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Working with joins

From: Access 2010: Queries in Depth

Video: Working with joins

When creating queries that include multiple tables, it's important to understand how those tables are related in order to get meaningful results. Typically, Access will be able to assist you in building these relationships if they're not previously defined. But there are occasions when you'll need to create or modify how your tables are grouped together. Let's take a look at a couple of examples. In the Chapter 2-6 group, let's go ahead and open that and we'll open up our Sales department query. You can see that we're getting Employee ID, FirstName, and the Department is Sales, and we have a total of 84 employees in the Sales department.

Working with joins

When creating queries that include multiple tables, it's important to understand how those tables are related in order to get meaningful results. Typically, Access will be able to assist you in building these relationships if they're not previously defined. But there are occasions when you'll need to create or modify how your tables are grouped together. Let's take a look at a couple of examples. In the Chapter 2-6 group, let's go ahead and open that and we'll open up our Sales department query. You can see that we're getting Employee ID, FirstName, and the Department is Sales, and we have a total of 84 employees in the Sales department.

If we look at this in Design view, we'll see that we've got the Employee ID, FirstName and Department, and the Criteria Sales. So 84 employees in our Sales department. Keep that number in mind. Let's go ahead and close this query out. We'll open up this query MaySales. Here we've got the employee ID, FirstName, and the May sales figures. But you can see we only have 31 records here. What happened to all the other employees? Let's go into Design view and take a look at this. This query is constructed with the Employees table, the Orders table, and the Products table.

We have the Employee ID and the FirstName coming from the Employees table, the Price coming from the Products table, and the OrderDate coming from the Orders table. Under Criteria, we have >=#5/1/2010# And <=#5/31/ 2010#, basically all of May. Finally, in the Price column, we have the total of Sum. So it's adding up all of the prices over the course of May. So why when we run this aren't getting all of our employees in the sales department? Well, we're only getting the employees that actually had sales in the month of May.

If I double-click on the join line between tbl_Employees and tbl_Orders, we'll see that the default view is to only include rows where the joined fields from both tables are equal. So that's why our results are only showing sales numbers for the employees that actually had sales. We're not seeing anything for the employees that didn't have sales. Now, we can construct a single query that fixes this issue. But if we were to try and change this here to include all records from the tbl_Employees and only those records from tbl_Orders where the joined fields are equal, Access won't allow us to do that.

If I try and run this query, Access is going to give us an error saying the SQL statement could not be executed because it contains ambiguous outer joins. In order to fix this, we need to create another query and use this MaySales query as an input. Okay, let me show you what I mean. Let's go ahead and close this MaySales query. Now we don't need to save changes to those. We'll create a new query in Design view. Go to Create > Create Design. First, we'll add our Employees table, and then we'll switch the tab to Queries and we will choose qry_MaySales.

Go ahead and close the Show Table window and you'll see that Access has generated a join between EmployeeID and EmployeeID here. Let's add a couple of fields to our query. Let's add our EmployeeID, FirstName, Department, and our May sales figure from that query. Now, if I run that query right now, we'll see the same number of records, 31, that we saw in the previous query. Again, we're only displaying the employees that had figures for May.

Go back into Design view and let's go ahead and change this relationship between the employees and the query that we had built by double-clicking on the line. What we want is the option to include all records from the tbl_Employees and only those records from MaySales where the joined fields are equal. Now depending on which order you put your tables in, these two options might be reversed. So yours might be 3 or 2. But what we're looking for is we're including all records from the Employees table.

Access updates the join line to indicate the choices that we made with the arrow pointing to the MaySales field. If we run this query now, we're going to get all of our employees regardless of whether they had sales or not. Now we're getting 200 and this is not the 84 people in our Sales department. So we have one more thing that we need to change here. You can see that we're also getting employees in the Advertising or Human Resources departments and it makes sense that they wouldn't any sales for the month of May. So let's go ahead and filter them out in Design view. Under Department, I'll change my Criteria to Sales.

Now, when I run this query, I'll get all 84 people that are in the Sales department, and for the people that had sales in the month of May we'll get their totals. If they didn't have any sales in May, it'll be a blank field here in MaySales column. So understanding how your data tables relate to one another is a critical component to creating well-structured queries that give meaningful results.

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This video is part of

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Access 2010: Queries in Depth

46 video lessons · 13837 viewers

Adam Wilbert
Author

 
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  1. 9m 9s
    1. Welcome
      1m 10s
    2. Using the exercise files
      41s
    3. Introducing the database
      4m 29s
    4. Previewing the course
      2m 49s
  2. 17m 17s
    1. Understanding queries
      3m 31s
    2. Following naming conventions and best practices
      2m 56s
    3. Using the Query Wizard
      5m 21s
    4. Exploring the design interface
      5m 29s
  3. 26m 39s
    1. Defining criteria
      5m 40s
    2. Understanding comparison operators
      3m 19s
    3. Defining the column headers
      2m 49s
    4. Exploring the property sheet
      7m 32s
    5. Printing query results
      2m 41s
    6. Working with joins
      4m 38s
  4. 14m 14s
    1. Understanding parameter queries
      4m 27s
    2. Obtaining parameters from forms
      5m 17s
    3. Creating a combo box
      4m 30s
  5. 23m 24s
    1. Understanding the Totals field
      5m 31s
    2. Creating aggregate calculations
      3m 31s
    3. Exploring the Expression Builder interface
      4m 28s
    4. Using mathematical operators
      5m 46s
    5. Applying text functions
      4m 8s
  6. 24m 23s
    1. Understanding dates as serial numbers
      2m 42s
    2. Specifying a range of dates or times
      3m 47s
    3. Formatting dates
      4m 31s
    4. Using other Date/Time functions
      3m 47s
    5. Defining today's date
      2m 41s
    6. Calculating time intervals
      6m 55s
  7. 20m 9s
    1. Introducing the conditional IIf function
      2m 57s
    2. Creating an IIf function
      7m 31s
    3. Nesting IIf functions
      4m 57s
    4. Using the Switch function
      4m 44s
  8. 20m 41s
    1. Understanding the reporting tool
      2m 13s
    2. Building the form
      6m 57s
    3. Building the query
      5m 4s
    4. Building the report
      3m 30s
    5. Finalizing the reporting tool
      2m 57s
  9. 25m 37s
    1. Finding duplicate records
      2m 17s
    2. Identifying unmatched records
      2m 29s
    3. Creating crosstab results
      2m 57s
    4. Creating backups
      1m 29s
    5. Creating update queries
      3m 22s
    6. Making, deleting, and appending records
      5m 36s
    7. Uniting tables
      3m 16s
    8. Embedding SQL code in queries
      4m 11s
  10. 1m 0s
    1. Next Steps
      1m 0s

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