查詢「on」子句不限制連接？

Question

我有如下的查詢，這是什麼意思做的是

所有提交的返回ID爲包括他們有多少次被讀取的計數的競爭2。

它做的一切都很好，除了它從所有其他競賽中返回行外;爲什麼它不僅加入`competition_id` = 2？

SELECT 
    `c`.`competition_id`, 
    `c`.`submission_id`, 
    COUNT(
     `submission_reads`.`submission_id` 
    ) `reads` 
    FROM 
    `submission_reads` 
    RIGHT JOIN `competition_submissions` c 
     ON `c`.`submission_id` = `submission_reads`.`submission_id` 
     AND c.top_round = 1 
     AND c.`competition_id` = 2 
    GROUP BY `c`.`submission_id` 

Answer 1

在RIGHT加入，你從右邊的表中的所有行; ON子句限制了左側表中的匹配行，即submission_reads表。如果要限制行數competition_submissions，則需要將連接從RIGHT更改爲LEFT。

評論：「我想從competition_submissions表中的所有行，無論它是否已在submission_reads的條目」

然後，你需要做的WHERE條款c.competition_id = 2一部分，因爲你希望它過濾外連接中「主」表的行：

SELECT 
    `c`.`competition_id`, 
    `c`.`submission_id`, 
    COUNT(
     `submission_reads`.`submission_id` 
    ) `reads` 
    FROM 
    `competition_submissions` c 
    LEFT JOIN `submission_reads` r 
     ON `c`.`submission_id` = `r`.`submission_id` 
     AND `c`.top_round = 1 
    WHERE `c`.`competition_id` = 2 
    GROUP BY `c`.`submission_id`