Updating Tables in Visual Studio .NET

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Updating Tables
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You can change information already in a table by using the Update statement, which was introduced in the last chapter Before exploring how to update a table, let's revise the Customers table so that you'll be able to follow along with examples shown within this section Insert the following data into the table: CustomerNumber CHAR(30) CustomerFirstName CHAR(30) CustomerLastName CHAR(30) Customerstreet CHAR(30) CustomerCity CHAR(30) Customerstate CHAR(2) CustomerZipCode CHAR(10) Discount LONG Price LONG Discountprice LONG
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As you'll recall from the preceding chapter, the Update statement is used to change values in one or more columns in one or more rows of the table The Update statement requires that you provide the name of the table and a Set clause that identifies column names and new values that will be placed in the column The new values overwrite the column's existing values An Update statement updates all rows unless you include a WHERE clause, which specifies a condition that must exist in a row before the row is updated
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Let's say that you want to change Bob Jones' address to 5 Main Street Here's what you need to write in the query:
UPDATE Customers SET Customerstreet = l Main Street1 5 WHERE CustomerFirstName = 'Bob1 and CustomerLastName
'Jones'
Only one row is updated by the previous query because only one customer is named Bob Jones If another customer has the same name, then his street address would also be changed In the real world, a customer number rather than a customer name is used to identify a customer so that only the proper customer information is updated Sometimes, you'll want to update multiple rows by replacing a sales representative with a new one You could find rows that contain the current sales representative and then replace it with the new sales representative There are four common WHERE clause expressions that are used to update multiple rows of a table These are
The IN test Updates only if a value matches a value in the IN clause The IS NULL test Rows that don't have a value in the specified column are updated when the IS NULL operator is used in the WHERE clause expression The comparison test You've seen this used in the preceding example All rows A query can direct the DBMS to update the specified column in all rows of a table by excluding the WHERE clause in the query
Be cautious whenever you execute a query that updates multiple rows, because an error in a query is multiplied by the number of rows in a table The IN clause provides two or more values that are compared to the value of the designated column in the IN clause Rows whose columns contain one of these values are updated by the UPDATE statement This is shown in the next example, where the value of the Discount column is changed to 25 if the current value of the Discount column is either 12 or 15:
UPDATE Customers SET Discount = 25 WHERE Discount IN (12,15)
The IS NULL test determines if the column is NULL, that is, if the column is devoid of any value If so, then the column is updated The next example uses the IS NULL test to update the Discount column if there isn't a last name in the CustomerLastName column:
UPDATE Customers SET Discount = 0 WHERE CustomerLastName IS NULL
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Another common use of the Update statement is to change the value of a column according to a calculation For example, we can calculate the discounted price of an item by using the value of the Price column and the value of the Discount column The result of the calculation can then be inserted into the DiscountPrice column Here's how this is done:
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