Most Access databases start out with a handful of saved queries, which is perfectly fine until users start asking for slightly different versions of the same list. One person wants customer names and email addresses. Another wants names and phone numbers. Before long, you have CustomerQ1, CustomerQ2, CustomerQ_Final, CustomerQ_Final2, and probably one called "UseThisOne." There is a better way.
With a QueryDef, VBA can create a query definition for you and save it right in the current Access database. Your code can decide which fields, criteria, sorting, joins, and other SQL pieces belong in the query. Access saves the resulting query object just as if you had built it manually in Query Design View.
A QueryDef is simply an Access object that represents a query definition. The important word there is definition. It stores the instructions for retrieving data, usually in SQL. It does not make a second copy of the records in your table.
Think of it as a recipe card. Your Customer table contains the actual ingredients. A saved query contains the recipe that tells Access which ingredients to retrieve, how to filter them, and how to display them. If somebody adds a new customer tomorrow, the saved query can return that customer because it is still reading the current data from the table.
When you create a query manually and save it in Access, you are creating a QueryDef behind the scenes. VBA gives you the ability to do the same thing programmatically. You can create a new query, modify an existing one, delete one you no longer need, or create a temporary query that exists only while your code is running.
The basic idea is wonderfully simple. You declare a variable as a DAO QueryDef, then use CurrentDb.CreateQueryDef to create it. That method needs two important things: a name for the saved query and the SQL statement you want Access to store in it.
For example, you might create a saved query named CustomerQ that selects CustomerID, FirstName, and LastName from your CustomerT table. The SQL can include a WHERE clause, an ORDER BY clause, calculated fields, joins, aliases, or whatever else the situation calls for. The full code walkthrough is in the embedded video, but the key point is that Access receives a query name and a SQL statement, then saves that definition as a query object.
If the QueryDef has a name, such as CustomerQ, it is saved in the database and eventually appears in the Navigation Pane. You may need to refresh the Navigation Pane before you see it. Clicking in the pane and pressing F5 is often enough. Once it appears, you can open it, inspect it in SQL View, or modify it in Query Design View like any other saved Access query.
There is one little gotcha: the query name must not already exist. If you try to create CustomerQ when CustomerQ is already in the database, Access will complain. It is not fond of duplicate object names, much like I am not fond of duplicate credit card charges.
For a one-time generated query, using a unique name may be enough. But if your application repeatedly generates the same type of query, you will usually want code that checks for the old query and deletes or replaces it before creating the new one. That keeps your Navigation Pane from turning into a junk drawer full of abandoned generated queries.
So why not just create the query manually? If the query will always have the same fields, conditions, and sorting, then by all means create it manually. There is nothing wrong with that. Not every Access problem needs VBA strapped to it.
The real value of QueryDefs appears when the structure of the query needs to change. Suppose one department needs FirstName, LastName, and EmailAddress. Another needs FirstName, LastName, and PhoneNumber. Those are not merely different search values. They are different SELECT lists.
VBA can build the SQL based on the user's choices. Your form might let users choose which fields they want to see, and the code can assemble the appropriate SELECT clause. One generated query may include email addresses, another may include phone numbers, and another may combine fields into a single display value using an SQL alias.
This is different from a parameter query. A parameter query is excellent when the query structure remains the same but the values change. For example, a query might ask for a customer ID, a start date, an end date, or a state. The SQL itself stays basically the same, and Access receives different values each time the query runs.
Parameters are for values. Query-building code is for definitions. You generally cannot safely pass a field name such as EmailAddress or PhoneNumber as a normal parameter and expect Access to treat it as part of the SQL structure. If the field list, table list, joins, sort order, or optional WHERE clauses need to change, then VBA needs to build the SQL statement itself.
Of course, the two techniques can work together. You can build a QueryDef whose structure is selected by your code and still use parameters for user-entered search values. That is often a cleaner approach than concatenating every date, ID, and text value directly into a SQL string.
A named QueryDef is useful when you want a query object that users can open later, use as the Record Source for a form or report, export, or inspect in the Navigation Pane. An unnamed QueryDef, created with an empty name, is temporary and exists only in memory. Those are handy when you need to run SQL from VBA without cluttering the database with another saved query object.
The big takeaway is simple: a QueryDef lets Access save SQL that your VBA code creates. The data remains safely in the underlying tables. The saved query is just the reusable set of instructions Access follows to retrieve that data.
Once you understand that distinction, dynamic query building becomes much less mysterious. You can create customer lists tailored to different users, build optional filters, generate export queries, create report sources, and avoid maintaining dozens of nearly identical saved queries by hand.
Watch the embedded video for the full demonstration of creating a saved QueryDef, refreshing the Navigation Pane, and opening the resulting query in Access.
Live long and prosper,
RR
No comments:
Post a Comment