doufei8250 2017-06-10 00:45
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SQL - 根据另一个表中的行数删除行

How would I go about deleting a row from the table 'subjects' that has a primary id 'subject_id' based on the number of rows in another table named 'replies' that uses a 'subject_id' column as a reference.

Example in pseudo code: If ('subject' has less than 1 reply){ delete 'subject'}

I don't know much about SQL triggers so I have no clue if I would be able to incorporate this directly in the database or if I'd have to write some PHP code to handle this...

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  • dopuzf0898 2017-06-10 01:20
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    To delete any subjects that have had no replies, this query should do the trick:

    DELETE s.* FROM subjects AS s
    WHERE NOT EXISTS 
        (
        SELECT r.subject_id
        FROM replies AS r
        WHERE r.subject_id = s.subject_id
        );
    

    Demo: DB Fiddle Example

    本回答被题主选为最佳回答 , 对您是否有帮助呢?
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