IE 423 – Design of Decision Support Systems Database development – Relationships and Queries

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Neighbor08a.mdb

Remember that we split our House Survey data into two relations (tables) …so now we need to create a table for the rest of the data Don’t forget that each record must have a unique house identifier …and what we will use this for?

New tables for Neighbor08 database

Neighbor08a.mdb Make a relationship between these two tables How do you do this? What kind of relationship is this?

Neighbor08a.mdb Then, we are also interested in who lives in these houses… …so create another table for people You will need – PersonID HouseID –to tell which house they live in FirstName LastName DOB (what data type?) CellPhone (use an input mask)

Neighbor08a.mdb

Make a relationship between what two tables? How do you do this? What kind of relationship is this?

Referential Integrity – what does it really mean? When we have a one-to-many relationship, we might call the table on the one side the parent table,… And the table on the many side the child table And for any given relationship, we can call the primary/foreign key pair in the related tables linking fields Referential Integrity – all foreign key values in a child table must come from (match) primary key values in the parent table

Referential Integrity – what does it really mean? Referential Integrity – Five Rules The Linking field in the parent table must be a primary key or must be indexed with unique value entries Linking fields must be of the same data type, exception autonumber and number (with length=ReplicationID) When you enforces Referential Integrity no existing records in the tables can violate referential integrity With Referential Integrity enforced, no record can be deleted where its key value matches a value use in foreign key in the child table With Referential Integrity enforced, we cannot change values of the linking field in the parent table there is a corresponding value in the linking field of the child table

College.mdb

Referential Integrity – what does it really mean?

Cascading So, how do we do these things? Access helps Cascading Deletes  If you delete the record in the parent table that contains the Primary key value, access will automatically delete matching records in the child table  Think about this Cascading Deletes  If you change a primary key value in the parent table, Access will automatically change the corresponding foreign key values in all records in child table

College.mdb Suppose you need to create a course registration system for college courses (or workshops, etc.) What are the objects that you need to work with? Students, Courses, ??? So, you have a table of students You have table of courses How do you relate these two tables? Does this present any problems?

College.mdb Solution – create two one-to-many relationships,… …but to do this you are missing something – What?

College.mdb ***Open College.mdb Build a registration system Enroll students in courses Any student can be in one or more courses Any course can have one or more (or zero) students

College.mdb Build a Registrations table In general terms this is sometimes called a linking table

College.mdb Define the relationships between Students & Registrations Courses & Registrations Register students for courses

Queries You’ve built tables,… You’ve added data… So, how do you use this data Queries (at least that’s one answer)

Queries Queries retrieve and, maybe, manipulate data in your database Several kinds of queries Select Query – retrieves data from tables  Simple Query – from a single table  Complex Query – from multiple tables Parameter Query – dynamic input of criteria Crosstab Queries – statistical aggregation of data …

Queries Several kinds of queries (…continued) Action Queries – operate on data in a table or multiple tables  Delete Queries – delete records based on selection criteria  Update Queries – modify field values based on selection criteria  Append Query – appends records from other tables to existing table

Queries Select Queries Single table query

Queries Select Queries Single table query Similar to defining a table, … …except query field come from other table fields (or other queries) To build a query you need  To create column (fields)  Pick the table from which to pull the field (no really an issue in a simple query  Sort or not  Show field or not  Define Select criteria

Queries Select Queries Fields and tables drag and drop Sort – left to right order – if checked Show – do you want to see it? Criteria  Logical test for record selection Criteria in multiple columns – “AND”ed Logical OR – add criteria to more rows

Queries Select Queries Build a simple query to Select all Senior Industrial Engineering students Select all Seniors and Juniors Select all Senior I.E. student and all Juniors

Queries Select Queries Build a simple query to Select all Senior Industrial Engineering students Select all Seniors and Juniors Select all Senior I.E. student and all Juniors

Queries Select Queries Complex Queries  Similar to simple queries, except…  Add more tables to the design grid  Tables need relationships  …may be already defined,…  …or you can design them in the query design grid

Queries Select Queries Complex Queries  Connecting multiple tables in a query is called a join operation  Two major kinds of joins Results include records from both source tables only if records match, otherwise not included Inner Join Include a selected record from one table (regardless of whether it has a match in the other table) and records from the other table that match records in the first table Outer Join

Queries Select Queries Complex Queries  OK, so what is a left outer join vs. a right outer join?