sql 工作台中的复合主键与自动递增主键

Question

I need the advice of someone who has a greeter experience.

I have an associative entity in my database, like that:

Table2-> CustomerID, ServiceID, DateSub

Since the same customer (with PK, for example 1111) can require the same service (with PK, for example 3) more than once but never in the same date , the composite PK of Table 2 can't be just (CustomerID, ServiceID).

Now I have 2 options:

1- Also "DateSub" will be a primary key , so the PK of table 2 will be (CustomerID, ServiceID, DateSub)

2- Create a specific PK for the associative entity (for example, Table2ID, and so CustomerID and Service ID will be FK)

Which of the 2 approach would you follow and why? Thank you

Answer 1

First of all you need to decide whether is it your requirement to make combination of CustomerID , ServiceID amd DateI column as unique? If so then you should go for firt option.

Otherwise I would go for second option.

With first option if DateI is of date data type you will not be able to insert same service for a customer twice. If it's datetime then it's doable though.

If you want to use this primary key (composite primary key) in any other table as foreign key then you need to use all three columns there too.

Answer 2

I tend to prefer the PK be "natural". You have 3 columns that, together, can uniquely define each row. I would consider using it.

The next question is what order to put the 3 columns in. This depends on the common queries. Please provide them.

An index (including the PK) is used only leftmost first. It may be desirable to have some secondary key(s), for efficient access to other columns. Again, let's see the queries.

If you have a lot of secondary indexes, it may be better to have a surrogate, AUTO_INCREMENT "id" as the PK. Again, let's see the queries.

If you ever use a date range, then it is probably best to have DateSub last in any index. (There are rare exceptions.)

How many rows in the table?

The table is ENGINE=InnoDB , correct?

Reminder: The PRIMARY KEY is a Unique key, which is an INDEX .

DateSub is of datatype DATE , correct?