将一对多关系转换为一对一关系 MySQL

Question

I have two MySQL tables we can call Foo and Bar .

Both tables have a column called PrizeGroupId . the goal is to create a one-to-one relationship between these columns, and I have created stored procedures to add/edit Foo that update the corresponding row via the one-to-one relationship in Bar .

The problem lies in the fact that the data wasn't always structured this way and I need to write a script to convert the data from it's previous state (which I'm about to describe) to a one-to-one relationship based off of PrizeGroupId .

Previously, multiple rows in Foo could have the same PrizeGroupId such that there was a one-to-many relationship between entries in Bar to Foo based off of PrizeGroupId . The script that I need to write has to break apart every one-to-many instance of this nature into many (almost identical) one-to-one relationships between Foo and Bar .

In principle, I want to:

1. Iterate through Foo
2. See if the current row's PrizeGroupId is not unique in Foo .
3. Assign it a unique value (perhaps the current items primary key)
4. Add a row in Bar with the new PrizeGroupId . Copy over all of the old row's other data into this new row such that it is "nearly identical".
5. After all is said and done, remove the old one-to-many row from Bar .

I understand the problem and how I could do this in pseudocode in a programming language, however I am still learning MySQL and am not sure how to go about solving a problem of this nature.

If you can provide me with help through MySQL code and/or what steps I can take/read about to go about solving this problem that would be, or at least point me to the kind of reading/SO question related to this kind of problem that would be appreciated, although I had a difficult time finding particular resources on my own.

Answer 1

What you are asking for is not that hard. Some of your thinking is getting in the way. First, one almost never iterates in SQL. SQL is not that kind of language. Everything in SQL is done via sets of something.

Your approach can be:

1. Identify the set of rows where the PrizeGroupId is already unique and move them to a new copy of the table.

To create a table, you can use "create table foo2 like foo;". Very useful.

To identify the rows where PrizeGroupId is already unique, use something like:

create table test_30602977 (id int primary key, other int);
insert into test_30602977 values (1, 1), (2, 2), (3, 2);
select other, count(*) as count from test_30602977 group by other having count = 1;

2. The rows left in the original table do not have a unique PrizeGroupId. Change the PrizeGroupId value so that they are unique.

3. Merge the two sets to reconstruct the table with the original rows and with PrizeGroupId unique.

One reason that this is hard is because if you had created the tables with the one-to-one join, you would have used the pk to join the tables. The pk is already unique so why use something else. Once you have the tables separated and the PrizeGroupId is unique, you might want to think about setting the pk of foo to the pk of bar and then removing the PrizeGroupId column.

Answer 2

What is required is for the Bar table to contain a single record for each record in Foo, and for the PrizeGroupId field in Foo to be unqiue. As Foo.Id is already unique, it makes sense to use that as the foreign key.

Running a query like SELECT Foo.id, Bar.* FROM Foo INNER JOIN Bar USING (PrizeGroupId) will give us a single record for each record in Foo, along with the data from the corresponding record from Bar. So, if we were to replace the data in Bar with what is returned by this query, and then use Foo.Id for PrizeGroupId, we'd acheive what is required.

1. Create a temporary table with the same structure as Bar - something like CREATE TABLE Bar_copy LIKE Bar

2. Fill the temporary table with one record for each record in Foo, joined to the corresponding record in Bar - you'll need to list all the columns in Bar, for example - INSERT INTO Bar_copy (id, field1, field2, field3) SELECT f.id, b.field1, b.field2, b.field3 FROM Foo AS f INNER JOIN Bar AS b USING (PrizeGroupId)

3. Clear the existing PrizeGroupID field from Foo - UPDATE Foo SET PrizeGroupId = NULL

4. Empty the existing Bar table and refill it with the records from the temporary table - INSERT INTO Bar (id, field1, field2, field3) SELECT id, field1, field2, field3 FROM Bar_copy

5. Update the foreign key values in Foo - UPDATE Foo SET PrizeGroupId = id

Obviously, take a back-up first!