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In addition to its role as an interactive database language, SQL is used for programmatic access to relational databases: I The most common technique for programmatic use of SQL is embedded SQL, where SQL statements are embedded into the application program, intermixed with the statements of a host programming language such as C or COBOL. I Embedded SQL statements are processed by a special SQL precompiler. They begin with a special introducer (usually EXEC SQL) and end with a terminator, which varies from one host language to another. I Variables from the application program, called host variables, can be used in embedded SQL statements wherever a constant can appear. These input host variables tailor the embedded SQL statement to the particular situation. I Host variables are also used to receive the results of database queries. The values of these output host variables can then be processed by the application program. I Queries that produce a single row of data are handled with the singleton SELECT statement of embedded SQL, which specifies both the query and the host variables to receive the retrieved data. I Queries that produce multiple rows of query results are handled with cursors in embedded SQL. The DECLARE CURSOR statement defines the query, the OPEN statement begins query processing, the FETCH statement retrieves successive rows of query results, and the CLOSE statement ends query processing. I The positioned UPDATE and DELETE statements can be used to update or delete the row currently selected by a cursor.
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SQL: The Complete Reference
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he embedded SQL programming features described in 17 are collectively known as static SQL. Static SQL is adequate for writing all of the programs typically required in a data processing application. For example, in the order-processing application of the sample database, you can use static SQL to write programs that handle order entry, order updates, order inquiries, customer inquiries, customer file maintenance, and programs that produce all types of reports. In every one of these programs, the pattern of database access is decided by the programmer and hard-coded into the program as a series of embedded SQL statements. There is an important class of applications, however, where the pattern of database access cannot be determined in advance. A graphic query tool or a report writer, for example, must be able to decide at runtime which SQL statements it will use to access the database. A personal computer spreadsheet that supports host database access must also be able to send a query to the host DBMS for execution on the fly. These programs and other general-purpose database front-ends cannot be written using static SQL techniques. They require an advanced form of embedded SQL, called dynamic SQL, described in this chapter.
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Limitations of Static SQL
As the name static SQL implies, a program built using the embedded SQL features described in 17 (host variables, cursors, and the DECLARE CURSOR, OPEN, FETCH, and CLOSE statements) has a predetermined, fixed pattern of database access. For each embedded SQL statement in the program, the tables and columns referenced by that statement are determined in advance by the programmer and hard-coded into the embedded SQL statement. Input host variables provide some flexibility in static SQL, but they don t fundamentally alter its static nature. Recall that a host variable can appear anywhere a constant is allowed in a SQL statement. You can use a host variable to alter a search condition:
exec sql select name, quota, sales from salesreps where quota > :cutoff_amount;
You can also use a host variable to change the data inserted or updated in a database:
exec sql update salesreps set quota = quota + :increase where quota >:cutoff_amount;
18:
Dynamic SQL *
However, you cannot use a host variable in place of a table name or a column reference. The attempted use of the host variables which_table and which_column in these statements is illegal:
exec sql update :which_table set :which_column = 0; exec sql declare cursor cursor7 for select * from :which_table;
Even if you could use a host variable in this way (and you cannot), another problem would immediately arise. The number of columns produced by the query in the second statement would vary, depending on which table was specified by the host variable. For the OFFICES table, the query results would have six columns; for the SALESREPS table, they would have nine columns. Furthermore, the data types of the columns would be different for the two tables. But to write a FETCH statement for the query, you must know in advance how many columns of query results there will be and their data types, because you must specify a host variable to receive each column:
exec sql fetch cursor7 into :var1, :var2, :var3;
As this discussion illustrates, if a program must be able to determine at runtime which SQL statements it will use, or which tables and columns it will reference, static SQL is inadequate for the task. Dynamic SQL overcomes these limitations. Dynamic SQL has been supported by the IBM SQL products since their introduction, and it has been supported for many years by the minicomputer-based and UNIX-based commercial RDBMS products. However, dynamic SQL was not specified by the original ANSI/ISO SQL1 standard; the standard defined only static SQL. The absence of dynamic SQL from the SQL1 standard is ironic, given the popular notion that the standard allowed you to build front-end database tools that are portable across many different DBMS brands. In fact, such front-end tools must almost always be built using dynamic SQL. In the absence of an ANSI/ISO standard, DB2 set the de facto standard for dynamic SQL. The other IBM databases of the day (SQL/DS and OS/2 Extended Edition) were nearly identical to DB2 in their dynamic SQL support, and most other SQL products also followed the DB2 standard. In 1992, the SQL2 standard added official support for dynamic SQL, mostly following the path set by IBM. The SQL2 standard does not require dynamic SQL support at the lowest level of compliance (Entry), but dynamic SQL support is required for products claiming the Intermediate or Full compliance levels to the SQL standard.
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