没有一个能按预期工作:

mysql> SELECT IFNULL(COUNT(*) AS freq, 0), IFNULL(column_name, 0) FROM table_name WHERE created_time >= CURDATE() GROUP BY column_name;
Empty set (0.40 sec)


mysql> SELECT IFNULL((SELECT COUNT(*), column_name FROM table_name WHERE created_time >= CURDATE() GROUP BY column_name), 'not found');
ERROR 1241 (21000): Operand should contain 1 column(s)


mysql> SELECT CASE WHEN ( (SELECT COUNT(*) FROM table_name WHERE created_time >= CURDATE()) > 0 )
            THEN (SELECT COUNT(*) AS freq, column_name FROM table_name WHERE created_time >= CURDATE() GROUP BY column_name)
            ELSE (SELECT 0,0) END;
ERROR 1241 (21000): Operand should contain 1 column(s)


mysql> SELECT IF(
    (SELECT COUNT(*) FROM table_name WHERE created_time >= CURDATE()) > 0,
    (SELECT COUNT(*) AS freq, column_name FROM table_name WHERE created_time >= CURDATE() GROUP BY column_name),
    (SELECT 0,0)
    );
ERROR 1241 (21000): Operand should contain 1 column(s)

我做错了什么？谢谢

Answer 1

作为解决方法，您可以使用 union 的技巧:

SELECT COUNT(*) AS freq, column_name 
FROM table_name 
WHERE created_time >= CURDATE() 
GROUP BY column_name
UNION
SELECT 0,0
FROM dual
WHERE (SELECT COUNT(*) FROM table_name WHERE created_time >= CURDATE()) = 0

我希望有人建议更优雅的方式来做到这一点