Question

当我有多个WHERE子句时，我在UNION语句中使用Count（*）时遇到问题。

以下是我试图开始工作的MySQL语法：

    SELECT Count(*)
    FROM   (SELECT title,
                   taskstatus,
                   Extract(year FROM finished)  AS year,
                   Extract(month FROM finished) AS month
            FROM   todo_languages
            UNION ALL
            SELECT title,
                   taskstatus,
                   Extract(year FROM finished)  AS year,
                   Extract(month FROM finished) AS month
            FROM   todo_main
            UNION ALL
            SELECT title,
                   taskstatus,
                   Extract(year FROM finished)  AS year,
                   Extract(month FROM finished) AS month
            FROM   todo_addons) AS a
    WHERE  taskstatus = 1
           AND year = 2014
           AND month = 04

在这里寻求帮助之前我有什么尝试？

首先，我测试了看看使用taskstatus进行计数是否像这样工作：

    SELECT Count(*)
    FROM   (SELECT title,
                   taskstatus,
                   Extract(year FROM finished)  AS year,
                   Extract(month FROM finished) AS month
            FROM   todo_languages
            UNION ALL
            SELECT title,
                   taskstatus,
                   Extract(year FROM finished)  AS year,
                   Extract(month FROM finished) AS month
            FROM   todo_main
            UNION ALL
            SELECT title,
                   taskstatus,
                   Extract(year FROM finished)  AS year,
                   Extract(month FROM finished) AS month
            FROM   todo_addons) AS a
    WHERE  taskstatus = 1

以上工作正常，并返回当前计数1.接下来，我测试以确保按月和按年计算得分：

SELECT Count(*)
FROM   (SELECT title,
               taskstatus,
               Extract(year FROM finished)  AS year,
               Extract(month FROM finished) AS month
        FROM   todo_languages
        UNION ALL
        SELECT title,
               taskstatus,
               Extract(year FROM finished)  AS year,
               Extract(month FROM finished) AS month
        FROM   todo_main
        UNION ALL
        SELECT title,
               taskstatus,
               Extract(year FROM finished)  AS year,
               Extract(month FROM finished) AS month
        FROM   todo_addons) AS a
WHERE  year = 2014
       AND month = 04

这也有效并返回行数1，但第一个查询返回的行数为0，这引出了我的观点。我有没有看到我做错了什么？提前谢谢。

Answer 1

你在哪里困惑？您可以记录WHERE taskstatus = 1，也可以记录WHERE year = 2014 AND month = 04但不记录WHERE taskstatus = 1 AND year = 2014 AND month = 04。

想象：

taskstatus     year       month
1              2000       05
2              2004       04

使用UNION时出现MySQL Count（*）问题

1 个答案: