如何在MySQL中實作依賴外鍵的序列列？

MySQL: Establishing a Sequence Column Dependent on a Foreign Field

In the realm of database management, it becomes imperative at times to modify existing structures to accommodate new data requirements. Such is the case when a sequence number needs to be assigned to records based on a specific foreign key.

Consider a table with columns ID, ACCOUNT, and some_other_stuff. The goal is to introduce a new column, SEQ, which increments uniquely for each distinct ACCOUNT value, like so:

ID     ACCOUNT     SEQ     some_other_stuff
1      1           1       ...
2      1           2       ...
3      1           3       ...
4      2           1       ...
5      2           2       ...
6      1           4       ...

SQL-Based Solution:

MySQL empowers us with triggers, a powerful mechanism to automatically execute predefined actions upon data manipulation events. In this instance, a trigger can be employed to populate the SEQ column with appropriate values during record insertion. The trigger code:

CREATE TRIGGER trg_mytable_bi
BEFORE INSERT ON mytable
FOR EACH ROW
BEGIN
      DECLARE nseq INT;
      SELECT COALESCE(MAX(seq), 0) + 1
      INTO nseq
      FROM mytable
      WHERE account = NEW.account;
      SET NEW.seq = nseq;
END;

Breakdown:

• The trigger trg_mytable_bi is invoked before each INSERT operation on the mytable.
• Inside the trigger, nseq is declared as an integer variable.
• A subquery fetches the maximum seq value for the matching account and adds 1 to it, effectively generating the next sequence number. This value is stored in nseq.
• Finally, NEW.seq (the seq value for the new record) is set to nseq, ensuring proper sequencing.

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