1. Queries - Access Cookbook, 2nd Edition [Book]
Selection from Access Cookbook [Book] You probably already know that you can use an update query to update the If you misspell the source field name, you run the risk of changing the values to the misspelled string rather than to the values in the source field. Many of the purchase values in tblAlbums are null. Most queries you design in Access will likely use multiple tables, allowing you to . to an Access query, this is what you'll see in the Object Relationship pane. Learn how to create a relationship between tables with this Microsoft Access tutorial. A primary feature of relational databases is that they contain multiple tables, . Creating relationships like the one above can assist in building queries Create a Totals Row · Create a Calculated Field · Create a Parameter Query.
Click Detail shows every field of every recordand then click Next. The query returns a list of orders, each with its shipping fee and the first and last name of the employee who handled it. Top of Page Connect the data in two tables by using their relationships with a third table Often, data in two tables are related to each other through a third table.
This is usually the case because the data between the first two tables are related in a many-to-many relationship.
Access Designing a Multi-Table Query
Often, it is good database design practice to split a many-to-many relationship between two tables into two one-to-many relationships involving three tables. You do this by creating a third table, called a junction table or a relationship table, that has a primary key and a foreign key for each of the other tables.
A one-to-many relationship is then created between each foreign key in the junction table and the corresponding primary key of one of the other tables. In such cases, you need to include all three tables in your query, even if you want to retrieve data from only two of them. Build a select query by using tables with a many-to-many relationship On the Create tab, in the Queries group, click Query Design. The Show Table dialog box opens. In the Show Table dialog box, double-click the two tables that contain the data you want to include in your query and also the junction table that links them, and then click Close.
All three tables appear in the query design workspace, joined on the appropriate fields. Double-click each of the fields that you want to use in your query results. Each field then appears in the query design grid.
In the query design grid, use the Criteria row to enter field criteria. To use a field criterion without displaying the field in the query results, clear the check box in the Show row for that field.
To sort the results based on the values in a field, in the query design grid, click Ascending or Descending depending on which way you want to sort the records in the Sort row for that field.
MS Access Many-To-Many Relationship
On the Design tab, in the Results group, click Run. Access displays the query output in Datasheet view. An example that uses the Northwind sample database Note: You may want to make a backup copy of the Northwind sample database, and then follow this example by using the backup copy. Suppose you have a new opportunity: They supply every category of food product that you broker. They are a fairly big business, and want your assurances that you can provide them access to enough potential sales to make it worthwhile: Can you provide them with the market they require?
The data you need to answer this question is found in two places: These tables are linked to each other by an Orders table. Relationships between the tables have already been defined. In the Orders table, each order can have only one customer, related to the Customers table on the CustomerID field.
Thus, a given customer can have many orders, each of which has many order details. Build the query in Design view Open the Northwind database. On the Create tab, in the Queries group, click Query Design. All three tables appear in the query design workspace. In the Customers table, double-click the City field to add it to the query design grid.
This causes only those records where the customer is in one of these two cities to be included in the query. The fields are added to the query design grid. In the ShippedDate column in the query design grid, select the Field row. Replace [ShippedDate] with Year: This creates a field alias, Year, that allows you to use just the year portion of the value in the ShippedDate field.
In the UnitPrice column in the query design grid, select the Field row. Replace [UnitPrice] with Sales: This creates a field alias, Sales, that calculates the sales for each record. On the Design tab, in the Query Type group, click Crosstab. Two new rows, Total and Crosstab, appear in the query design grid.
In the City column in the query design grid, click the Crosstab row, and then click Row Heading. This makes city values appear as the row headings that is, the query returns one row for each city.
Create relationships with the Lookup Wizard
In the Year column, click the Crosstab row, and then click Column Heading. This makes year values appear as the column headings that is, the query returns one column for each year. In the Sales column, click the Crosstab row, and then click Value. This makes sales values appear at the intersection of rows and column that is, the query returns one sales value for each combination of city and year. In the Sales column, click the Totals row, and then click Sum.
This causes the query to sum the values in this column. You can leave the Totals row for the other two columns at the default value of Group By, because you want to see each value for these columns, not aggregate values. Top of Page View all the records from two similar tables Sometimes, you will want to combine data from two tables that are identical in structure, but one of them is located in another database.
Consider the following scenario. Suppose you are an analyst working with student data. You are embarking on a data sharing initiative between your school and another school, so that both schools can improve their curricula. As you can see in the image above, our criteria requiring phone numbers to begin with must be typed like this: To learn how to write additional criteria, consult our printable Query Criteria Quick Reference Guidewhich includes several of the most common criteria used in Access queries.
Joining tables in queries The final thing you need to consider when designing a query is the way you link—or join—the tables you're working with.
When you add two tables to an Access query, this is what you'll see in the Object Relationship pane: Joined tables in the Object Relationship Pane The line connecting the two tables is called the join line. See how the join line is actually an arrow? This is because it indicates the order in which the query looks at data from the two tables.
In the image above, the arrow is pointing from left to right, which means the query will look at data in the left table first, then look at only the data in the right table that relates to the records it's already seen in the left table. Your tables won't always be joined this way. Sometimes Access will join them right to left. In either case, you might need to change the direction of the join to make sure your query includes the correct information.
The join direction can affect which information your query retrieves. To understand what this means, consider the query we're designing. For our query, we need to see customers who have placed orders, so we've included the Customers table and the Orders table.
Let's take a look at some of the data contained in these tables. Related data stored in the Customers and Orders tables What do you notice when you look at these lists?
First of all, every single order in the Orders table is linked to someone in the Customers table—the customer who placed that order.
However, when you look at the Customers table, you'll see that the customers who've placed multiple orders are linked to more than one order, and those who've never placed an order are linked to no orders.
As you can see, even when two tables are linked it's possible to have records in one table that have no relationship to any record in the other table. So what happens when Access tries to run our query with the current join, left to right? It pulls every record from the table to the left: The Left to Right join retrieves all the records from the table on the left first.
Create a query based on multiple tables
It then retrieves every record from the right table that has a relationship with a record Access has already taken from the left table. The query then retrieves the orders linked to the customer records it already pulled. Because our join began with the Customers table, our query will include records for all of our customers, including those who've never placed orders.
This is more information than we need. We only want to see records for customers who have placed orders. Fortunately, we can fix this problem by changing the direction of the join line. If we join the tables from right to left instead, Access will first retrieve the orders from the right table, our Orders table: The Right to Left join retrieves all the records from the table on the right first.
Then Access will look at the left table and retrieve only the records of customers who are linked to an order on the right. Next, Access retrieves only the records from the left table that are linked to existing orders from the right.
We now have exactly the information we want: As you can see, we had to join our tables in the correct direction to obtain the information we wanted. Now that we understand which join direction we need to use, we're ready to build our query!
In our query, we needed to use the right-to-left join, but the correct join direction for the tables in your queries will depend on what information you want to see and where that information is stored.
When you add tables to a query, Access will automatically join the tables for you, but it often doesn't join them in the correct direction. This is why it's important to always review the joins between your tables before you build a query.
Creating a multi-table query Now that we've planned our query, we're ready to design and run it. If you have created written plans for your query, be sure to reference them often throughout the query design process. To create a multi-table query: Select the Query Design command from the Create tab on the Ribbon. Clicking the Query Design Command In the dialog box that appears, select each table you want to include in your query and click Add.
You can press and hold the Ctrl key on your keyboard to select more than one table. When we planned our query, we decided we needed information from the Customers and Orders table, so we'll add these. Selecting tables to use in the query After you have added all of the tables you want, click Close.
The tables will appear in the Object Relationship pane, linked by a join line. Double-click the thin section of the join line between two tables to edit its join direction. Clicking the join line to edit its direction The Join Properties dialog box will appear.
- Access Cookbook by Paul Litwin, Ken Getz, Andy Baron
- Access Cookbook, 2nd Edition by Andy Baron, Paul Litwin, Ken Getz
- Define relationships between tables using Access SQL
Select an option to choose the direction of your join. In our query, the left table is the Customers table, so choosing this would mean all of the customers who met our location criteria—whether or not they had placed an order—would be included in our results.
We don't want to choose this option for our query. Because our right table is our Orders table, selecting this option will let us work with records for all of the orders and only the customers who've placed orders. We'll choose this option for our query because this is exactly the data we want to see. In the table windows, double-click the field names you want to include in your query. They will be added to the design grid in the bottom part of the screen.
In our example, we'll include most of the fields from the Customers table: We'll also include the ID number from the Orders table. Adding table fields to the query Set field criteria by entering the desired criteria in the criteria row of each field.
We want to set two criteria: First, to find customers who do not live in Raleigh, we'll type Not in "Raleigh" in the City field.