Retrieving Single Tuples with Embedded SQL
To illustrate the concepts of embedded SQL, we will use C as the host programming language. When using C as the host language, an embedded SQL statement is distinguished from programming language statements by prefixing it with the key-words EXEC SQL so that a preprocessor (or precompiler) can separate embedded SQL statements from the host language code. The SQL statements within a program are terminated by a matching END-EXEC or by a semicolon (;). Similar rules apply to embedding SQL in other programming languages.
Within an embedded SQL command, we may refer to specially declared C program variables. These are called shared variables because they are used in both the C pro-gram and the embedded SQL statements. Shared variables are prefixed by a colon (:) when they appear in an SQL statement. This distinguishes program variable names from the names of database schema constructs such as attributes (column names) and relations (table names). It also allows program variables to have the same names as attribute names, since they are distinguishable by the colon (:) prefix in the SQL statement. Names of database schema constructs—such as attributes and relations—can only be used within the SQL commands, but shared program variables can be used elsewhere in the C program without the colon (:) prefix.
Suppose that we want to write C programs to process the COMPANY database in Figure 3.5. We need to declare program variables to match the types of the database attributes that the program will process. The programmer can choose the names of the program variables; they may or may not have names that are identical to their corresponding database attributes. We will use the C program variables declared in Figure 13.1 for all our examples and show C program segments without variable declarations. Shared variables are declared within a declare section in the program, as shown in Figure 13.1 (lines 1 through 7). A few of the common bindings of C types to SQL types are as follows. The SQL types INTEGER, SMALLINT, REAL, and DOUBLE are mapped to the C types long, short, float, and double, respectively. Fixed-length and varying-length strings (CHAR[i], VARCHAR[i]) in SQL can be mapped to arrays of characters (char [i+1], varchar [i+1]) in C that are one character longer than the SQL type because strings in C are terminated by a NULL
character (\0), which is not part of the character string itself. Although varchar is not a standard C data type, it is permitted when C is used for SQL database programming.
Notice that the only embedded SQL commands in Figure 13.1 are lines 1 and 7, which tell the precompiler to take note of the C variable names between BEGIN DECLARE and END DECLARE because they can be included in embedded SQL statements—as long as they are preceded by a colon (:). Lines 2 through 5 are regular C program declarations. The C program variables declared in lines 2 through 5 correspond to the attributes of the EMPLOYEE and DEPARTMENT tables from the COMPANY database in Figure 3.5 that was declared by the SQL DDL in Figure 4.1. The variables declared in line 6—SQLCODE and SQLSTATE—are used to communicate errors and exception conditions between the database system and the executing program. Line 0 shows a program variable loop that will not be used in any embed-ded SQL statement, so it is declared outside the SQL declare section.
Connecting to the Database. The SQL command for establishing a connection to a database has the following form:
CONNECT TO <server name>AS <connection name>
AUTHORIZATION <user account name and password> ;
In general, since a user or program can access several database servers, several connections can be established, but only one connection can be active at any point in time. The programmer or user can use the <connection name> to change from the currently active connection to a different one by using the following command:
SET CONNECTION <connection name> ;
Once a connection is no longer needed, it can be terminated by the following command:
DISCONNECT <connection name> ;
In the examples in this chapter, we assume that the appropriate connection has already been established to the COMPANY database, and that it is the currently active connection.
Communicating between the Program and the DBMS Using SQLCODE and SQLSTATE. The two special communication variables that are used by the DBMS to communicate exception or error conditions to the program are SQLCODE and SQLSTATE. The SQLCODE variable shown in Figure 13.1 is an integer variable. After each database command is executed, the DBMS returns a value in SQLCODE. A value of 0 indicates that the statement was executed successfully by the DBMS. If SQLCODE > 0 (or, more specifically, if SQLCODE = 100), this indicates that no more data (records) are available in a query result. If SQLCODE < 0, this indicates some error has occurred. In some systems—for example, in the Oracle RDBMS— SQLCODE is a field in a record structure called SQLCA (SQL communication area), so it is referenced as SQLCA.SQLCODE. In this case, the definition of SQLCA must be included in the C program by including the following line:
EXEC SQL include SQLCA ;
In later versions of the SQL standard, a communication variable called SQLSTATE was added, which is a string of five characters. A value of ‘00000’ in SQLSTATE indicates no error or exception; other values indicate various errors or exceptions. For example, ‘02000’ indicates ‘no more data’ when using SQLSTATE. Currently, both SQLSTATE and SQLCODE are available in the SQL standard. Many of the error and exception codes returned in SQLSTATE are supposed to be standardized for all SQL vendors and platforms, whereas the codes returned in SQLCODE are not standardized but are defined by the DBMS vendor. Hence, it is generally better to use SQLSTATE because this makes error handling in the application programs inde-pendent of a particular DBMS. As an exercise, the reader should rewrite the exam-ples given later in this chapter using SQLSTATE instead of SQLCODE.
Example of Embedded SQL Programming. Our first example to illustrate embedded SQL programming is a repeating program segment (loop) that takes as input a Social Security number of an employee and prints some information from the corresponding EMPLOYEE record in the database. The C program code is shown as program segment E1 in Figure 13.2. The program reads (inputs) an Ssn value and then retrieves the EMPLOYEE tuple with that Ssn from the database via the embedded SQL command. The INTO clause (line 5) specifies the program variables into which attribute values from the database record are retrieved. C program variables in the INTO clause are prefixed with a colon (:), as we discussed earlier. The INTO clause can be used in this way only when the query result is a single record; if multi-ple records are retrieved, an error will be generated. We will see how multiple records are handled in Section 13.2.2.
Line 7 in E1 illustrates the communication between the database and the program through the special variable SQLCODE. If the value returned by the DBMS in SQLCODE is 0, the previous statement was executed without errors or exception conditions. Line 7 checks this and assumes that if an error occurred, it was because
no EMPLOYEE tuple existed with the given Ssn; therefore it outputs a message to that effect (line 8).
In E1 a single record is selected by the embedded SQL query (because Ssn is a key attribute of EMPLOYEE);. When a single record is retrieved, the programmer can assign its attribute values directly to C program variables in the INTO clause, as in line 5. In general, an SQL query can retrieve many tuples. In that case, the C pro-gram will typically go through the retrieved tuples and process them one at a time. The concept of a cursor is used to allow tuple-at-a-time processing of a query result by the host language program. We describe cursors next.