Building Queries

When initially opening the tool, it will open a database connection to the database server currently selected in the server browser. Alternitavely, you may right mouse-click on the server desired and select the "Query Builder" menu item to launch the tool. Adding Tables To the Query - When the Query Builder is initially opened, the add table dialog will open, allowing you to choose tables you would like in your query.

You may also choose the "Add Table" menu item in the toolbar to add more tables at any time if necessary. The Add Table chooser allows you to automatically include columns in the query, or you may choose them individually from the table entity in the diagram by clicking on the check box left of the column. The table entity contains a list of columns within the table and one extra row to indicate all columns of the table.

The primary key columns are bolded , and columns participating in indexes are in italics. This will limit options in the schema combo box to the user schema. This can provide a performance boost in scenarios when the database has a large number of user schemas and tables, by limiting the list of tables and views to the users objects. Creating JOINs with drag and drop - JOINs for your query are created by dragging table columns in one table entity and dropping them off on columns of another table entity in table diagram pane.

JOINs are indicated by connecting lines. JOINs can be removed by right clicking the line and selecting Remove. The JOIN icon changes to indicate all rows of a table have been selected. Adding Columns to the Query - Columns are added by checking their check box on the left-hand side of a Table Entity. When a column is checked, it appears in the Columns Pane. Aliases can be created and edited by entering text within the Alias column inside the Columns Pane. The SQL pane updates instantly to show the aliases in script form for review. Adding Filter criteria - You may add filters to your query by adding criteria to the [criteria] column of the columns grid pane.

The [Advanced Mode] offers a [Criteria] free form column for operator and values, and an extended list of [Or For most relational algebra operations, the order of execution does not matter, which means that the same result can be reached by forming and combining intermediate results in different ways. In practice, database queries are pretty far made with the help of operations that resemble the relational algebra operations. The order of executing operations and producing intermediate results is determined by a query optimizer.

The order of execution and the intermediate results are important for the efficiency of the query. The query optimizers try to arrange the operations so that the whole execution will be as efficient as possible. In practice, this usually means that they try to minimize the intermediate results as quickly as possible. Information about cars of year model, where faults have been found in the inspection for year Information about inspections is stored in table Inspection and if faults are found they are registered in table Problem.

Thus we need these three tables to obtain the information we want.

Only cars of year model are interesting. The model year of a car is represented as the value of attribute ModelYear in table Car. Our first intermediate result consists of tuples representing cars of model year This is obtained using selection. The cars we are interested in should have been inspected for the period Thus we need only the rows that cover that period.

We use selection to retrieve them. Now we have the cars and inspections we were interested in. We have to connect the rows. We use the join operation. On the Design tab, click Run. The query runs, and displays a list of products and their prices. For example, if you have a database for a store that sells food items and you want to review orders for customers who live in a particular city. Say that the data about orders and data about customers are stored in two tables named Customers and Orders respectively. If each table has a Customer ID field, which forms the basis of a one-to-many relationship between the two tables.

You can create a query that returns orders for customers in a particular city, for example, Las Vegas, by using the following procedure:.

On the Create tab, in the Query group, click Query Design. Close the Show Table dialog box. This line shows the relationship between the two tables. In the Customers table, double-click Company and City to add these fields to the query design grid. In the query design grid, in the City column, clear the check box in the Show row.

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In the Criteria row of the City column, type Las Vegas. Clearing the Show check box prevents the query from displaying the city in its results, and typing Las Vegas in the Criteria row specifies that you want to see only records where the value of the City field is Las Vegas. In this case, the query returns only the customers that are located in Las Vegas.

In the Orders table, double-click Order ID and Order Date to add these fields to the next two columns of the query design grid. On the Design tab, in the Results group, click Run. The query runs, and then displays a list of orders for customers in Las Vegas. If you frequently want to run variations of a particular query, consider using a parameter query.

When you run a parameter query, the query prompts you for field values, and then uses the values that you supply to create criteria for your query. You cannot create a parameter query in an Access web app. Continuing from the previous example where you learnt to create a select query that returns orders for customers located in Las Vegas, you can modify the select query to prompt you to specify the city each time that you run the query. To follow along, open the database that you created in the previous example:. In the Navigation Pane, right-click the query named Orders by City that you created in the previous section , and then click Design View on the shortcut menu.

In the query design grid, in the Criteria row of the City column, delete Las Vegas , and then type [For what city? The string [For what city? The square brackets indicate that you want the query to ask for input, and the text in this case, For what city? Select the check box in the Show row of the City column, so that the query results will display the city.

The query prompts you to enter a value for City.

Queries help you find and work with your data

What if you don't know what values you can specify? You can use wildcard characters as part of the prompt:. In the query design grid, in the Criteria row of the City column, type Like [For what city? You can also specify what type of data a parameter should accept.

Create a simple select query

When you specify the data type that a parameter should accept, users see a more helpful error message if they enter the wrong type of data, such as entering text when currency is expected. If a parameter is set to accept text data, any input is interpreted as text, and no error message is displayed. To specify the data type for parameters in a query, use the following procedure:. In the Query Parameters dialog box, in the Parameter column, type the prompt for each parameter for which you want to specify the data type. Make sure that each parameter matches the prompt that you use in the Criteria row of the query design grid.

In the Data Type column, select the data type for each parameter. For more information, see using parameters to ask for input when running a query. The Total row in a datasheet is very useful, but for more complex questions, you use a totals query. A totals query is a select query that allows you to group and summarize data, like when you want to see total sales per product. In a totals query, you can use the Sum function an aggregate function , to see total sales per product. You cannot use aggregate functions in an Access web app. Use the following procedure to modify the Product Subtotals query that you created in the previous example so that it summarizes product subtotals by product.

Although they have similar names, the Totals row in the design grid and the Total row in a datasheet are not the same:. When you use the Totals row in the design grid, you must choose an aggregate function for each field. If you do not want to perform a calculation on a field, you can group by the field. In the second column of the design grid, in the Total row, select Sum from the drop-down list. The query runs, and then displays a list of products with subtotals. For more information, see Display column totals in a datasheet using a Totals row.

You usually would not use tables to store calculated values, like subtotals, even if they are based on data in the same database, because calculated values can become outdated if the values that they are based on changes. For example, you would not store someone's age in a table, because every year you would have to update the value; instead, you store the person's date of birth, and then use a query to calculate the person's age.

Building Queries

This database has a table called Orders Details that has information about the products in fields such as, price of each product and the quantities. You can calculate the subtotal by using a query that multiplies the quantity of each product by the unit price for that product, multiplies the quantity of each product by the unit price and discount for that product, and then subtracts the total discount from the total unit price. If you created the sample database in the previous example, open it and follow along:.

In the Order Details table, double-click Product ID to add this field to the first column of the query design grid.