php - 我如何使用 mysql COUNT() 执行此操作

Question

我正在使用这个 UNION 查询来计算查询给出的记录。

这是我的查询:

// Count the number of records:
$q = "SELECT COUNT( DISTINCT i.institute_id)    
          FROM institutes AS i
            INNER JOIN institute_category_subject AS ics 
                ON ics.institute_id = i.institute_id
            INNER JOIN subjects AS s 
                ON ics.subject_id = s.subject_id
          WHERE s.subject_name LIKE '%mathematics%'
        UNION
        SELECT COUNT( DISTINCT t.tutor_id)
          FROM tutors AS t
            INNER JOIN tutor_category_subject  AS tcs
                ON tcs.tutor_id = t.tutor_id
            INNER JOIN subjects AS s
                ON tcs.subject_id = s.subject_id
          WHERE s.subject_name LIKE '%mathematics%'";

执行此查询后，我的输出结果如下。

+---------------------------------+
| COUNT( DISTINCT i.institute_id) |
+---------------------------------+
|                               3 |
|                               2 |
+---------------------------------+

这不是我期望的结果。我需要通过添加 3 + 2 得到 5 作为结果。添加两个选择查询。

谁能告诉我我是怎么弄明白的？

想想你。

Answer 1

用子查询包装 UNIONed 查询

SELECT SUM(total) totalSum
FROM
    (
        SELECT  COUNT( DISTINCT i.institute_id) total
        FROM    institutes AS i
                INNER JOIN institute_category_subject AS ics 
                    ON ics.institute_id = i.institute_id
                INNER JOIN subjects AS s 
                    ON ics.subject_id = s.subject_id
        WHERE   s.subject_name LIKE '%mathematics%'
        UNION
        SELECT  COUNT( DISTINCT t.tutor_id)   total
        FROM    tutors AS t
                INNER JOIN tutor_category_subject  AS tcs
                    ON tcs.tutor_id = t.tutor_id
                INNER JOIN subjects AS s
                    ON tcs.subject_id = s.subject_id
        WHERE   s.subject_name LIKE '%mathematics%'
    ) s