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SQL: A Beginner s Guide
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varchar v_CD_NAME[60]; EXEC SQL END DECLARE SECTION;
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As you can see, the variable declarations are enclosed in the two declaration-related SQL statements. Notice that these statements are treated just like any other embedded SQL statements in C. Each statement begins with EXEC SQL and ends with a semicolon. Two host variables are being declared in this section. The first one, v_CD_ID, is declared with the long data type, and the second variable, v_CD_NAME, is declared with the varchar data type. The two variable declarations follow the conventions of the host language. Notice that a comment follows each declaration. The comments also adhere to the conventions of the host language. When host variables are used in SQL statements, an impedance mismatch can occur as a result of the differences between host language data types and SQL data types. As you can see in the preceding example, the variables are declared with C data types; however, the variables will be used in SQL statements to pass data to columns that are configured with SQL data types. If the data types are compatible, then data can be passed through the variables; otherwise, the impedance mismatch between the data types prevents values from being passed. For example, the v_CD_ID variable is configured with the long data type, which is compatible with the INTEGER data type in SQL, and the v_CD_NAME variable is configured with the varchar data type, which is compatible with the CHARACTER VARYING data type (usually abbreviated as VARCHAR) in SQL. As a result, you can pass data through these variables as long as the receiving columns are configured with the compatible data types.
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You state that data can be passed from the variable to the SQL statement if the data types are compatible. How do you pass data if they re not compatible Most programming languages contain at least some data types that do not match up with SQL data types. If this situation arises, you can use the CAST value expression within the SQL statement to convert the variable value into a value that can be used by the SQL statement. In effect, the CAST value expression changes the data type of the value. For example, we can modify the previous embedded statement to convert the v_CD_ID host variable, as shown in the following example:
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EXEC SQL SELECT CD_NAME, IN_STOCK FROM CD_INVENTORY WHERE CD_ID = CAST(:v_CD_ID AS INT);
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As you can see, CAST is used to convert the value in the host variable to an INTEGER data type. (For more information about CAST, see 10.) To determine which data types in a host language are compatible with data types in SQL, you should refer to the SQL:2006 standard, language-specific documentation, or product-specific documentation.
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Accessing SQL Data from Your Host Program
Retrieving SQL Data
As you have seen throughout this book, the process of querying data in an SQL database involves executing a SELECT statement that in turn retrieves data from the applicable table or tables and returns that data in a result set. A result set can be made up of one or more rows and one or more columns. When you are querying data interactively, the multiple rows present no problems because your client application can handle more than one row. However, when querying data from an embedded SQL statement, multiple rows have to be handled through a cursor in order to allow that host language to work with one row at a time. A cursor, as you ll recall, acts as a pointer to specific rows in the result set. The cursor declaration defines the SELECT statement that retrieves data from the database, and cursor-related statements are then used to retrieve the individual rows from that result set. (For more information about cursors, see 15.) Cursors, then, provide a solution to one type of impedance mismatch that can occur between SQL and the host language. Specifically, SQL returns data in sets, and most programming languages cannot handle sets. By using some sort of looping construct within the programming language and then using the SQL FETCH statement, you can cycle through each row in the result set to retrieve the data that you need. Despite the availability of cursors to embedded SQL, there are often times when you know that your database query will return only one row. For example, you might want to retrieve data about a specific CD or performing artist, in which case, a cursor is unnecessary. To facilitate single-row retrievals, embedded SQL supports the singleton SELECT statement. A singleton SELECT statement is similar to a regular SELECT statement except in two ways:
You do not include a GROUP BY, HAVING, or ORDER BY clause. You include an INTO clause that specifies the host variables that will receive the data returned by the SELECT statement to the host program.
For example, suppose your SELECT statement returns the name of the CD and the number in stock, as shown in the following embedded statement:
EXEC SQL SELECT INTO FROM WHERE CD_NAME, IN_STOCK :v_CD_NAME, :v_IN_STOCK CD_INVENTORY CD_ID = :v_CD_ID;
As you can see in this statement, the v_CD_ID variable is used to specify which CD should be returned. The value is entered by the user, and the variable passes that value from the host program into the SELECT statement. Now let s take a look at the INTO clause. Notice that the clause contains two variables, the same number of variables as the number of columns retrieved from the CD_INVENTORY table. These variables are declared in the same way as the other host variables that we ve seen. Because this SELECT statement returns only one row and two columns, only two values are returned. These values are transferred to the variables. The variables must be specified in the same order as the column names are specified.
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