MySQL中如何根據外鍵關係產生唯一序列列？

Database Enhancement: Generating Sequence Columns Based on Foreign Key Relations in MySQL

In a relational database like MySQL, it may become necessary to add a column to a table that captures a sequence number. This can be a challenge when the sequence must be unique to specific related values in another column. Let's explore a solution for this problem.

Problem Statement

Consider the example of a database table with the following structure:

CREATE TABLE mytable (
  ID INT NOT NULL,
  ACCOUNT INT NOT NULL,
  some_other_stuff TEXT
);

The goal is to add a seq column that assigns unique sequence numbers for each distinct ACCOUNT value.

SQL Solution

One elegant solution to this problem is to utilize a trigger in MySQL. A trigger is a database object that automatically executes a pre-defined set of actions when specific events occur on a table.

Creating the Trigger

To create a trigger that generates the sequence column, execute the following SQL statement:

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;

How the Trigger Works

This trigger is executed before each row is inserted into the mytable. It calculates a new sequence number for the row by:

1. Selecting the maximum value of seq for the specified account or returning 0 if no rows exist.
2. Incrementing the maximum value by 1.
3. Setting the seq column of the new row to the calculated sequence number.

Example

Consider the initial table data:

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

Inserting a new row into the table with ACCOUNT = 1 would generate a seq value of 4.

ID	ACCOUNT	seq	some_other_stuff
1	1	1	...
2	1	2	...
3	1	3	...
4	2	1	...
5	2	2	...
**6	1	4	...	**

Conclusion

By leveraging the power of SQL triggers, we can achieve the desired functionality of adding a sequence column based on another field. This technique is efficient, flexible, and easy to implement.

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