ASP.NET 4 in VB 2010 : ADO.NET Fundamentals (part 2) - SQL Basics

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When you interact with a data source through ADO.NET, you use SQL to retrieve, modify, and update information. In some cases, ADO.NET will hide some of the details for you or even generate required SQL statements automatically. However, to design an efficient database application with a minimal amount of frustration, you need to understand the basic concepts of SQL.

SQL (Structured Query Language) is a standard data access language used to interact with relational databases. Different databases differ in their support of SQL or add other features, but the core commands used to select, add, and modify data are common. In a database product such as SQL Server, it's possible to use SQL to create fairly sophisticated SQL scripts for stored procedures and triggers (although they have little of the power of a full object-oriented programming language). When working with ADO.NET, however, you'll probably use only the following standard types of SQL statements:

  • A Select statement retrieves records.

  • An Update statement modifies existing records.

  • An Insert statement adds a new record.

  • A Delete statement deletes existing records.

If you already have a good understanding of SQL, you can skip the next few sections. Otherwise, read on for a quick tour of SQL fundamentals.

3.1. Running Queries in Visual Studio

If you've never used SQL before, you may want to play around with it and create some sample queries before you start using it in an ASP.NET site. Most database products provide some sort of tool for testing queries. If you're using a full version of SQL Server, you can try SQL Server Management Studio. If you don't want to use an extra tool, you can run your queries using the Server Explorer window described earlier. Just follow these steps in Visual Studio:

  1. Right-click your connection, and choose New Query.

  2. Choose the table (or tables) you want to use in your query from the Add Table dialog box (as shown in Figure 5), click Add, and then click Close.

    Figure 5. Adding tables to a query
  3. You'll now see a handy query-building window. You can create your query by adding check marks next to the fields you want, or you can edit the SQL by hand in the lower portion of the window. Best of all, if you edit the SQL directly, you can type in anything—you don't need to stick to the tables you selected in step 2, and you don't need to restrict yourself to Select statements.

  4. When you're ready to run the query, select Query Designer => Execute SQL from the menu. Assuming your query doesn't have any errors, you'll get one of two results. If you're selecting records, the results will appear at the bottom of the window. If you're deleting or updating records, a message box will appear informing you how many records were affected (see Figure 6).

Figure 6. Executing a query

When programming with ADO.NET, it always helps to know your database. If you have information on hand about the data types it uses, the stored procedures it provides, and the user account you need to use, you'll be able to work more quickly and with less chance of error.

3.2. The Select Statement

To retrieve one or more rows of data, you use a Select statement. A basic Select statement has the following structure:

SELECT [columns]
  FROM [tables]
  WHERE [search_condition]
  ORDER BY [order_expression ASC | DESC]

This format really just scratches the surface of SQL. If you want, you can create more sophisticated queries that use subgrouping, averaging and totaling, and other options (such as setting a maximum number of returned rows). By performing this work in a query (instead of in your application), you can often create far more efficient applications.

The next few sections present sample Select statements. After each example, a series of bulleted points breaks the SQL down to explain how each part of it works.

3.2.1. A Sample Select Statement

The following is a typical (and rather inefficient) Select statement for the pubs database. It works with the Authors table, which contains a list of authors:


  • The asterisk (*) retrieves all the columns in the table. This isn't the best approach for a large table if you don't need all the information. It increases the amount of data that has to be transferred and can slow down your server.

  • The From clause identifies that the Authors table is being used for this statement.

  • The statement doesn't have a Where clause. This means all the records will be retrieved from the database, regardless of whether it has 10 or 10 million records. This is a poor design practice, because it often leads to applications that appear to work fine when they're first deployed but gradually slow down as the database grows. In general, you should always include a Where clause to limit the possible number of rows (unless you absolutely need them all). Often, queries are limited by a date field (for example, including all orders that were placed in the last three months).

  • The statement doesn't have an Order By clause. This is a perfectly acceptable approach, especially if order doesn't matter or you plan to sort the data on your own using the tools provided in ADO.NET.

3.2.2. Improving the Select Statement

Here's another example that retrieves a list of author names:

SELECT au_lname, au_fname FROM Authors WHERE State='CA' ORDER BY au_lname ASC


  • Only two columns are retrieved (au_lname and au_fname). They correspond to the first and last names of the author.

  • A Where clause restricts results to those authors who live in the specified state (California). Note that the Where clause requires apostrophes around the value you want to match, because it's a text value.

  • An Order By clause sorts the information alphabetically by the author's last name. The ASC part (for ascending) is optional, because that's the default sort order.

3.2.3. An Alternative Select Statement

Here's one last example:

SELECT TOP 100 * FROM Sales ORDER BY ord_date DESC

This example uses the Top clause instead of a Where statement. The database rows will be sorted in descending order by order date, and the first 100 matching results will be retrieved. In this case, it's the 100 most recent orders. You could also use this type of statement to find the most expensive items you sell or the best-performing employees.

3.2.4. The Where Clause

In many respects, the Where clause is the most important part of the Select statement. You can find records that match several conditions using the And keyword, and you can find records that match any one of a series of conditions using the Or keyword. You can also specify greater-than and less-than comparisons by using the greater-than (>) and less-than (<) operators.

The following is an example with a different table and a more sophisticated Where statement:

SELECT * FROM Sales WHERE ord_date < '2000/01/01' AND ord_date > '1987/01/01'


This example uses the international date format to compare date values. Although SQL Server supports many date formats, yyyy/mm/dd is recommended to prevent ambiguity.

If you were using Microsoft Access, you would need to use the U.S. date format, mm/dd/yyyy, and replace the apostrophes around the date with the number (#) symbol.

3.2.5. String Matching with the Like Operator

The Like operator allows you to perform partial string matching to filter records where a particular field starts with, ends with, or contains a certain set of characters. For example, if you want to see all store names that start with B, you could use the following statement:

SELECT * FROM Stores WHERE stor_name LIKE 'B%'

To see a list of all stores ending with S, you would put the percent sign before the S, like this:

SELECT * FROM Stores WHERE stor_name LIKE '%S'

The third way to use the Like operator is to return any records that contain a certain character or sequence of characters. For example, suppose you want to see all stores that have the word book somewhere in the name. In this case, you could use a SQL statement like this:

SELECT * FROM Stores WHERE stor_name LIKE '%book%'

By default, SQL is not case sensitive, so this syntax finds instances of BOOK, book, or any variation of mixed case.

Finally, you can indicate one of a set of characters, rather than just any character, by listing the allowed characters within square brackets. Here's an example:

SELECT * FROM Stores WHERE stor_name LIKE '[abcd]%'

This SQL statement will return stores with names starting with A, B, C, or D.

3.2.6. Aggregate Queries

The SQL language also defines special aggregate functions. Aggregate functions work with a set of values but return only a single value. For example, you can use an aggregate function to count the number of records in a table or to calculate the average price of a product. Table 1 lists the most commonly used aggregate functions.

Table 1. SQL Aggregate Functions
Avg(fieldname)Calculates the average of all values in a given numeric field
Sum(fieldname)Calculates the sum of all values in a given numeric field
Min(fieldname) and Max(fieldname)Finds the minimum or maximum value in a number field
Count(*)Returns the number of rows in the result set
Count(DISTINCT fieldname)Returns the number of unique (and non-null) rows in the result set for the specified field

For example, here's a query that returns a single value—the number of records in the Authors table:


And here's how you could calculate the total quantity of all sales by adding together the qty field in each record:


3.3. The SQL Update Statement

The SQL Update statement selects all the records that match a specified search expression and then modifies them all according to an update expression. At its simplest, the Update statement has the following format:

UPDATE [table] SET [update_expression] WHERE [search_condition]

Typically, you'll use an Update statement to modify a single record. The following example adjusts the phone column in a single author record. It uses the unique author ID to find the correct row.

UPDATE Authors SET phone='408 496-2222' WHERE au_id='172-32-1176'

This statement returns the number of affected rows. (See Figure 7 for an example in Visual Studio.) However, it won't display the change. To do that, you need to request the row by performing another Select statement:

Figure 7. Executing an update query in Visual Studio

SELECT phone FROM Authors WHERE au_id='172-32-1176'

As with a Select statement, you can use an Update statement with several criteria:

UPDATE Authors SET au_lname='Whiteson', au_fname='John'
    WHERE au_lname='White' AND au_fname='Johnson'

You can even use the Update statement to update an entire range of matching records. The following example increases the price of every book in the Titles table that was published in 1991 by one dollar:

UPDATE Titles SET price=price+1
    WHERE pubdate >= '1991/01/01' AND pubdate < '1992/01/01'

3.4. The SQL Insert Statement

The SQL Insert statement adds a new record to a table with the information you specify. It takes the following form:

INSERT INTO [table] ([column_list]) VALUES ([value_list])

You can provide the information in any order you want, as long as you make sure the list of column names and the list of values correspond exactly:

INSERT INTO Authors (au_id, au_lname, au_fname, zip, contract)
       VALUES ('998-72-3566', 'Khan', 'John', 84152, 0)

This example leaves out some information, such as the city and address, in order to provide a simple example. However, it provides the minimum information that's required to create a new record in the Authors table.

Remember, database tables often have requirements that can prevent you from adding a record unless you fill in all the fields with valid information. Alternatively, some fields may be configured to use a default value if left blank. In the Authors table, some fields are required, and a special format is defined for the ZIP code and author ID.

One feature the Authors table doesn't use is an automatically incrementing identity field. This feature, which is supported in most relational database products, assigns a unique value to a specified field when you perform an insert operation. When you insert a record into a table that has a unique incrementing ID, you shouldn't specify a value for the ID. Instead, allow the database to choose one automatically.


If you're designing a database, it's a good design to add an auto-incrementing identity field to every table. This is the fastest, easiest, and least error-prone way to assign a unique identification number to every record. Without an automatically generated identity field, you'll need to go to considerable effort to create and maintain your own unique field. Often programmers fall into the trap of using a data field for a unique identifier, such as a Social Security number (SSN) or a name. This almost always leads to trouble at some inconvenient time far in the future, when you need to add a person who doesn't have an SSN (for example, a foreign national) or you need to account for an SSN or name change (which will cause problems for other related tables, such as a purchase order table that identifies the purchaser by the name or SSN field). A much better approach is to use a unique identifier and have the database engine assign an arbitrary unique number to every row automatically.

If you create a table without a unique identification column, you'll have trouble when you need to select that specific row for deletion or updates. Selecting records based on a text field can also lead to problems if the field contains special embedded characters (such as apostrophes). You'll also find it extremely awkward to create table relationships.

3.5. The SQL Delete Statement

The Delete statement is even easier to use. It specifies criteria for one or more rows that you want to remove. Be careful: once you delete a row, it's gone for good!

DELETE FROM [table] WHERE [search_condition]

The following example removes a single matching row from the Authors table:

DELETE FROM Authors WHERE au_id='172-32-1176'


If you attempt to run this specific Delete statement, you'll run into a database error. The problem is that this author record is linked to one or more records in the TitleAuthor table. The author record can't be removed unless the linked records are deleted first. (After all, it wouldn't make sense to have a book linked to an author that doesn't exist.)

The Delete and Update commands return a single piece of information: the number of affected records. You can examine this value and use it to determine whether the operation is successful or executed as expected.

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