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How to Implement a Sequence Column Dependent on a Foreign Key in MySQL?
How to Implement a Sequence Column Dependent on a Foreign Key in MySQL?
Nov 15, 2024 am 03:02 AM
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.
The above is the detailed content of How to Implement a Sequence Column Dependent on a Foreign Key in MySQL?. For more information, please follow other related articles on the PHP Chinese website!
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