 Review quiz. Answer questions.  Discuss queries: ◦ What is a query? Turning data stored in a database into information for decision making. ◦ You: Completed two tutorials with step-by-step instructions for creating queries in MS Access. Now must apply knowledge and skills learned in the tutorials. ◦ Class today:  Highlight key issues about queries.  Explain aspects of MS Access queries in more detail.  Answer any questions about queries in preparation for completion of the next part of the project (due 10/04).

 What is the difference between a query and a table?  What is the difference between a query datasheet and a table datasheet?  Why do we create queries when we already have data in tables?

Query Underlying Tables Result Table

A query reduces the number of rows and columns in the underlying tables to provide information for decision making. A query enhances the data in the underlying tables by adding calculations and logical conditions.

 Table contains structure of data, constraints and actual data. ◦ Table is referred to as “underlying data”.  Query is a way to look at the data. ◦ Queries seldom look at the complete contents of a table because tables are usually very big, with many columns and many rows. ◦ The goal of creating a query is to provide appropriate data for decision making. ◦ Queries “filter” the data; fewer columns, fewer rows, calculated fields, summarized information.

 Individual row queries. ◦ Using one table. ◦ Using multiple tables.  Aggregate queries. ◦ Creating one line in the result table. ◦ Creating multiple groups in the result table.  Parameter queries.

 Pre-written functions exist to do common summary calculations: ◦ Sum, count ◦ Max, min ◦ Avg, stDev, var ◦ First, last  Can do calculations for all data in a result table, or grouped data in a result table

 Criteria. ◦ And vs. Or ◦ Relational operators., =, IN, LIKE ◦ Wildcards  Multiple tables.  Calculations.  Logical conditions. ◦ IIF ◦ NOT

 Each value in a field has very specific data coded for a computer to read.  Humans can discern vague similarities and differences among data fairly easily. Computers are more exacting.  Computers need you to tell them when data is a date, or a character, or a number.  A zero is not the same as a blank which is not the same as a null.  A null is a special character assigned to a field that technically has “no value”. It is very useful because we can search for a null value with special operators.

 Computers require very explicit instructions.  MS Access has default instructions, but that is because it is considered a very friendly, user-oriented package.  Normally, must be very explicit about relational operators on the conditions of queries. ◦ =, >, =, <= ◦ Like ◦ Between ◦ In ◦ Is  Wildcard is an asterisk.

 Referred to as “joining” tables.  Can produce confusing results.  Very dependent on a well-designed database. The tables must be related with appropriate foreign keys or the tables cannot be joined correctly for queries.

 Frequently want to see if something is TRUE or FALSE.  Example: If a training event has > $500 in expenses, then it is a high expense event.  Logical condition for Access: ◦ IIF(trainingexpenses + travelexpenses > 500, “high expense”) ◦ IIF(ISNULL(trainingexpenses + travelexpenses), 0) ◦ IIF(ISNULL(trainingexpenses + travelexpenses), 0, trainingexpenses + travelexpenses)

 5 minutes.  Turn to the person or people next to you.  Describe at least 3 queries that would be relevant to the training database we use for the Access project. ◦ None of the queries can be from the Access Project Part 2. ◦ Make sure that one of the queries requires calculations.

 Design view: Used to structure a query. Referred to as “query by example” or QBE.  Result table: The table produced by the query. Shown in the datasheet view.  SELECT query window: The window displayed in design view that is filled out to produce a result table. Also called the query design grid.  Field row: The area in the SELECT query window used to define what columns should appear in the result table.  Criteria row: The area in the SELECT query window used to identify which rows should appear in the result table.

 Can do calculations for a column based on the data in other columns for that same row.  Can use mathematical operators.  Can use pre-written functions in MS Access. Many different types of pre-written functions for date handling, data type conversion, calculations, etc. ◦ See the pre-written functions in the expression builder.  Can be very simple to very complicated.