Question

We have two tables. The first contains a name (varchar) field. The second contains a field that references the name field from the first table. This foreign key in the second table will be repeated for every row associated with that name. Is it generally discouraged to use a varchar/string field as a join between two tables? When is the best case where a string field can be used as a join field?

Answer 1

It’s certainly possible to use a varchar as a key field (or simply something to join on). The main problems with it are based on what you normally store in a varchar field; mutable data. Strictly speaking, it’s not advisable to have key fields change. A person’s name, telephone number, even their SSN can all change. However, the employee with internal ID 3 will always be ID 3, even if there are two John Smiths.

Second, string comparison is dependent on a number of nit-picky details, such as culture, collation, whitespace translation, etc. that can break a join for no immediately-apparent reason. Say you use a tabspace character \t for a certain string you’re joining on. Later, you change your software to replace \t with 3 spaces to reduce character escapes in your raw strings. You have now broken any functionality requiring a string with escaped tabs to be matched to an identical-looking, but differently-composed, string.

Lastly, even given two perfectly identical strings, there is a slight performance benefit to comparing two integer numbers than comparing two strings. Integer comparison is effectively constant-time. String comparison is linear at best, based on the length of the string.