FAQ: SQL Queries in Visual Basic

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1. Q. How can I test my SQL query quickly and easily?
   
   A. Use VisData. VisData is a sample application that ships with Visual Basic 4.0. You'll find it in C:\VB\Samples\VisData\VisData.vbp. It has a window to test SQL queries, can open all of the database types that Visual Basic supports, and has proven to be very stable.
2. Q. How do I write an SQL query that has values that contain single or double quotation marks?
   
   A. Single quotation marks ('), double quotation marks ("), and the pipe symbol (|) are all special characters in Microsoft Jet SQL. Double up the quotation marks or, if you have only one type of quotation mark (single or double) in your data, use the other one as the delimiter. All other characters are handled as is.
   
   
3. Q. How do I use a Visual Basic variable in an SQL query?
   
   A. Visual Basic variables and properties must be concatenated into the SQL string. If you embed the name of the variable, your code will search using the variable name rather than the variable value.
   
   
4. Q. How can I call stored procedures (queries) for SQL Server databases?
   
   A. You can pass the stored procedure name to an ExecuteSQL, a CreateDynaset, or a DataControl's RecordSource property.
   
   
5. Q. How can I select a specific list of values or exclude certain records in a Select statement?
   
   A. Use the In keyword to identify the values you want to include or exclude, adding the Not keyword to exclude, as in this example:

         Select * From Authors Where Au_ID In (1, 3, 5, 7, 9)
         Select * From Authors Where Au_ID Not In (1, 3, 5, 7, 9)
     

6. Q. How can I build a Select statement that works with multiple tables?
   
   A. A simple Select statement across unrelated tables returns a useless cross product of all the records in the tables listed in the From clause:

         Select * From Publishers, Titles
      

   To get a useful list of data, your data must have a common key field (or set of fields) that relate the tables. You can retrieve data from your tables by simply ensuring that these key fields match for each row you retrieve:

         Select * From Publishers, Titles
            Where Publishers.PubID=Titles.PubID
      

   However, this returns rows without recognizing the relationship between the records in the separate tables, so the resulting RecordSet will not be updatable. To return an updatable recordset, you need to join the tables as in this example:

         Select * From Publishers
            Inner Join Titles On Publishers.PubID=Titles.PubID
      

   For more information, please see the "Joins" topic in the Visual Basic Help file and the following articles in the Microsoft Knowledge Base:

   102681 : PRB: Error When Updating Fields in Dynaset That Has 2+ Tables

   
   
   
7. Q. How can I build a Select statement that includes multiple tables in more than one database?
   
   A. You can specify an external database with the <table> IN <database> syntax, specify the connect string in your SQL query, or attach the tables to a Microsoft Access database.
   
   
8. Q. How can I return a default value in my Select statement result when the real value stored in the table is null?
   
   A. You can construct a SQL statement that returns a default value rather than Null for those records where the value is actually null and use the Union operator to add these records to those where the value is not null. For example, the following query returns the word Anonymous for any record in the Authors table where the Author field is null. (To make this example work, you need to add a record with an AU_ID value but no Author value.)
   
   (Select Author, Au_ID From Authors Where Not IsNull(Author)) Union (Select "Anonymous" As Author, Au_ID From Authors Where IsNull(Author))
9. Q. How can I make my SQL statement run faster?
   
   A. For a list of hints, please see the following articles in the Microsoft Knowledge Base:

   112112 : How to Optimize Microsoft Access Version 2.0 Queries
   
   129882 : How to Optimize SQL Queries in Visual Basic 3.0 and 4.0
   
   99321 : Tips for Optimizing Queries on Attached SQL Tables

10. Q. Why can't I get a parameter query to work on the Data Control?
    
    A. This is a limitation of the Data Control. It does not support parameter queries. The Properties window lists any available non-parameter queries, but does not list any parameter queries in the database, so it's not possible to make the mistake of using a parameter query for a Data Control at design time.
    
    The work around to this limitation is to run the parameterized query into a scratch recordset variable and then set that result into the recordset of the Data Control.