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# A list of IDs in a column

I need to create a table which has a field that contains a list of id's to rows in another table. For example,...

the table portfolioAcc has a field which holds a list of holding accounts for a customer. I would really appreciate any help - thanks in advance.

This type of structure, in which one table is related to another through multiple IDs or keys that are stored within a single column, usually in a comma-separated list, can easily get you into trouble. For example,

```Customer

id  name     accounts
71  J.Smith   12
72  B.White   14,24
73  K.Baker   16,20,26
75  A.Brown   22

Account
id  type     amount
12  bonds   1047.00
14  stock    937.21
16  bonds   3700.00
20  cash      42.98
22  stock  10456.34
24  cash     211.03
26  stock   2044.65```

So B.White has two accounts, a stock account for 937.21, and a cash account for 211.03.

It is easy enough to see how someone can come up with this design. After all, it's called a one-to-many relationship, and sure enough, each customer has one or more accounts. So at first blush, it does indeed look like a one-to-many relationship.

But, alas, the foreign keys are in the wrong table.

The way to relate tables in a one-to-many relationship is to remember that:

• the "one" table (a.k.a. parent) contains the primary key

• the "many" table (a.k.a. child) contains the foreign key

So the preferred design is:

```Customer

id  name
71  J.Smith
72  B.White
73  K.Baker
75  A.Brown

Account
id  custid  type     amount
12    71    bonds   1047.00
14    72    stock    937.21
16    73    bonds   3700.00
20    73    cash      42.98
22    75    stock  10456.34
24    72    cash     211.03
26    73    stock   2044.65```

Space does not permit a full explanation of all the reasons why the comma-separated list of IDs is a poor design. The main problem is that it makes table joins difficult, if not impossible.

This was last published in March 2003

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