I have a table that has an AUTO_INCREMENT
field. Currently, it is also a PRIMARY KEY.
However, there are situations where I need this AUTO_INCREMENT
column to permit duplicates. In other words - two different rows can have the same value inside the AUTO_INCREMENT
column. This would mean having an AUTO_INCREMENT field that is not a PRIMARY KEY.
Is this possible?
I'm guessing it's not, since whenever I try to do it, I get this error:
ERROR 1075 (42000) at line 130: Incorrect table definition; there can be only one auto column and it must be defined as a key
I like to have the AUTO_INCREMENT field because it saves me from having to manually store / increment a separate counter elsewhere in my database. I can just insert into the table and grab the value that was inserted. However, if I can't have duplicates, it seems like I'm going to be stuck with using a separate table to track and manually increment this field.
UPDATE: As a quick clarification, I am already familiar with grouping the AUTO_INCREMENT field with another key, as described here . Let's assume for the sake of argument that this solution won't work due to other constraints in the database.
An auto-increment field in MySQL must be part of a key (ie an index), but not necessarily part of a primary key or unique key.
CREATE TABLE mytable (
id INT PRIMARY KEY,
otto INT AUTO_INCREMENT,
KEY (otto)
);
-- allow the auto-increment to generate a value
INSERT INTO mytable (id, otto) VALUES (123, DEFAULT);
SELECT * FROM mytable;
> 123, 1
-- specify a duplicate value, overriding the auto-increment mechanism
INSERT INTO mytable (id, otto) VALUES (456, 1);
SELECT * FROM mytable;
> 123, 1
> 456, 1
-- allow the auto-increment to generate another value
INSERT INTO mytable (id, otto) VALUES (789, DEFAULT);
SELECT * FROM mytable;
> 123, 1
> 456, 1
> 789, 2
Sounds like 'subtask' is a table to which 'task' has a FK reference to. That is, if subtasks are reused.
OTOH if a task can have many subtasks, and a subtask can be linked to more than one task then you're looking at many-to-many in a seperate table.
in either case I don't think you want the DB autogenerating these 'linked-IDs'.
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