在MySQL数据库中存储枚举

Question

I need the most clean and efficient solution for my case. I'm trying to store Marks of student in a MySQL DB. A student may be "ABSENT" or "COPY CASE", I want to store such info also in DB.

I thought of assigning codes for above case like -1 for ABSENT, -2 for COPY CASE, etc. This codes will be stored in Marks column only.

More ever, while reading them with a select query, I should get their display values ie ABSENT , COPY CASE , etc only.

Can all these be achieved at DB end only?

Or do I need to implement these things at Application level only?

Are there any API's available for Java for such functionality?

Answer 1

mysql supports enums:

CREATE TABLE students (name varchar(64), mark ENUM('ABSENT','COPY CASE' ));
INSERT INTO students VALUES ('john', 'COPY CASE');

In java, you can treat this column as a string type column, but in the database, values are stored efficiently, and trying to store a value not contained in the enum will result in an error.

Answer 2

如何将其name() （String版本）存储到DB并使用valueOf()从DB读取时创建它

Answer 3

If the enum string representations are pretty string heavy, and the database is large (checks against these enums are frequent), I'd probably map each enum to an integer constant (no, not the ordinal() , but your own custom integer value) and have methods on the enum to get and create an enum from these values. This way, you'll be able to store the enums pretty efficiently.

public enum Status {

  ABSENT(1), COPY_PASTE(2);

  int value;

  private Status(int value) {
      this.value = value;
  }

  public int getValue() {
      return this.value;
  }

  public static Status ofStatusCode(int value) {
      // retrieve status from status code
  }

}

Answer 4

I thought of storing codes for above case like -1 for ABSENT, -2 for COPY CASE, etc. This codes will be stored in Marks column only.

I'd rather prefer having a separate column, say status which holds values such as ABSENT , COPY CASE and a default value such as APPEARED

And marks column would stay separate.

In the view layer, you could do something like

if(status is 'APPEARED')
show marks field
else
show status field

Answer 5

MySQL has it own ENUM type . And as @Jigar Pointed out, you can use Enum.getName to get Java's a String representing the enum for persistence, retrieve the field from the database as a String and use Enum.valueOf to load the enum constant back.

You can also use Enum.ordinal() + 1 to get a integer for persistence (MySQL also support using integer indexes directly with ENUM type) and issue a query like:

SELECT enum_col-1 FROM tbl_name;

to retrieve the ordinal back (it will retrieve -1 for rows with invalid enum values).

Plus MyEnumType value = MyEnumType.values()[ordinal] to convert it back to Java enum type.

Anyway, I think that the String based solution is cleaner and more elegant, plus safer on the long run (if you ever need to change the order of the java Enum, or add new enum values at the middle of the existing ones, you will thank yourself for going with String based strategy to begin with).