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SQL: A Beginner s Guide
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In 15, when I discuss SQL cursors, I introduce you to embedded SQL. As you ll recall from that discussion, embedded SQL refers to SQL statements that are interspersed in some type of application programming language. The SQL statements are blended into the host language to allow the source program to be able to access and modify SQL data and the underlying database structure. According to the SQL:2006 standard, you can embed SQL statements in the following programming languages:
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Ada C COBOL Fortran MUMPS Pascal PL/I
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Although the standard supports embedded SQL statements in these languages, SQL implementations rarely support embedding statements in all these languages. An implementation might be limited to only one or two programming languages, and some implementations might not support embedded SQL at all (although most implementations provide embedded SQL for at least one language). In addition, many implementations support embedded SQL in languages other than those specified in the SQL standard. For example, Oracle offers SQLJ, which supports embedded SQL in Java programs. When a program contains embedded SQL statements, it must be compiled in a manner different from regular programs. Figure 17-2 illustrates the process followed when compiling these programs. As you can see from the figure, we start with a program file that contains the host programming language and the embedded SQL statements. Before the program is compiled, it is submitted to a precompiler that is specific to the host programming language and the SQL implementation. The precompiler strips the SQL statements out of the host language code, often converting them into comments to document the original statements, and replaces them with calls to vendor-supplied library routines that access the SQL statements. As a result, two files are created, one for the host language and one for the SQL statements. Once a file is created for the host language, the source program is compiled in its normal way, as would be expected from a specific language. The output from the host language compiler is the object code, which is linked to various library routines. From this, an executable program is generated that links to the application plan. The application plan is created by a bind utility that validates and optimizes the SQL statements. The plan contains the SQL statements and information that the program needs to access the database.
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Accessing SQL Data from Your Host Program
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Source program containing host language and SQL statements Specific to SQL implementation and host language
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SQL statements
Host language compiler
Bind program
Object code
Application plan
Library routines
Linker
Executable program
Figure 17-2
Compiling programs that contain embedded SQL
Creating an Embedded SQL Statement
When you develop a program that contains embedded SQL, you must follow specific conventions that determine how the SQL code is added to the program. These conventions are based on a combination of special SQL language elements and the requirements of the host programming language. In order to be used in a host language, an embedded SQL statement must conform to the following guidelines:
Each SQL statement must begin with a qualified prefix. Each SQL statement may or may not require a qualified terminator, depending on the host language. Line breaks within the SQL statement must be handled according to the style of the host language. The placement of comments must be handled according to the style of the host language.
SQL: A Beginner s Guide
Language
Ada C COBOL Fortran MUMPS Pascal PL/I
Prefix
EXEC SQL EXEC SQL EXEC SQL EXEC SQL &SQL( EXEC SQL EXEC SQL
Terminator
; ; END-EXEC (no terminator) ) ; ;
Table 17-1 Beginning and Ending an SQL Statement Most embedded SQL statements require a qualified prefix and terminator. Table 17-1 provides the prefix and terminator for each supported programming language. As you can see from the table, the Ada, C, Pascal, and PL/I languages all handle an embedded SQL statement in the same way. For example, suppose you want to embed a SELECT statement that retrieves the CD_NAME and IN_STOCK columns from the CD_INVENTORY table. To do so, you would use the following statement:
EXEC SQL SELECT CD_NAME, IN_STOCK FROM CD_INVENTORY;
Notice that the statement is preceded by the EXEC SQL prefix and ended with the semicolon terminator. If you were to create a similar statement in another language, your prefix or terminator might be different. In the case of MUMPS, they would both be different.
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