Modify the E-R diagram for the Student Registration System to include co-requisite and prerequisite relationships that exist over multiple periods of time. Each period begins in a certain semester and year and ends in a certain semester and year, or it continues into the present. Modify the translation into the relational model appropriately.
What will be an ideal response?
Instead of the relationship Requires with a single attribute EnforcedSince,we use two attributes, EnforcedSince and EnforcedUntil. If the latter is NULL, then the prerequisite is in force at present time. A similar relationship is needed for co-requisites. The corresponding relational representation requires that we add the attribute EnforcedUntil to the de?nition of the table Requires and create a similar table for co-requisites.
```
EnforcedUntil DATE,
```
Note that unlike EnforcedSince, we do not use the NOT NULL speci?cation, because NULL is meaningful — it represents prerequisites that are valid at present time.
You might also like to view...
Consider Figure 2.1. In addition to constraints relating the values of columns in one table to columns in another table, there are also constraints that impose restrictions on values in a column or a combination of columns within a table. One such constraint forces that a column or a group of columns must be unique across all rows in the table. For example, in the STUDENT table, the StudentNumber column must be unique (to prevent two different students from having the same StudentNumber). Identify the column or the group of columns in the other tables that must be unique across all rows in the table?
What will be an ideal response?
If the computer has been rebooted and you cannot find your document, you can use the Windows Explorer ________ command to look for it
Fill in the blank(s) with correct word
Using good grammar, explain why technicians must be able to identify common ports
What will be an ideal response?
The specfication "_____" is a relative unit that represents the current font size.
A. m B. em C. mem D. me