SQL Unit 12 Creating Tables, Keys, and Constraints in Access

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SQL Unit 12Creating Tables, Keys, and Constraints in Access

Kirk Scott

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• 12.1 Creating a Blank Database in Access• 12.2 Creating a Table Using Access's

Graphical User Interface• 12.3 Specifying Referential Integrity Using

Access's Graphical User Interface• 12.4 Entering Data• 12.5 Assignment Description

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12.2 Creating a Blank Database in Access

• In order to create tables, you need a blank database to put them in.

• In order to do this in Access, click on the Office Icon in the upper left hand corner.

• In the menu there will be a New option. •

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• If you click on the New option, the screen shown on the following overhead will appear.

• The option to name and create the new database is on the right hand side of the screen.

• By default, the database will probably be saved to the Documents folder on the machine you're using.

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• You can type in a different database name if you want to.

• You can also click on the little folder icon and browse for a different destination now

• Or later on you can take the Save option in the menu and specify a location to save it to at that time

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• In any case, to finish creating a new database, you will need to click on the Create button

• When you do so, the screen shown on the following overhead will appear.

• This is one place/way that tables can be defined.

• Access is trying to be helpful, but as you’ll see, we don’t really want to make use of this screen.

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• The system has provided a table named Table1 as part of the new database you’re creating.

• Click on the X in the upper right hand corner of Table1, closing it.

• This will take you to a completely blank Access screen.

• In that screen, click on the Create tab.• The blank screen with the Create tab highlighted

is shown on the following overhead.

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• There will be several options listed under the Create tab.

• 1. The leftmost icon will be labeled Table. – DO NOT TAKE THIS ONE.

• 2. The icon fourth from the left will be labeled Table Design. – THIS IS THE ONE TO TAKE.

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• If you mistakenly took number 1 above, the system would take you to a table design screen where a key field has already been put into the table.

• The system is trying to be helpful, but it isn't a help, it's a hindrance…

• The system assumes that you don’t know what a key field is

• We know, and we want to define our own

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• If you take the fourth icon from the left, as desired, you will end up with a completely blank table design screen

• This screen is shown on the following overhead

• If you reach this point, you are ready for the next section, where table definition is covered.

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12.2 Creating a Table Using Access's Graphical User Interface

• On the following overheads, the SQL commands are given for creating two tables, Mother and Person, in a one-to-many relationship.

• The primary key of the Mother table is embedded as a foreign key in the Person table.

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• Following the table definitions is a query that would create an index on the lastname field of the Mother table.

• This is done to refresh your memory.• The goal of this unit is to show how to do all of

this using MS Access's graphical user interface.

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• CREATE TABLE Mother• (SSN TEXT(9),• lastname TEXT(24)• CONSTRAINT motherpkSSN PRIMARY

KEY(SSN))

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• CREATE TABLE Person• (SSN TEXT(9),• lastname TEXT(24),• motherSSN TEXT(9),• dob DATE,• CONSTRAINT personpkSSN PRIMARY KEY(SSN),• CONSTRAINT personfkmother FOREIGN KEY(motherSSN)• REFERENCES Mother(SSN)• ON DELETE RESTRICT• ON UPDATE CASCADE)

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• CREATE INDEX motherSSNindexasc• ON Person(motherSSN)

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• SQL is the preferred alternative for writing queries, but for designing and creating tables, the graphical user interface is preferred.

• If you have understood the discussion of table definitions, constraints, and indexes as described using SQL, then you know the general concepts.

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• These concepts appear in a different form, and in some cases, using different words, in the graphical user interface.

• This interface is preferred for two reasons, one good and one bad.

• The good reason is that it can be easier to use than the SQL syntax.

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• The bad reason is that in Microsoft Access table design, if you attempt to do something using SQL, there is no guarantee that in all cases it will work.

• When Microsoft designed Access, the graphical user interface was the primary interface, so if you do something there, it should work.

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• Here is a logical outline of what is involved in creating tables.

• Specifying referential integrity is done after the related tables are created using a different view in the graphical user interface.

• That will be covered in the next section of the unit.

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• On the following overheads an outline is given of the steps you would follow when using the graphical user interface.

• It is organized in an order that makes sense when using the graphical user interface

• It includes options that correspond to the points listed above.

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• This outline is also a preview of everything you will find in the interface, and it lists options that can be skipped.

• After everything is summarized in the outline, the things listed in the outline will be shown with screenshots.

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Creating tables

• At the top of the design view for a table:– Give each field a name. – As soon as you enter this first field, the bottom of the

design view will expand to include the options listed under the following black bullet.

– If the field is the primary key field, right click on it and specify that

– Alternatively, with that field highlighted, click on the key symbol above

– Give each field a data type

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• At the bottom of the design view there will be a long list of options:– DO: Specify a Field Size for those data types where the user

can specify a size (like the width in characters of a text field)– SKIP: Specify a field Format– SKIP: Specify a field Input Mask– SKIP: Specify a field Caption– SKIP: Specify a field Default Value– SKIP: Specify a field Validation Rule– SKIP: Specify a field Validation Text

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– DO: Specify whether the field is required or not. – This is the equivalent of specifying NOT NULL. – If the field is not required, then NULL is allowed. – Notice this unpleasant fact: – Even though you indicate that a field is the

primary key field, this does not automatically change the setting to "Required—Yes".

– To be on the safe side, it would be wise to check "Required—Yes" on the primary key field.

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– DO: Specify whether Allow Zero Length is OK. – This is a detail that is related to the question of nulls.

– A zero length data value is virtually a null. – Notice this unpleasant fact: – Even though you indicate that a field is the primary

key field, this does not automatically change the setting to "Allow Zero Length—No".

– To be on the safe side, it would be wise to check "Allow Zero Length—No" on the primary key field.

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– DO: Specify whether the field is Indexed. – There are two options, No Duplicates and Duplicates

Allowed. – Either way, the field is indexed. – Duplicates vs. No Duplicates corresponds to

specifying NOT UNIQUE or UNIQUE in SQL. – Note that when you make something the primary

key, by default, this becomes indexed with no duplicates allowed.

– Other fields can be indexed at will.

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– SKIP: Specify Unicode Compression– SKIP: Specify IME Mode– SKIP: Specify IME Sentence Mode– SKIP: Specify Smart Tags

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• When you finish the design, you should close the table by clicking on the X in the upper right hand corner.

• At this time you can save the table in the database and give it a name.

• This is sort of like closing and naming queries. • What follows are screen shots for the completed

designs for the Mother and Person tables.

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• Keep in mind when looking at the screenshots that the general characteristics listed at the bottom apply to whichever field in the table design is highlighted at the top.

• In the Mother table design, the primary key field, SSN, is highlighted.

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• The Person table design screenshot follows. • Notice that in this screenshot, it's the foreign

key field, motherSSN, which is highlighted. • In addition to other characteristics, it is

indexed, and for the foreign key field, "Duplicates OK" is the desired setting.

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12.3 Specifying Referential Integrity Using Access's Graphical User Interface

• In order to include referential integrity in the database design, you have to go to the rightmost tab at the top of Access

• This is the Database Tools tab• Under this tab is the Relationships icon• This is shown on the following overhead

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• To create a primary key-foreign key relationship between tables in a database, you have to click on the relationships button.

• A dialog box will appear where you get to add the tables of your database to a graphical representation of the database.

• The screenshot below shows the dialog box.

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• This is what the Relationships screen looks like after adding both tables.

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• To create a one-to-many relationship between Mother and Person:– Click on the primary key of the Mother table, SSN– Drag the mouse to the foreign key field of Person,

motherSSN– Drop there

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• Remember that a primary to foreign key relationship is captured by embedding the primary key of the one table as a foreign key in the many table.

• When using the graphical user interface you click, drag, and drop, from the one/primary key field to the many/foreign key field

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• After you have taken the first step in creating the relationship, the system will prompt you to see if you want to enforce referential integrity.

• This is done by means of a dialog box, which is shown on the following overhead.

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• Then you have to check the Enforce Referential Integrity box and choose whether updates or deletes should cascade.

• If you do not choose the cascade option, the default is to restrict.

• A representative choice is shown.

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• After you click the create button, the relationships screen will look as shown on the following overhead.

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• The 1 and the infinity symbol on the ends of the line connecting the table are the Access equivalent of a crow's foot.

• The infinity symbol represents the "many" side of the relationship.

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• When you close the relationships view, you will be prompted whether you want to save the changes.

• Again, this is analogous to closing and saving a query.

• If you did everything correctly, don’t forget to save the changes.

• If you made a mistake on the relationship, do not save the changes, and start over.

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12.4 Entering Data

• In a previous unit the SQL INSERT key word was explained.

• If you only had an SQL interface, this would be the only way to enter data into a table.

• It can be useful if you need to set up queries that automatically put more than one record into a table at a time, but for a single record the graphical user interface is more convenient.

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• To insert data using the graphical user interface, simply open a table.

• The default view is the datasheet view. • In this view you can enter the values into the

rows. • It looks and feels pretty much like entering

data into a spreadsheet.

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Data entry and referential integrity

• When doing data entry you have one last thing to worry about:

• Referential integrity. • Referential integrity says that you can't put a

value into a foreign key field that doesn't exist in the corresponding primary key field.

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• In practical terms, this restriction has this consequence:

• Suppose you are enforcing referential integrity on tables in a one-to-many relationship

• You have to put the records into the "one" table before putting the corresponding records into the "many" table.

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• In this example, you have to enter Mother information before entering Person information.

• If you try to do it in the other order, you'll get an error message.

• A screenshot of the Mother table is given below with some data in it.

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• Here is a screenshot of the Person table with some matching data.

• A given motherSSN may occur more than once. • It's also possible to have a record with a null

motherSSN.• What is not possible is a Person record with a

MotherSSN value that doesn’t exist in the Mother table.

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12.5 Assignment Description

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12.5.1 Background

• The assignment for Unit 12 is different from the preceding assignments.

• Rather than a list of queries, it is a small project where you have to create some tables, and so on.

• The assignment instructions will be reviewed• Then an example will be covered.

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12.5.2 Assignment Instructions

• This section covers the instructions for the assignment.

• The next section gives a specific example of an assignment problem.

• There is a general instruction sheet.• Also, each individual version of the

assignment comes with specific instructions

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Version Specific Instructions

• You should have received a sheet telling you which version of the assignment you’ll be doing

• This sheet includes:• 1. A small E-R diagram which illustrates a

database scenario.• 2. A brief verbal description of anything that

might not be clear from the names of the tables and fields in the scenario.

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• 3. Schemas for the tables, including field names and types along with an indication of whether they're primary key or foreign key fields.

• 4. One example record for each table.

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• This is what you're not given:• 1. You are not given the widths of the text

fields. • You will have to pick reasonable values for these

yourselves.• 2. Aside from the one example record for each

table, you are not given data for the tables. • You will need to make some up.

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• This assignment includes table creation and some constraints.

• You should be using Access's graphical user interface for this assignment.

• In the long run it doesn't make much sense to do these things using SQL when a graphical alternative is available.

• If you are ever really going to use Access, it is worth trying the graphical user interface and getting a little practice.

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• For this part of the assignment you will make up a new .mdb or .accdb file and turn that in for credit.

• Do the following things:• 1. Create all of the tables in the given database

scenario. • Do this by taking the "Create table in design

view" option in Access.• 2. Make up suitable widths for the text fields.

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• 3. When you create the tables, make sure you specify the primary key field.

• You can do this by right clicking on the field and choosing the primary key option.

• 4. When you create the tables, make sure you specify that foreign keys can't be null.

• In Access, this is signified by "Required: Yes".

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• 5. Also put an index on every foreign key field.

• Notice that there are such things as indexes with duplicates and those with no duplicates.

• For a foreign key field you want to allow duplicates.

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• 6. Once the tables have been created, go to the Database Tools tab and click on the "Relationships" tool in the toolbar.

• Add your tables to the relationships area and enforce referential integrity on all pk-fk pairs.

• You do this by clicking on the "one" field, dragging, and dropping on the "many" field.

• Then check the box to enforce referential integrity and check the box to cascade on updates.

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• 7. Once you've created the tables and set up referential integrity, pop the tables open and enter data into them.

• Enter the example record given for each, and make up 2 more rows containing suitable data for each table.

• You have to be careful when making up data so that foreign key fields do not contain values that are not in the corresponding primary key fields.

• If you violate referential integrity when entering data, you will get an error message.

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• Remember that in order to do this successfully, you will also have to enter primary key data before foreign key data

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12.5.3 Example Assignment

Doctor Diagnosis Patient Insurance Company

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• Verbal Description of Scenario: • The insurance company's payment rate is a

percent, like 80%, expressed as a decimal, .80.

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Table Schemas and Sample Records:

• Doctor, (123, William Smith, ophthalmology, 562-3478)

• Idno text pk• Docname text• Specialty text• Phone text

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• Patient, (789, Sara Jones, 456 Karluk St., 111)• Idno text pk• Patname text• Address text• Insurancecoidno text fk

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• Insuranceco, (111, Blue Cross Blue Shield, 782-3406, 100.00, .80)

• Idno text pk• Coname text• Phone text• Deductible currency• Paymentrate number

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• Diagnosis, (101, astigmatism, 5/1/2007, 123, 789)

• Idno text pk• Date date• Doctoridno text fk• Patientidno fk

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The End