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Normally, it doesn t matter whether your embedded SQL statements appear in uppercase or lowercase. Programmers generally follow the conventions of the host language. However, for the purposes of this chapter, I ll treat embedded SQL statements as I have other SQL statements throughout the book: I ll use uppercase for both SQL keywords and SQL identifiers.
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In order to use embedded SQL effectively, you must be able to pass values between the host program and the SQL statements. For example, your embedded SQL statement might include a WHERE clause that requires a specific value in order to evaluate the search condition. The value might be supplied by a user or by an operation within the host program. In either case, that value must in some way be passed from the program to the SQL statement. To pass values to and from an SQL statement, you can use host variables. A host variable is a type of parameter that is declared within the host language and is then referenced within the embedded SQL statement. When a host variable is used within an SQL statement, the name of
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the variable must be preceded by a colon. The colon signals to the precompiler that the named item is a variable and not a database object, such as a table or a column. As a result, you do not have to worry about whether the host variable shares the same name as a database object. You can use a host variable in an embedded SQL statement in any place where you might expect to define a value. For example, the following SELECT statement can be embedded in a C program:
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EXEC SQL SELECT CD_NAME, IN_STOCK FROM CD_INVENTORY WHERE CD_ID = :v_CD_ID;
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Notice that the v_CD_ID host variable is preceded by a colon. The colon is used only within the SQL statement. When the variable is declared earlier in the host program no colon is used.
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The preceding example begins with EXEC SQL and ends with a semicolon. In addition, no specific continuation character is required to indicate a line break in the SQL statement. These conventions are consistent with what you would expect to find in a C program. For the examples in this chapter, I will be using embedded SQL statements as they would appear within a C program. Embedded SQL is well supported in C by a number of SQL implementations.
You re not limited to referencing a host variable in the WHERE clause of a SELECT statement. For example, you can reference a variable in the SET clause of an UPDATE statement, the VALUES clause of an INSERT statement, or the WHERE clause of a DELETE or UPDATE statement. However, you cannot use a host variable in place of an SQL identifier. In other words, you cannot pass an object name, such as a table name or column name, through a host variable.
Declaring Host Variables
As I mentioned earlier, you must declare your host variables within the host program. You can declare them anywhere in the program where you would normally declare variables in a particular language. In addition, you must declare the variables according to the conventions of the host language. The only difference is that you must begin the declarations with the BEGIN DECLARE SECTION statement and end the declarations with the END DECLARE SECTION statement. These two statements notify the precompiler that the variables enclosed in the statements are to be used in the other embedded SQL statements. Let s take a look at an example of what I mean. Suppose you want to declare two variables, one to receive a value that identifies the CD and one that receives the name of the CD. Your variable declaration in C might look like the following:
EXEC SQL BEGIN DECLARE SECTION; long v_CD_ID;
/* compact disc ID */
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