我該如何使用mysql COUNT（）

Question

我正在使用此UNION查詢來計算查詢提供的記錄。

這是我的查詢：

// 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%'";

執行此查詢后，我得到以下結果作為輸出。

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

這不是我的預期結果。 我需要通過添加3 + 2得到5作為結果。添加兩個select查詢。

有人可以告訴我如何解決嗎？

想你。

Answer 1

用子查詢包裝UNION ed查詢

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

Answer 2

由於您要對行進行計數，因此不需要額外的總和+計數+計數開銷。

只要這樣做：

$q = "SELECT COUNT(*) FROM (
    SELECT 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 ALL
    SELECT 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%'
  ) mysubquery";